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diff --git a/REORG.TODO/manual/stdio.texi b/REORG.TODO/manual/stdio.texi new file mode 100644 index 0000000000..29f3fed89b --- /dev/null +++ b/REORG.TODO/manual/stdio.texi @@ -0,0 +1,5663 @@ +@node I/O on Streams, Low-Level I/O, I/O Overview, Top +@c %MENU% High-level, portable I/O facilities +@chapter Input/Output on Streams +@c fix an overfull: +@tex +\hyphenation{which-ever} +@end tex + +This chapter describes the functions for creating streams and performing +input and output operations on them. As discussed in @ref{I/O +Overview}, a stream is a fairly abstract, high-level concept +representing a communications channel to a file, device, or process. + +@menu +* Streams:: About the data type representing a stream. +* Standard Streams:: Streams to the standard input and output + devices are created for you. +* Opening Streams:: How to create a stream to talk to a file. +* Closing Streams:: Close a stream when you are finished with it. +* Streams and Threads:: Issues with streams in threaded programs. +* Streams and I18N:: Streams in internationalized applications. +* Simple Output:: Unformatted output by characters and lines. +* Character Input:: Unformatted input by characters and words. +* Line Input:: Reading a line or a record from a stream. +* Unreading:: Peeking ahead/pushing back input just read. +* Block Input/Output:: Input and output operations on blocks of data. +* Formatted Output:: @code{printf} and related functions. +* Customizing Printf:: You can define new conversion specifiers for + @code{printf} and friends. +* Formatted Input:: @code{scanf} and related functions. +* EOF and Errors:: How you can tell if an I/O error happens. +* Error Recovery:: What you can do about errors. +* Binary Streams:: Some systems distinguish between text files + and binary files. +* File Positioning:: About random-access streams. +* Portable Positioning:: Random access on peculiar ISO C systems. +* Stream Buffering:: How to control buffering of streams. +* Other Kinds of Streams:: Streams that do not necessarily correspond + to an open file. +* Formatted Messages:: Print strictly formatted messages. +@end menu + +@node Streams +@section Streams + +For historical reasons, the type of the C data structure that represents +a stream is called @code{FILE} rather than ``stream''. Since most of +the library functions deal with objects of type @code{FILE *}, sometimes +the term @dfn{file pointer} is also used to mean ``stream''. This leads +to unfortunate confusion over terminology in many books on C. This +manual, however, is careful to use the terms ``file'' and ``stream'' +only in the technical sense. +@cindex file pointer + +@pindex stdio.h +The @code{FILE} type is declared in the header file @file{stdio.h}. + +@comment stdio.h +@comment ISO +@deftp {Data Type} FILE +This is the data type used to represent stream objects. A @code{FILE} +object holds all of the internal state information about the connection +to the associated file, including such things as the file position +indicator and buffering information. Each stream also has error and +end-of-file status indicators that can be tested with the @code{ferror} +and @code{feof} functions; see @ref{EOF and Errors}. +@end deftp + +@code{FILE} objects are allocated and managed internally by the +input/output library functions. Don't try to create your own objects of +type @code{FILE}; let the library do it. Your programs should +deal only with pointers to these objects (that is, @code{FILE *} values) +rather than the objects themselves. +@c !!! should say that FILE's have "No user-serviceable parts inside." + +@node Standard Streams +@section Standard Streams +@cindex standard streams +@cindex streams, standard + +When the @code{main} function of your program is invoked, it already has +three predefined streams open and available for use. These represent +the ``standard'' input and output channels that have been established +for the process. + +These streams are declared in the header file @file{stdio.h}. +@pindex stdio.h + +@comment stdio.h +@comment ISO +@deftypevar {FILE *} stdin +The @dfn{standard input} stream, which is the normal source of input for the +program. +@end deftypevar +@cindex standard input stream + +@comment stdio.h +@comment ISO +@deftypevar {FILE *} stdout +The @dfn{standard output} stream, which is used for normal output from +the program. +@end deftypevar +@cindex standard output stream + +@comment stdio.h +@comment ISO +@deftypevar {FILE *} stderr +The @dfn{standard error} stream, which is used for error messages and +diagnostics issued by the program. +@end deftypevar +@cindex standard error stream + +On @gnusystems{}, you can specify what files or processes correspond to +these streams using the pipe and redirection facilities provided by the +shell. (The primitives shells use to implement these facilities are +described in @ref{File System Interface}.) Most other operating systems +provide similar mechanisms, but the details of how to use them can vary. + +In @theglibc{}, @code{stdin}, @code{stdout}, and @code{stderr} are +normal variables which you can set just like any others. For example, +to redirect the standard output to a file, you could do: + +@smallexample +fclose (stdout); +stdout = fopen ("standard-output-file", "w"); +@end smallexample + +Note however, that in other systems @code{stdin}, @code{stdout}, and +@code{stderr} are macros that you cannot assign to in the normal way. +But you can use @code{freopen} to get the effect of closing one and +reopening it. @xref{Opening Streams}. + +The three streams @code{stdin}, @code{stdout}, and @code{stderr} are not +unoriented at program start (@pxref{Streams and I18N}). + +@node Opening Streams +@section Opening Streams + +@cindex opening a stream +Opening a file with the @code{fopen} function creates a new stream and +establishes a connection between the stream and a file. This may +involve creating a new file. + +@pindex stdio.h +Everything described in this section is declared in the header file +@file{stdio.h}. + +@comment stdio.h +@comment ISO +@deftypefun {FILE *} fopen (const char *@var{filename}, const char *@var{opentype}) +@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@acsmem{} @acsfd{} @aculock{}}} +@c fopen may leak the list lock if cancelled within _IO_link_in. +The @code{fopen} function opens a stream for I/O to the file +@var{filename}, and returns a pointer to the stream. + +The @var{opentype} argument is a string that controls how the file is +opened and specifies attributes of the resulting stream. It must begin +with one of the following sequences of characters: + +@table @samp +@item r +Open an existing file for reading only. + +@item w +Open the file for writing only. If the file already exists, it is +truncated to zero length. Otherwise a new file is created. + +@item a +Open a file for append access; that is, writing at the end of file only. +If the file already exists, its initial contents are unchanged and +output to the stream is appended to the end of the file. +Otherwise, a new, empty file is created. + +@item r+ +Open an existing file for both reading and writing. The initial contents +of the file are unchanged and the initial file position is at the +beginning of the file. + +@item w+ +Open a file for both reading and writing. If the file already exists, it +is truncated to zero length. Otherwise, a new file is created. + +@item a+ +Open or create file for both reading and appending. If the file exists, +its initial contents are unchanged. Otherwise, a new file is created. +The initial file position for reading is at the beginning of the file, +but output is always appended to the end of the file. +@end table + +As you can see, @samp{+} requests a stream that can do both input and +output. When using such a stream, you must call @code{fflush} +(@pxref{Stream Buffering}) or a file positioning function such as +@code{fseek} (@pxref{File Positioning}) when switching from reading +to writing or vice versa. Otherwise, internal buffers might not be +emptied properly. + +Additional characters may appear after these to specify flags for the +call. Always put the mode (@samp{r}, @samp{w+}, etc.) first; that is +the only part you are guaranteed will be understood by all systems. + +@Theglibc{} defines additional characters for use in @var{opentype}: + +@table @samp +@item c +The file is opened with cancellation in the I/O functions disabled. + +@item e +The underlying file descriptor will be closed if you use any of the +@code{exec@dots{}} functions (@pxref{Executing a File}). (This is +equivalent to having set @code{FD_CLOEXEC} on that descriptor. +@xref{Descriptor Flags}.) + +@item m +The file is opened and accessed using @code{mmap}. This is only +supported with files opened for reading. + +@item x +Insist on creating a new file---if a file @var{filename} already +exists, @code{fopen} fails rather than opening it. If you use +@samp{x} you are guaranteed that you will not clobber an existing +file. This is equivalent to the @code{O_EXCL} option to the +@code{open} function (@pxref{Opening and Closing Files}). + +The @samp{x} modifier is part of @w{ISO C11}. +@end table + +The character @samp{b} in @var{opentype} has a standard meaning; it +requests a binary stream rather than a text stream. But this makes no +difference in POSIX systems (including @gnusystems{}). If both +@samp{+} and @samp{b} are specified, they can appear in either order. +@xref{Binary Streams}. + +@cindex stream orientation +@cindex orientation, stream +If the @var{opentype} string contains the sequence +@code{,ccs=@var{STRING}} then @var{STRING} is taken as the name of a +coded character set and @code{fopen} will mark the stream as +wide-oriented with appropriate conversion functions in place to convert +from and to the character set @var{STRING}. Any other stream +is opened initially unoriented and the orientation is decided with the +first file operation. If the first operation is a wide character +operation, the stream is not only marked as wide-oriented, also the +conversion functions to convert to the coded character set used for the +current locale are loaded. This will not change anymore from this point +on even if the locale selected for the @code{LC_CTYPE} category is +changed. + +Any other characters in @var{opentype} are simply ignored. They may be +meaningful in other systems. + +If the open fails, @code{fopen} returns a null pointer. + +When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a +32 bit machine this function is in fact @code{fopen64} since the LFS +interface replaces transparently the old interface. +@end deftypefun + +You can have multiple streams (or file descriptors) pointing to the same +file open at the same time. If you do only input, this works +straightforwardly, but you must be careful if any output streams are +included. @xref{Stream/Descriptor Precautions}. This is equally true +whether the streams are in one program (not usual) or in several +programs (which can easily happen). It may be advantageous to use the +file locking facilities to avoid simultaneous access. @xref{File +Locks}. + +@comment stdio.h +@comment Unix98 +@deftypefun {FILE *} fopen64 (const char *@var{filename}, const char *@var{opentype}) +@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@acsmem{} @acsfd{} @aculock{}}} +This function is similar to @code{fopen} but the stream it returns a +pointer for is opened using @code{open64}. Therefore this stream can be +used even on files larger than @twoexp{31} bytes on 32 bit machines. + +Please note that the return type is still @code{FILE *}. There is no +special @code{FILE} type for the LFS interface. + +If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 +bits machine this function is available under the name @code{fopen} +and so transparently replaces the old interface. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypevr Macro int FOPEN_MAX +The value of this macro is an integer constant expression that +represents the minimum number of streams that the implementation +guarantees can be open simultaneously. You might be able to open more +than this many streams, but that is not guaranteed. The value of this +constant is at least eight, which includes the three standard streams +@code{stdin}, @code{stdout}, and @code{stderr}. In POSIX.1 systems this +value is determined by the @code{OPEN_MAX} parameter; @pxref{General +Limits}. In BSD and GNU, it is controlled by the @code{RLIMIT_NOFILE} +resource limit; @pxref{Limits on Resources}. +@end deftypevr + +@comment stdio.h +@comment ISO +@deftypefun {FILE *} freopen (const char *@var{filename}, const char *@var{opentype}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{} @acsfd{}}} +@c Like most I/O operations, this one is guarded by a recursive lock, +@c released even upon cancellation, but cancellation may leak file +@c descriptors and leave the stream in an inconsistent state (e.g., +@c still bound to the closed descriptor). Also, if the stream is +@c part-way through a significant update (say running freopen) when a +@c signal handler calls freopen again on the same stream, the result is +@c likely to be an inconsistent stream, and the possibility of closing +@c twice file descriptor number that the stream used to use, the second +@c time when it might have already been reused by another thread. +This function is like a combination of @code{fclose} and @code{fopen}. +It first closes the stream referred to by @var{stream}, ignoring any +errors that are detected in the process. (Because errors are ignored, +you should not use @code{freopen} on an output stream if you have +actually done any output using the stream.) Then the file named by +@var{filename} is opened with mode @var{opentype} as for @code{fopen}, +and associated with the same stream object @var{stream}. + +If the operation fails, a null pointer is returned; otherwise, +@code{freopen} returns @var{stream}. On Linux, @code{freopen} may also +fail and set @code{errno} to @code{EBUSY} when the kernel structure for +the old file descriptor was not initialized completely before @code{freopen} +was called. This can only happen in multi-threaded programs, when two +threads race to allocate the same file descriptor number. To avoid the +possibility of this race, do not use @code{close} to close the underlying +file descriptor for a @code{FILE}; either use @code{freopen} while the +file is still open, or use @code{open} and then @code{dup2} to install +the new file descriptor. + +@code{freopen} has traditionally been used to connect a standard stream +such as @code{stdin} with a file of your own choice. This is useful in +programs in which use of a standard stream for certain purposes is +hard-coded. In @theglibc{}, you can simply close the standard +streams and open new ones with @code{fopen}. But other systems lack +this ability, so using @code{freopen} is more portable. + +When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a +32 bit machine this function is in fact @code{freopen64} since the LFS +interface replaces transparently the old interface. +@end deftypefun + +@comment stdio.h +@comment Unix98 +@deftypefun {FILE *} freopen64 (const char *@var{filename}, const char *@var{opentype}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{} @acsfd{}}} +This function is similar to @code{freopen}. The only difference is that +on 32 bit machine the stream returned is able to read beyond the +@twoexp{31} bytes limits imposed by the normal interface. It should be +noted that the stream pointed to by @var{stream} need not be opened +using @code{fopen64} or @code{freopen64} since its mode is not important +for this function. + +If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 +bits machine this function is available under the name @code{freopen} +and so transparently replaces the old interface. +@end deftypefun + +In some situations it is useful to know whether a given stream is +available for reading or writing. This information is normally not +available and would have to be remembered separately. Solaris +introduced a few functions to get this information from the stream +descriptor and these functions are also available in @theglibc{}. + +@comment stdio_ext.h +@comment GNU +@deftypefun int __freadable (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} +The @code{__freadable} function determines whether the stream +@var{stream} was opened to allow reading. In this case the return value +is nonzero. For write-only streams the function returns zero. + +This function is declared in @file{stdio_ext.h}. +@end deftypefun + +@comment stdio_ext.h +@comment GNU +@deftypefun int __fwritable (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} +The @code{__fwritable} function determines whether the stream +@var{stream} was opened to allow writing. In this case the return value +is nonzero. For read-only streams the function returns zero. + +This function is declared in @file{stdio_ext.h}. +@end deftypefun + +For slightly different kinds of problems there are two more functions. +They provide even finer-grained information. + +@comment stdio_ext.h +@comment GNU +@deftypefun int __freading (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} +The @code{__freading} function determines whether the stream +@var{stream} was last read from or whether it is opened read-only. In +this case the return value is nonzero, otherwise it is zero. +Determining whether a stream opened for reading and writing was last +used for writing allows to draw conclusions about the content about the +buffer, among other things. + +This function is declared in @file{stdio_ext.h}. +@end deftypefun + +@comment stdio_ext.h +@comment GNU +@deftypefun int __fwriting (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} +The @code{__fwriting} function determines whether the stream +@var{stream} was last written to or whether it is opened write-only. In +this case the return value is nonzero, otherwise it is zero. + +This function is declared in @file{stdio_ext.h}. +@end deftypefun + + +@node Closing Streams +@section Closing Streams + +@cindex closing a stream +When a stream is closed with @code{fclose}, the connection between the +stream and the file is canceled. After you have closed a stream, you +cannot perform any additional operations on it. + +@comment stdio.h +@comment ISO +@deftypefun int fclose (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@aculock{} @acsmem{} @acsfd{}}} +@c After fclose, it is undefined behavior to use the stream it points +@c to. Therefore, one must only call fclose when the stream is +@c otherwise unused. Concurrent uses started before will complete +@c successfully because of the lock, which makes it MT-Safe. Calling it +@c from a signal handler is perfectly safe if the stream is known to be +@c no longer used, which is a precondition for fclose to be safe in the +@c first place; since this is no further requirement, fclose is safe for +@c use in async signals too. After calling fclose, you can no longer +@c use the stream, not even to fclose it again, so its memory and file +@c descriptor may leak if fclose is canceled before @c releasing them. +@c That the stream must be unused and it becomes unused after the call +@c is what would enable fclose to be AS- and AC-Safe while freopen +@c isn't. However, because of the possibility of leaving __gconv_lock +@c taken upon cancellation, AC-Safety is lost. +This function causes @var{stream} to be closed and the connection to +the corresponding file to be broken. Any buffered output is written +and any buffered input is discarded. The @code{fclose} function returns +a value of @code{0} if the file was closed successfully, and @code{EOF} +if an error was detected. + +It is important to check for errors when you call @code{fclose} to close +an output stream, because real, everyday errors can be detected at this +time. For example, when @code{fclose} writes the remaining buffered +output, it might get an error because the disk is full. Even if you +know the buffer is empty, errors can still occur when closing a file if +you are using NFS. + +The function @code{fclose} is declared in @file{stdio.h}. +@end deftypefun + +To close all streams currently available @theglibc{} provides +another function. + +@comment stdio.h +@comment GNU +@deftypefun int fcloseall (void) +@safety{@prelim{}@mtunsafe{@mtasurace{:streams}}@asunsafe{}@acsafe{}} +@c Like fclose, using any previously-opened streams after fcloseall is +@c undefined. However, the implementation of fcloseall isn't equivalent +@c to calling fclose for all streams: it just flushes and unbuffers all +@c streams, without any locking. It's the flushing without locking that +@c makes it unsafe. +This function causes all open streams of the process to be closed and +the connections to corresponding files to be broken. All buffered data +is written and any buffered input is discarded. The @code{fcloseall} +function returns a value of @code{0} if all the files were closed +successfully, and @code{EOF} if an error was detected. + +This function should be used only in special situations, e.g., when an +error occurred and the program must be aborted. Normally each single +stream should be closed separately so that problems with individual +streams can be identified. It is also problematic since the standard +streams (@pxref{Standard Streams}) will also be closed. + +The function @code{fcloseall} is declared in @file{stdio.h}. +@end deftypefun + +If the @code{main} function to your program returns, or if you call the +@code{exit} function (@pxref{Normal Termination}), all open streams are +automatically closed properly. If your program terminates in any other +manner, such as by calling the @code{abort} function (@pxref{Aborting a +Program}) or from a fatal signal (@pxref{Signal Handling}), open streams +might not be closed properly. Buffered output might not be flushed and +files may be incomplete. For more information on buffering of streams, +see @ref{Stream Buffering}. + +@node Streams and Threads +@section Streams and Threads + +@cindex threads +@cindex multi-threaded application +Streams can be used in multi-threaded applications in the same way they +are used in single-threaded applications. But the programmer must be +aware of the possible complications. It is important to know about +these also if the program one writes never use threads since the design +and implementation of many stream functions are heavily influenced by the +requirements added by multi-threaded programming. + +The POSIX standard requires that by default the stream operations are +atomic. I.e., issuing two stream operations for the same stream in two +threads at the same time will cause the operations to be executed as if +they were issued sequentially. The buffer operations performed while +reading or writing are protected from other uses of the same stream. To +do this each stream has an internal lock object which has to be +(implicitly) acquired before any work can be done. + +But there are situations where this is not enough and there are also +situations where this is not wanted. The implicit locking is not enough +if the program requires more than one stream function call to happen +atomically. One example would be if an output line a program wants to +generate is created by several function calls. The functions by +themselves would ensure only atomicity of their own operation, but not +atomicity over all the function calls. For this it is necessary to +perform the stream locking in the application code. + +@comment stdio.h +@comment POSIX +@deftypefun void flockfile (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@assafe{}@acunsafe{@aculock{}}} +@c There's no way to tell whether the lock was acquired before or after +@c cancellation so as to unlock only when appropriate. +The @code{flockfile} function acquires the internal locking object +associated with the stream @var{stream}. This ensures that no other +thread can explicitly through @code{flockfile}/@code{ftrylockfile} or +implicitly through the call of a stream function lock the stream. The +thread will block until the lock is acquired. An explicit call to +@code{funlockfile} has to be used to release the lock. +@end deftypefun + +@comment stdio.h +@comment POSIX +@deftypefun int ftrylockfile (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@assafe{}@acunsafe{@aculock{}}} +The @code{ftrylockfile} function tries to acquire the internal locking +object associated with the stream @var{stream} just like +@code{flockfile}. But unlike @code{flockfile} this function does not +block if the lock is not available. @code{ftrylockfile} returns zero if +the lock was successfully acquired. Otherwise the stream is locked by +another thread. +@end deftypefun + +@comment stdio.h +@comment POSIX +@deftypefun void funlockfile (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@assafe{}@acunsafe{@aculock{}}} +The @code{funlockfile} function releases the internal locking object of +the stream @var{stream}. The stream must have been locked before by a +call to @code{flockfile} or a successful call of @code{ftrylockfile}. +The implicit locking performed by the stream operations do not count. +The @code{funlockfile} function does not return an error status and the +behavior of a call for a stream which is not locked by the current +thread is undefined. +@end deftypefun + +The following example shows how the functions above can be used to +generate an output line atomically even in multi-threaded applications +(yes, the same job could be done with one @code{fprintf} call but it is +sometimes not possible): + +@smallexample +FILE *fp; +@{ + @dots{} + flockfile (fp); + fputs ("This is test number ", fp); + fprintf (fp, "%d\n", test); + funlockfile (fp) +@} +@end smallexample + +Without the explicit locking it would be possible for another thread to +use the stream @var{fp} after the @code{fputs} call returns and before +@code{fprintf} was called with the result that the number does not +follow the word @samp{number}. + +From this description it might already be clear that the locking objects +in streams are no simple mutexes. Since locking the same stream twice +in the same thread is allowed the locking objects must be equivalent to +recursive mutexes. These mutexes keep track of the owner and the number +of times the lock is acquired. The same number of @code{funlockfile} +calls by the same threads is necessary to unlock the stream completely. +For instance: + +@smallexample +void +foo (FILE *fp) +@{ + ftrylockfile (fp); + fputs ("in foo\n", fp); + /* @r{This is very wrong!!!} */ + funlockfile (fp); +@} +@end smallexample + +It is important here that the @code{funlockfile} function is only called +if the @code{ftrylockfile} function succeeded in locking the stream. It +is therefore always wrong to ignore the result of @code{ftrylockfile}. +And it makes no sense since otherwise one would use @code{flockfile}. +The result of code like that above is that either @code{funlockfile} +tries to free a stream that hasn't been locked by the current thread or it +frees the stream prematurely. The code should look like this: + +@smallexample +void +foo (FILE *fp) +@{ + if (ftrylockfile (fp) == 0) + @{ + fputs ("in foo\n", fp); + funlockfile (fp); + @} +@} +@end smallexample + +Now that we covered why it is necessary to have locking it is +necessary to talk about situations when locking is unwanted and what can +be done. The locking operations (explicit or implicit) don't come for +free. Even if a lock is not taken the cost is not zero. The operations +which have to be performed require memory operations that are safe in +multi-processor environments. With the many local caches involved in +such systems this is quite costly. So it is best to avoid the locking +completely if it is not needed -- because the code in question is never +used in a context where two or more threads may use a stream at a time. +This can be determined most of the time for application code; for +library code which can be used in many contexts one should default to be +conservative and use locking. + +There are two basic mechanisms to avoid locking. The first is to use +the @code{_unlocked} variants of the stream operations. The POSIX +standard defines quite a few of those and @theglibc{} adds a few +more. These variants of the functions behave just like the functions +with the name without the suffix except that they do not lock the +stream. Using these functions is very desirable since they are +potentially much faster. This is not only because the locking +operation itself is avoided. More importantly, functions like +@code{putc} and @code{getc} are very simple and traditionally (before the +introduction of threads) were implemented as macros which are very fast +if the buffer is not empty. With the addition of locking requirements +these functions are no longer implemented as macros since they would +expand to too much code. +But these macros are still available with the same functionality under the new +names @code{putc_unlocked} and @code{getc_unlocked}. This possibly huge +difference of speed also suggests the use of the @code{_unlocked} +functions even if locking is required. The difference is that the +locking then has to be performed in the program: + +@smallexample +void +foo (FILE *fp, char *buf) +@{ + flockfile (fp); + while (*buf != '/') + putc_unlocked (*buf++, fp); + funlockfile (fp); +@} +@end smallexample + +If in this example the @code{putc} function would be used and the +explicit locking would be missing the @code{putc} function would have to +acquire the lock in every call, potentially many times depending on when +the loop terminates. Writing it the way illustrated above allows the +@code{putc_unlocked} macro to be used which means no locking and direct +manipulation of the buffer of the stream. + +A second way to avoid locking is by using a non-standard function which +was introduced in Solaris and is available in @theglibc{} as well. + +@comment stdio_ext.h +@comment GNU +@deftypefun int __fsetlocking (FILE *@var{stream}, int @var{type}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asulock{}}@acsafe{}} +@c Changing the implicit-locking status of a stream while it's in use by +@c another thread may cause a lock to be implicitly acquired and not +@c released, or vice-versa. This function should probably hold the lock +@c while changing this setting, to make sure we don't change it while +@c there are any concurrent uses. Meanwhile, callers should acquire the +@c lock themselves to be safe, and even concurrent uses with external +@c locking will be fine, as long as functions that require external +@c locking are not called without holding locks. + +The @code{__fsetlocking} function can be used to select whether the +stream operations will implicitly acquire the locking object of the +stream @var{stream}. By default this is done but it can be disabled and +reinstated using this function. There are three values defined for the +@var{type} parameter. + +@vtable @code +@item FSETLOCKING_INTERNAL +The stream @code{stream} will from now on use the default internal +locking. Every stream operation with exception of the @code{_unlocked} +variants will implicitly lock the stream. + +@item FSETLOCKING_BYCALLER +After the @code{__fsetlocking} function returns, the user is responsible +for locking the stream. None of the stream operations will implicitly +do this anymore until the state is set back to +@code{FSETLOCKING_INTERNAL}. + +@item FSETLOCKING_QUERY +@code{__fsetlocking} only queries the current locking state of the +stream. The return value will be @code{FSETLOCKING_INTERNAL} or +@code{FSETLOCKING_BYCALLER} depending on the state. +@end vtable + +The return value of @code{__fsetlocking} is either +@code{FSETLOCKING_INTERNAL} or @code{FSETLOCKING_BYCALLER} depending on +the state of the stream before the call. + +This function and the values for the @var{type} parameter are declared +in @file{stdio_ext.h}. +@end deftypefun + +This function is especially useful when program code has to be used +which is written without knowledge about the @code{_unlocked} functions +(or if the programmer was too lazy to use them). + +@node Streams and I18N +@section Streams in Internationalized Applications + +@w{ISO C90} introduced the new type @code{wchar_t} to allow handling +larger character sets. What was missing was a possibility to output +strings of @code{wchar_t} directly. One had to convert them into +multibyte strings using @code{mbstowcs} (there was no @code{mbsrtowcs} +yet) and then use the normal stream functions. While this is doable it +is very cumbersome since performing the conversions is not trivial and +greatly increases program complexity and size. + +The Unix standard early on (I think in XPG4.2) introduced two additional +format specifiers for the @code{printf} and @code{scanf} families of +functions. Printing and reading of single wide characters was made +possible using the @code{%C} specifier and wide character strings can be +handled with @code{%S}. These modifiers behave just like @code{%c} and +@code{%s} only that they expect the corresponding argument to have the +wide character type and that the wide character and string are +transformed into/from multibyte strings before being used. + +This was a beginning but it is still not good enough. Not always is it +desirable to use @code{printf} and @code{scanf}. The other, smaller and +faster functions cannot handle wide characters. Second, it is not +possible to have a format string for @code{printf} and @code{scanf} +consisting of wide characters. The result is that format strings would +have to be generated if they have to contain non-basic characters. + +@cindex C++ streams +@cindex streams, C++ +In the @w{Amendment 1} to @w{ISO C90} a whole new set of functions was +added to solve the problem. Most of the stream functions got a +counterpart which take a wide character or wide character string instead +of a character or string respectively. The new functions operate on the +same streams (like @code{stdout}). This is different from the model of +the C++ runtime library where separate streams for wide and normal I/O +are used. + +@cindex orientation, stream +@cindex stream orientation +Being able to use the same stream for wide and normal operations comes +with a restriction: a stream can be used either for wide operations or +for normal operations. Once it is decided there is no way back. Only a +call to @code{freopen} or @code{freopen64} can reset the +@dfn{orientation}. The orientation can be decided in three ways: + +@itemize @bullet +@item +If any of the normal character functions are used (this includes the +@code{fread} and @code{fwrite} functions) the stream is marked as not +wide oriented. + +@item +If any of the wide character functions are used the stream is marked as +wide oriented. + +@item +The @code{fwide} function can be used to set the orientation either way. +@end itemize + +It is important to never mix the use of wide and not wide operations on +a stream. There are no diagnostics issued. The application behavior +will simply be strange or the application will simply crash. The +@code{fwide} function can help avoid this. + +@comment wchar.h +@comment ISO +@deftypefun int fwide (FILE *@var{stream}, int @var{mode}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{}}} +@c Querying is always safe, but changing the stream when it's in use +@c upthread may be problematic. Like most lock-acquiring functions, +@c this one may leak the lock if canceled. + +The @code{fwide} function can be used to set and query the state of the +orientation of the stream @var{stream}. If the @var{mode} parameter has +a positive value the streams get wide oriented, for negative values +narrow oriented. It is not possible to overwrite previous orientations +with @code{fwide}. I.e., if the stream @var{stream} was already +oriented before the call nothing is done. + +If @var{mode} is zero the current orientation state is queried and +nothing is changed. + +The @code{fwide} function returns a negative value, zero, or a positive +value if the stream is narrow, not at all, or wide oriented +respectively. + +This function was introduced in @w{Amendment 1} to @w{ISO C90} and is +declared in @file{wchar.h}. +@end deftypefun + +It is generally a good idea to orient a stream as early as possible. +This can prevent surprise especially for the standard streams +@code{stdin}, @code{stdout}, and @code{stderr}. If some library +function in some situations uses one of these streams and this use +orients the stream in a different way the rest of the application +expects it one might end up with hard to reproduce errors. Remember +that no errors are signal if the streams are used incorrectly. Leaving +a stream unoriented after creation is normally only necessary for +library functions which create streams which can be used in different +contexts. + +When writing code which uses streams and which can be used in different +contexts it is important to query the orientation of the stream before +using it (unless the rules of the library interface demand a specific +orientation). The following little, silly function illustrates this. + +@smallexample +void +print_f (FILE *fp) +@{ + if (fwide (fp, 0) > 0) + /* @r{Positive return value means wide orientation.} */ + fputwc (L'f', fp); + else + fputc ('f', fp); +@} +@end smallexample + +Note that in this case the function @code{print_f} decides about the +orientation of the stream if it was unoriented before (will not happen +if the advice above is followed). + +The encoding used for the @code{wchar_t} values is unspecified and the +user must not make any assumptions about it. For I/O of @code{wchar_t} +values this means that it is impossible to write these values directly +to the stream. This is not what follows from the @w{ISO C} locale model +either. What happens instead is that the bytes read from or written to +the underlying media are first converted into the internal encoding +chosen by the implementation for @code{wchar_t}. The external encoding +is determined by the @code{LC_CTYPE} category of the current locale or +by the @samp{ccs} part of the mode specification given to @code{fopen}, +@code{fopen64}, @code{freopen}, or @code{freopen64}. How and when the +conversion happens is unspecified and it happens invisibly to the user. + +Since a stream is created in the unoriented state it has at that point +no conversion associated with it. The conversion which will be used is +determined by the @code{LC_CTYPE} category selected at the time the +stream is oriented. If the locales are changed at the runtime this +might produce surprising results unless one pays attention. This is +just another good reason to orient the stream explicitly as soon as +possible, perhaps with a call to @code{fwide}. + +@node Simple Output +@section Simple Output by Characters or Lines + +@cindex writing to a stream, by characters +This section describes functions for performing character- and +line-oriented output. + +These narrow stream functions are declared in the header file +@file{stdio.h} and the wide stream functions in @file{wchar.h}. +@pindex stdio.h +@pindex wchar.h + +@comment stdio.h +@comment ISO +@deftypefun int fputc (int @var{c}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{} @aculock{}}} +@c If the stream is in use when interrupted by a signal, the recursive +@c lock won't help ensure the stream is consistent; indeed, if fputc +@c gets a signal precisely before the post-incremented _IO_write_ptr +@c value is stored, we may overwrite the interrupted write. Conversely, +@c depending on compiler optimizations, the incremented _IO_write_ptr +@c may be stored before the character is stored in the buffer, +@c corrupting the stream if async cancel hits between the two stores. +@c There may be other reasons for AS- and AC-unsafety in the overflow +@c cases. +The @code{fputc} function converts the character @var{c} to type +@code{unsigned char}, and writes it to the stream @var{stream}. +@code{EOF} is returned if a write error occurs; otherwise the +character @var{c} is returned. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun wint_t fputwc (wchar_t @var{wc}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{} @aculock{}}} +The @code{fputwc} function writes the wide character @var{wc} to the +stream @var{stream}. @code{WEOF} is returned if a write error occurs; +otherwise the character @var{wc} is returned. +@end deftypefun + +@comment stdio.h +@comment POSIX +@deftypefun int fputc_unlocked (int @var{c}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +@c The unlocked functions can't possibly satisfy the MT-Safety +@c requirements on their own, because they require external locking for +@c safety. +The @code{fputc_unlocked} function is equivalent to the @code{fputc} +function except that it does not implicitly lock the stream. +@end deftypefun + +@comment wchar.h +@comment POSIX +@deftypefun wint_t fputwc_unlocked (wchar_t @var{wc}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{fputwc_unlocked} function is equivalent to the @code{fputwc} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int putc (int @var{c}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{} @aculock{}}} +This is just like @code{fputc}, except that most systems implement it as +a macro, making it faster. One consequence is that it may evaluate the +@var{stream} argument more than once, which is an exception to the +general rule for macros. @code{putc} is usually the best function to +use for writing a single character. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun wint_t putwc (wchar_t @var{wc}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{} @aculock{}}} +This is just like @code{fputwc}, except that it can be implement as +a macro, making it faster. One consequence is that it may evaluate the +@var{stream} argument more than once, which is an exception to the +general rule for macros. @code{putwc} is usually the best function to +use for writing a single wide character. +@end deftypefun + +@comment stdio.h +@comment POSIX +@deftypefun int putc_unlocked (int @var{c}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{putc_unlocked} function is equivalent to the @code{putc} +function except that it does not implicitly lock the stream. +@end deftypefun + +@comment wchar.h +@comment GNU +@deftypefun wint_t putwc_unlocked (wchar_t @var{wc}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{putwc_unlocked} function is equivalent to the @code{putwc} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int putchar (int @var{c}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{} @aculock{}}} +The @code{putchar} function is equivalent to @code{putc} with +@code{stdout} as the value of the @var{stream} argument. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun wint_t putwchar (wchar_t @var{wc}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{} @aculock{}}} +The @code{putwchar} function is equivalent to @code{putwc} with +@code{stdout} as the value of the @var{stream} argument. +@end deftypefun + +@comment stdio.h +@comment POSIX +@deftypefun int putchar_unlocked (int @var{c}) +@safety{@prelim{}@mtunsafe{@mtasurace{:stdout}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{putchar_unlocked} function is equivalent to the @code{putchar} +function except that it does not implicitly lock the stream. +@end deftypefun + +@comment wchar.h +@comment GNU +@deftypefun wint_t putwchar_unlocked (wchar_t @var{wc}) +@safety{@prelim{}@mtunsafe{@mtasurace{:stdout}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{putwchar_unlocked} function is equivalent to the @code{putwchar} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int fputs (const char *@var{s}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{} @aculock{}}} +The function @code{fputs} writes the string @var{s} to the stream +@var{stream}. The terminating null character is not written. +This function does @emph{not} add a newline character, either. +It outputs only the characters in the string. + +This function returns @code{EOF} if a write error occurs, and otherwise +a non-negative value. + +For example: + +@smallexample +fputs ("Are ", stdout); +fputs ("you ", stdout); +fputs ("hungry?\n", stdout); +@end smallexample + +@noindent +outputs the text @samp{Are you hungry?} followed by a newline. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun int fputws (const wchar_t *@var{ws}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{} @aculock{}}} +The function @code{fputws} writes the wide character string @var{ws} to +the stream @var{stream}. The terminating null character is not written. +This function does @emph{not} add a newline character, either. It +outputs only the characters in the string. + +This function returns @code{WEOF} if a write error occurs, and otherwise +a non-negative value. +@end deftypefun + +@comment stdio.h +@comment GNU +@deftypefun int fputs_unlocked (const char *@var{s}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{fputs_unlocked} function is equivalent to the @code{fputs} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. +@end deftypefun + +@comment wchar.h +@comment GNU +@deftypefun int fputws_unlocked (const wchar_t *@var{ws}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{fputws_unlocked} function is equivalent to the @code{fputws} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int puts (const char *@var{s}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +The @code{puts} function writes the string @var{s} to the stream +@code{stdout} followed by a newline. The terminating null character of +the string is not written. (Note that @code{fputs} does @emph{not} +write a newline as this function does.) + +@code{puts} is the most convenient function for printing simple +messages. For example: + +@smallexample +puts ("This is a message."); +@end smallexample + +@noindent +outputs the text @samp{This is a message.} followed by a newline. +@end deftypefun + +@comment stdio.h +@comment SVID +@deftypefun int putw (int @var{w}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function writes the word @var{w} (that is, an @code{int}) to +@var{stream}. It is provided for compatibility with SVID, but we +recommend you use @code{fwrite} instead (@pxref{Block Input/Output}). +@end deftypefun + +@node Character Input +@section Character Input + +@cindex reading from a stream, by characters +This section describes functions for performing character-oriented +input. These narrow stream functions are declared in the header file +@file{stdio.h} and the wide character functions are declared in +@file{wchar.h}. +@pindex stdio.h +@pindex wchar.h + +These functions return an @code{int} or @code{wint_t} value (for narrow +and wide stream functions respectively) that is either a character of +input, or the special value @code{EOF}/@code{WEOF} (usually -1). For +the narrow stream functions it is important to store the result of these +functions in a variable of type @code{int} instead of @code{char}, even +when you plan to use it only as a character. Storing @code{EOF} in a +@code{char} variable truncates its value to the size of a character, so +that it is no longer distinguishable from the valid character +@samp{(char) -1}. So always use an @code{int} for the result of +@code{getc} and friends, and check for @code{EOF} after the call; once +you've verified that the result is not @code{EOF}, you can be sure that +it will fit in a @samp{char} variable without loss of information. + +@comment stdio.h +@comment ISO +@deftypefun int fgetc (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +@c Same caveats as fputc, but instead of losing a write in case of async +@c signals, we may read the same character more than once, and the +@c stream may be left in odd states due to cancellation in the underflow +@c cases. +This function reads the next character as an @code{unsigned char} from +the stream @var{stream} and returns its value, converted to an +@code{int}. If an end-of-file condition or read error occurs, +@code{EOF} is returned instead. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun wint_t fgetwc (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function reads the next wide character from the stream @var{stream} +and returns its value. If an end-of-file condition or read error +occurs, @code{WEOF} is returned instead. +@end deftypefun + +@comment stdio.h +@comment POSIX +@deftypefun int fgetc_unlocked (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{fgetc_unlocked} function is equivalent to the @code{fgetc} +function except that it does not implicitly lock the stream. +@end deftypefun + +@comment wchar.h +@comment GNU +@deftypefun wint_t fgetwc_unlocked (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{fgetwc_unlocked} function is equivalent to the @code{fgetwc} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int getc (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This is just like @code{fgetc}, except that it is permissible (and +typical) for it to be implemented as a macro that evaluates the +@var{stream} argument more than once. @code{getc} is often highly +optimized, so it is usually the best function to use to read a single +character. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun wint_t getwc (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This is just like @code{fgetwc}, except that it is permissible for it to +be implemented as a macro that evaluates the @var{stream} argument more +than once. @code{getwc} can be highly optimized, so it is usually the +best function to use to read a single wide character. +@end deftypefun + +@comment stdio.h +@comment POSIX +@deftypefun int getc_unlocked (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{getc_unlocked} function is equivalent to the @code{getc} +function except that it does not implicitly lock the stream. +@end deftypefun + +@comment wchar.h +@comment GNU +@deftypefun wint_t getwc_unlocked (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{getwc_unlocked} function is equivalent to the @code{getwc} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int getchar (void) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +The @code{getchar} function is equivalent to @code{getc} with @code{stdin} +as the value of the @var{stream} argument. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun wint_t getwchar (void) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +The @code{getwchar} function is equivalent to @code{getwc} with @code{stdin} +as the value of the @var{stream} argument. +@end deftypefun + +@comment stdio.h +@comment POSIX +@deftypefun int getchar_unlocked (void) +@safety{@prelim{}@mtunsafe{@mtasurace{:stdin}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{getchar_unlocked} function is equivalent to the @code{getchar} +function except that it does not implicitly lock the stream. +@end deftypefun + +@comment wchar.h +@comment GNU +@deftypefun wint_t getwchar_unlocked (void) +@safety{@prelim{}@mtunsafe{@mtasurace{:stdin}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{getwchar_unlocked} function is equivalent to the @code{getwchar} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. +@end deftypefun + +Here is an example of a function that does input using @code{fgetc}. It +would work just as well using @code{getc} instead, or using +@code{getchar ()} instead of @w{@code{fgetc (stdin)}}. The code would +also work the same for the wide character stream functions. + +@smallexample +int +y_or_n_p (const char *question) +@{ + fputs (question, stdout); + while (1) + @{ + int c, answer; + /* @r{Write a space to separate answer from question.} */ + fputc (' ', stdout); + /* @r{Read the first character of the line.} + @r{This should be the answer character, but might not be.} */ + c = tolower (fgetc (stdin)); + answer = c; + /* @r{Discard rest of input line.} */ + while (c != '\n' && c != EOF) + c = fgetc (stdin); + /* @r{Obey the answer if it was valid.} */ + if (answer == 'y') + return 1; + if (answer == 'n') + return 0; + /* @r{Answer was invalid: ask for valid answer.} */ + fputs ("Please answer y or n:", stdout); + @} +@} +@end smallexample + +@comment stdio.h +@comment SVID +@deftypefun int getw (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function reads a word (that is, an @code{int}) from @var{stream}. +It's provided for compatibility with SVID. We recommend you use +@code{fread} instead (@pxref{Block Input/Output}). Unlike @code{getc}, +any @code{int} value could be a valid result. @code{getw} returns +@code{EOF} when it encounters end-of-file or an error, but there is no +way to distinguish this from an input word with value -1. +@end deftypefun + +@node Line Input +@section Line-Oriented Input + +Since many programs interpret input on the basis of lines, it is +convenient to have functions to read a line of text from a stream. + +Standard C has functions to do this, but they aren't very safe: null +characters and even (for @code{gets}) long lines can confuse them. So +@theglibc{} provides the nonstandard @code{getline} function that +makes it easy to read lines reliably. + +Another GNU extension, @code{getdelim}, generalizes @code{getline}. It +reads a delimited record, defined as everything through the next +occurrence of a specified delimiter character. + +All these functions are declared in @file{stdio.h}. + +@comment stdio.h +@comment GNU +@deftypefun ssize_t getline (char **@var{lineptr}, size_t *@var{n}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@aculock{} @acucorrupt{} @acsmem{}}} +@c Besides the usual possibility of getting an inconsistent stream in a +@c signal handler or leaving it inconsistent in case of cancellation, +@c the possibility of leaving a dangling pointer upon cancellation +@c between reallocing the buffer at *lineptr and updating the pointer +@c brings about another case of @acucorrupt. +This function reads an entire line from @var{stream}, storing the text +(including the newline and a terminating null character) in a buffer +and storing the buffer address in @code{*@var{lineptr}}. + +Before calling @code{getline}, you should place in @code{*@var{lineptr}} +the address of a buffer @code{*@var{n}} bytes long, allocated with +@code{malloc}. If this buffer is long enough to hold the line, +@code{getline} stores the line in this buffer. Otherwise, +@code{getline} makes the buffer bigger using @code{realloc}, storing the +new buffer address back in @code{*@var{lineptr}} and the increased size +back in @code{*@var{n}}. +@xref{Unconstrained Allocation}. + +If you set @code{*@var{lineptr}} to a null pointer, and @code{*@var{n}} +to zero, before the call, then @code{getline} allocates the initial +buffer for you by calling @code{malloc}. This buffer remains allocated +even if @code{getline} encounters errors and is unable to read any bytes. + +In either case, when @code{getline} returns, @code{*@var{lineptr}} is +a @code{char *} which points to the text of the line. + +When @code{getline} is successful, it returns the number of characters +read (including the newline, but not including the terminating null). +This value enables you to distinguish null characters that are part of +the line from the null character inserted as a terminator. + +This function is a GNU extension, but it is the recommended way to read +lines from a stream. The alternative standard functions are unreliable. + +If an error occurs or end of file is reached without any bytes read, +@code{getline} returns @code{-1}. +@end deftypefun + +@comment stdio.h +@comment GNU +@deftypefun ssize_t getdelim (char **@var{lineptr}, size_t *@var{n}, int @var{delimiter}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@aculock{} @acucorrupt{} @acsmem{}}} +@c See the getline @acucorrupt note. +This function is like @code{getline} except that the character which +tells it to stop reading is not necessarily newline. The argument +@var{delimiter} specifies the delimiter character; @code{getdelim} keeps +reading until it sees that character (or end of file). + +The text is stored in @var{lineptr}, including the delimiter character +and a terminating null. Like @code{getline}, @code{getdelim} makes +@var{lineptr} bigger if it isn't big enough. + +@code{getline} is in fact implemented in terms of @code{getdelim}, just +like this: + +@smallexample +ssize_t +getline (char **lineptr, size_t *n, FILE *stream) +@{ + return getdelim (lineptr, n, '\n', stream); +@} +@end smallexample +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun {char *} fgets (char *@var{s}, int @var{count}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +The @code{fgets} function reads characters from the stream @var{stream} +up to and including a newline character and stores them in the string +@var{s}, adding a null character to mark the end of the string. You +must supply @var{count} characters worth of space in @var{s}, but the +number of characters read is at most @var{count} @minus{} 1. The extra +character space is used to hold the null character at the end of the +string. + +If the system is already at end of file when you call @code{fgets}, then +the contents of the array @var{s} are unchanged and a null pointer is +returned. A null pointer is also returned if a read error occurs. +Otherwise, the return value is the pointer @var{s}. + +@strong{Warning:} If the input data has a null character, you can't tell. +So don't use @code{fgets} unless you know the data cannot contain a null. +Don't use it to read files edited by the user because, if the user inserts +a null character, you should either handle it properly or print a clear +error message. We recommend using @code{getline} instead of @code{fgets}. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun {wchar_t *} fgetws (wchar_t *@var{ws}, int @var{count}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +The @code{fgetws} function reads wide characters from the stream +@var{stream} up to and including a newline character and stores them in +the string @var{ws}, adding a null wide character to mark the end of the +string. You must supply @var{count} wide characters worth of space in +@var{ws}, but the number of characters read is at most @var{count} +@minus{} 1. The extra character space is used to hold the null wide +character at the end of the string. + +If the system is already at end of file when you call @code{fgetws}, then +the contents of the array @var{ws} are unchanged and a null pointer is +returned. A null pointer is also returned if a read error occurs. +Otherwise, the return value is the pointer @var{ws}. + +@strong{Warning:} If the input data has a null wide character (which are +null bytes in the input stream), you can't tell. So don't use +@code{fgetws} unless you know the data cannot contain a null. Don't use +it to read files edited by the user because, if the user inserts a null +character, you should either handle it properly or print a clear error +message. +@comment XXX We need getwline!!! +@end deftypefun + +@comment stdio.h +@comment GNU +@deftypefun {char *} fgets_unlocked (char *@var{s}, int @var{count}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{fgets_unlocked} function is equivalent to the @code{fgets} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. +@end deftypefun + +@comment wchar.h +@comment GNU +@deftypefun {wchar_t *} fgetws_unlocked (wchar_t *@var{ws}, int @var{count}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{fgetws_unlocked} function is equivalent to the @code{fgetws} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefn {Deprecated function} {char *} gets (char *@var{s}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +The function @code{gets} reads characters from the stream @code{stdin} +up to the next newline character, and stores them in the string @var{s}. +The newline character is discarded (note that this differs from the +behavior of @code{fgets}, which copies the newline character into the +string). If @code{gets} encounters a read error or end-of-file, it +returns a null pointer; otherwise it returns @var{s}. + +@strong{Warning:} The @code{gets} function is @strong{very dangerous} +because it provides no protection against overflowing the string +@var{s}. @Theglibc{} includes it for compatibility only. You +should @strong{always} use @code{fgets} or @code{getline} instead. To +remind you of this, the linker (if using GNU @code{ld}) will issue a +warning whenever you use @code{gets}. +@end deftypefn + +@node Unreading +@section Unreading +@cindex peeking at input +@cindex unreading characters +@cindex pushing input back + +In parser programs it is often useful to examine the next character in +the input stream without removing it from the stream. This is called +``peeking ahead'' at the input because your program gets a glimpse of +the input it will read next. + +Using stream I/O, you can peek ahead at input by first reading it and +then @dfn{unreading} it (also called @dfn{pushing it back} on the stream). +Unreading a character makes it available to be input again from the stream, +by the next call to @code{fgetc} or other input function on that stream. + +@menu +* Unreading Idea:: An explanation of unreading with pictures. +* How Unread:: How to call @code{ungetc} to do unreading. +@end menu + +@node Unreading Idea +@subsection What Unreading Means + +Here is a pictorial explanation of unreading. Suppose you have a +stream reading a file that contains just six characters, the letters +@samp{foobar}. Suppose you have read three characters so far. The +situation looks like this: + +@smallexample +f o o b a r + ^ +@end smallexample + +@noindent +so the next input character will be @samp{b}. + +@c @group Invalid outside @example +If instead of reading @samp{b} you unread the letter @samp{o}, you get a +situation like this: + +@smallexample +f o o b a r + | + o-- + ^ +@end smallexample + +@noindent +so that the next input characters will be @samp{o} and @samp{b}. +@c @end group + +@c @group +If you unread @samp{9} instead of @samp{o}, you get this situation: + +@smallexample +f o o b a r + | + 9-- + ^ +@end smallexample + +@noindent +so that the next input characters will be @samp{9} and @samp{b}. +@c @end group + +@node How Unread +@subsection Using @code{ungetc} To Do Unreading + +The function to unread a character is called @code{ungetc}, because it +reverses the action of @code{getc}. + +@comment stdio.h +@comment ISO +@deftypefun int ungetc (int @var{c}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +The @code{ungetc} function pushes back the character @var{c} onto the +input stream @var{stream}. So the next input from @var{stream} will +read @var{c} before anything else. + +If @var{c} is @code{EOF}, @code{ungetc} does nothing and just returns +@code{EOF}. This lets you call @code{ungetc} with the return value of +@code{getc} without needing to check for an error from @code{getc}. + +The character that you push back doesn't have to be the same as the last +character that was actually read from the stream. In fact, it isn't +necessary to actually read any characters from the stream before +unreading them with @code{ungetc}! But that is a strange way to write a +program; usually @code{ungetc} is used only to unread a character that +was just read from the same stream. @Theglibc{} supports this +even on files opened in binary mode, but other systems might not. + +@Theglibc{} only supports one character of pushback---in other +words, it does not work to call @code{ungetc} twice without doing input +in between. Other systems might let you push back multiple characters; +then reading from the stream retrieves the characters in the reverse +order that they were pushed. + +Pushing back characters doesn't alter the file; only the internal +buffering for the stream is affected. If a file positioning function +(such as @code{fseek}, @code{fseeko} or @code{rewind}; @pxref{File +Positioning}) is called, any pending pushed-back characters are +discarded. + +Unreading a character on a stream that is at end of file clears the +end-of-file indicator for the stream, because it makes the character of +input available. After you read that character, trying to read again +will encounter end of file. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun wint_t ungetwc (wint_t @var{wc}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +The @code{ungetwc} function behaves just like @code{ungetc} just that it +pushes back a wide character. +@end deftypefun + +Here is an example showing the use of @code{getc} and @code{ungetc} to +skip over whitespace characters. When this function reaches a +non-whitespace character, it unreads that character to be seen again on +the next read operation on the stream. + +@smallexample +#include <stdio.h> +#include <ctype.h> + +void +skip_whitespace (FILE *stream) +@{ + int c; + do + /* @r{No need to check for @code{EOF} because it is not} + @r{@code{isspace}, and @code{ungetc} ignores @code{EOF}.} */ + c = getc (stream); + while (isspace (c)); + ungetc (c, stream); +@} +@end smallexample + +@node Block Input/Output +@section Block Input/Output + +This section describes how to do input and output operations on blocks +of data. You can use these functions to read and write binary data, as +well as to read and write text in fixed-size blocks instead of by +characters or lines. +@cindex binary I/O to a stream +@cindex block I/O to a stream +@cindex reading from a stream, by blocks +@cindex writing to a stream, by blocks + +Binary files are typically used to read and write blocks of data in the +same format as is used to represent the data in a running program. In +other words, arbitrary blocks of memory---not just character or string +objects---can be written to a binary file, and meaningfully read in +again by the same program. + +Storing data in binary form is often considerably more efficient than +using the formatted I/O functions. Also, for floating-point numbers, +the binary form avoids possible loss of precision in the conversion +process. On the other hand, binary files can't be examined or modified +easily using many standard file utilities (such as text editors), and +are not portable between different implementations of the language, or +different kinds of computers. + +These functions are declared in @file{stdio.h}. +@pindex stdio.h + +@comment stdio.h +@comment ISO +@deftypefun size_t fread (void *@var{data}, size_t @var{size}, size_t @var{count}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function reads up to @var{count} objects of size @var{size} into +the array @var{data}, from the stream @var{stream}. It returns the +number of objects actually read, which might be less than @var{count} if +a read error occurs or the end of the file is reached. This function +returns a value of zero (and doesn't read anything) if either @var{size} +or @var{count} is zero. + +If @code{fread} encounters end of file in the middle of an object, it +returns the number of complete objects read, and discards the partial +object. Therefore, the stream remains at the actual end of the file. +@end deftypefun + +@comment stdio.h +@comment GNU +@deftypefun size_t fread_unlocked (void *@var{data}, size_t @var{size}, size_t @var{count}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{fread_unlocked} function is equivalent to the @code{fread} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun size_t fwrite (const void *@var{data}, size_t @var{size}, size_t @var{count}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function writes up to @var{count} objects of size @var{size} from +the array @var{data}, to the stream @var{stream}. The return value is +normally @var{count}, if the call succeeds. Any other value indicates +some sort of error, such as running out of space. +@end deftypefun + +@comment stdio.h +@comment GNU +@deftypefun size_t fwrite_unlocked (const void *@var{data}, size_t @var{size}, size_t @var{count}, FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{fwrite_unlocked} function is equivalent to the @code{fwrite} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. +@end deftypefun + +@node Formatted Output +@section Formatted Output + +@cindex format string, for @code{printf} +@cindex template, for @code{printf} +@cindex formatted output to a stream +@cindex writing to a stream, formatted +The functions described in this section (@code{printf} and related +functions) provide a convenient way to perform formatted output. You +call @code{printf} with a @dfn{format string} or @dfn{template string} +that specifies how to format the values of the remaining arguments. + +Unless your program is a filter that specifically performs line- or +character-oriented processing, using @code{printf} or one of the other +related functions described in this section is usually the easiest and +most concise way to perform output. These functions are especially +useful for printing error messages, tables of data, and the like. + +@menu +* Formatted Output Basics:: Some examples to get you started. +* Output Conversion Syntax:: General syntax of conversion + specifications. +* Table of Output Conversions:: Summary of output conversions and + what they do. +* Integer Conversions:: Details about formatting of integers. +* Floating-Point Conversions:: Details about formatting of + floating-point numbers. +* Other Output Conversions:: Details about formatting of strings, + characters, pointers, and the like. +* Formatted Output Functions:: Descriptions of the actual functions. +* Dynamic Output:: Functions that allocate memory for the output. +* Variable Arguments Output:: @code{vprintf} and friends. +* Parsing a Template String:: What kinds of args does a given template + call for? +* Example of Parsing:: Sample program using @code{parse_printf_format}. +@end menu + +@node Formatted Output Basics +@subsection Formatted Output Basics + +The @code{printf} function can be used to print any number of arguments. +The template string argument you supply in a call provides +information not only about the number of additional arguments, but also +about their types and what style should be used for printing them. + +Ordinary characters in the template string are simply written to the +output stream as-is, while @dfn{conversion specifications} introduced by +a @samp{%} character in the template cause subsequent arguments to be +formatted and written to the output stream. For example, +@cindex conversion specifications (@code{printf}) + +@smallexample +int pct = 37; +char filename[] = "foo.txt"; +printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n", + filename, pct); +@end smallexample + +@noindent +produces output like + +@smallexample +Processing of `foo.txt' is 37% finished. +Please be patient. +@end smallexample + +This example shows the use of the @samp{%d} conversion to specify that +an @code{int} argument should be printed in decimal notation, the +@samp{%s} conversion to specify printing of a string argument, and +the @samp{%%} conversion to print a literal @samp{%} character. + +There are also conversions for printing an integer argument as an +unsigned value in octal, decimal, or hexadecimal radix (@samp{%o}, +@samp{%u}, or @samp{%x}, respectively); or as a character value +(@samp{%c}). + +Floating-point numbers can be printed in normal, fixed-point notation +using the @samp{%f} conversion or in exponential notation using the +@samp{%e} conversion. The @samp{%g} conversion uses either @samp{%e} +or @samp{%f} format, depending on what is more appropriate for the +magnitude of the particular number. + +You can control formatting more precisely by writing @dfn{modifiers} +between the @samp{%} and the character that indicates which conversion +to apply. These slightly alter the ordinary behavior of the conversion. +For example, most conversion specifications permit you to specify a +minimum field width and a flag indicating whether you want the result +left- or right-justified within the field. + +The specific flags and modifiers that are permitted and their +interpretation vary depending on the particular conversion. They're all +described in more detail in the following sections. Don't worry if this +all seems excessively complicated at first; you can almost always get +reasonable free-format output without using any of the modifiers at all. +The modifiers are mostly used to make the output look ``prettier'' in +tables. + +@node Output Conversion Syntax +@subsection Output Conversion Syntax + +This section provides details about the precise syntax of conversion +specifications that can appear in a @code{printf} template +string. + +Characters in the template string that are not part of a conversion +specification are printed as-is to the output stream. Multibyte +character sequences (@pxref{Character Set Handling}) are permitted in a +template string. + +The conversion specifications in a @code{printf} template string have +the general form: + +@smallexample +% @r{[} @var{param-no} @r{$]} @var{flags} @var{width} @r{[} . @var{precision} @r{]} @var{type} @var{conversion} +@end smallexample + +@noindent +or + +@smallexample +% @r{[} @var{param-no} @r{$]} @var{flags} @var{width} . @r{*} @r{[} @var{param-no} @r{$]} @var{type} @var{conversion} +@end smallexample + +For example, in the conversion specifier @samp{%-10.8ld}, the @samp{-} +is a flag, @samp{10} specifies the field width, the precision is +@samp{8}, the letter @samp{l} is a type modifier, and @samp{d} specifies +the conversion style. (This particular type specifier says to +print a @code{long int} argument in decimal notation, with a minimum of +8 digits left-justified in a field at least 10 characters wide.) + +In more detail, output conversion specifications consist of an +initial @samp{%} character followed in sequence by: + +@itemize @bullet +@item +An optional specification of the parameter used for this format. +Normally the parameters to the @code{printf} function are assigned to the +formats in the order of appearance in the format string. But in some +situations (such as message translation) this is not desirable and this +extension allows an explicit parameter to be specified. + +The @var{param-no} parts of the format must be integers in the range of +1 to the maximum number of arguments present to the function call. Some +implementations limit this number to a certain upper bound. The exact +limit can be retrieved by the following constant. + +@defvr Macro NL_ARGMAX +The value of @code{NL_ARGMAX} is the maximum value allowed for the +specification of a positional parameter in a @code{printf} call. The +actual value in effect at runtime can be retrieved by using +@code{sysconf} using the @code{_SC_NL_ARGMAX} parameter @pxref{Sysconf +Definition}. + +Some systems have a quite low limit such as @math{9} for @w{System V} +systems. @Theglibc{} has no real limit. +@end defvr + +If any of the formats has a specification for the parameter position all +of them in the format string shall have one. Otherwise the behavior is +undefined. + +@item +Zero or more @dfn{flag characters} that modify the normal behavior of +the conversion specification. +@cindex flag character (@code{printf}) + +@item +An optional decimal integer specifying the @dfn{minimum field width}. +If the normal conversion produces fewer characters than this, the field +is padded with spaces to the specified width. This is a @emph{minimum} +value; if the normal conversion produces more characters than this, the +field is @emph{not} truncated. Normally, the output is right-justified +within the field. +@cindex minimum field width (@code{printf}) + +You can also specify a field width of @samp{*}. This means that the +next argument in the argument list (before the actual value to be +printed) is used as the field width. The value must be an @code{int}. +If the value is negative, this means to set the @samp{-} flag (see +below) and to use the absolute value as the field width. + +@item +An optional @dfn{precision} to specify the number of digits to be +written for the numeric conversions. If the precision is specified, it +consists of a period (@samp{.}) followed optionally by a decimal integer +(which defaults to zero if omitted). +@cindex precision (@code{printf}) + +You can also specify a precision of @samp{*}. This means that the next +argument in the argument list (before the actual value to be printed) is +used as the precision. The value must be an @code{int}, and is ignored +if it is negative. If you specify @samp{*} for both the field width and +precision, the field width argument precedes the precision argument. +Other C library versions may not recognize this syntax. + +@item +An optional @dfn{type modifier character}, which is used to specify the +data type of the corresponding argument if it differs from the default +type. (For example, the integer conversions assume a type of @code{int}, +but you can specify @samp{h}, @samp{l}, or @samp{L} for other integer +types.) +@cindex type modifier character (@code{printf}) + +@item +A character that specifies the conversion to be applied. +@end itemize + +The exact options that are permitted and how they are interpreted vary +between the different conversion specifiers. See the descriptions of the +individual conversions for information about the particular options that +they use. + +With the @samp{-Wformat} option, the GNU C compiler checks calls to +@code{printf} and related functions. It examines the format string and +verifies that the correct number and types of arguments are supplied. +There is also a GNU C syntax to tell the compiler that a function you +write uses a @code{printf}-style format string. +@xref{Function Attributes, , Declaring Attributes of Functions, +gcc.info, Using GNU CC}, for more information. + +@node Table of Output Conversions +@subsection Table of Output Conversions +@cindex output conversions, for @code{printf} + +Here is a table summarizing what all the different conversions do: + +@table @asis +@item @samp{%d}, @samp{%i} +Print an integer as a signed decimal number. @xref{Integer +Conversions}, for details. @samp{%d} and @samp{%i} are synonymous for +output, but are different when used with @code{scanf} for input +(@pxref{Table of Input Conversions}). + +@item @samp{%o} +Print an integer as an unsigned octal number. @xref{Integer +Conversions}, for details. + +@item @samp{%u} +Print an integer as an unsigned decimal number. @xref{Integer +Conversions}, for details. + +@item @samp{%x}, @samp{%X} +Print an integer as an unsigned hexadecimal number. @samp{%x} uses +lower-case letters and @samp{%X} uses upper-case. @xref{Integer +Conversions}, for details. + +@item @samp{%f} +Print a floating-point number in normal (fixed-point) notation. +@xref{Floating-Point Conversions}, for details. + +@item @samp{%e}, @samp{%E} +Print a floating-point number in exponential notation. @samp{%e} uses +lower-case letters and @samp{%E} uses upper-case. @xref{Floating-Point +Conversions}, for details. + +@item @samp{%g}, @samp{%G} +Print a floating-point number in either normal or exponential notation, +whichever is more appropriate for its magnitude. @samp{%g} uses +lower-case letters and @samp{%G} uses upper-case. @xref{Floating-Point +Conversions}, for details. + +@item @samp{%a}, @samp{%A} +Print a floating-point number in a hexadecimal fractional notation with +the exponent to base 2 represented in decimal digits. @samp{%a} uses +lower-case letters and @samp{%A} uses upper-case. @xref{Floating-Point +Conversions}, for details. + +@item @samp{%c} +Print a single character. @xref{Other Output Conversions}. + +@item @samp{%C} +This is an alias for @samp{%lc} which is supported for compatibility +with the Unix standard. + +@item @samp{%s} +Print a string. @xref{Other Output Conversions}. + +@item @samp{%S} +This is an alias for @samp{%ls} which is supported for compatibility +with the Unix standard. + +@item @samp{%p} +Print the value of a pointer. @xref{Other Output Conversions}. + +@item @samp{%n} +Get the number of characters printed so far. @xref{Other Output Conversions}. +Note that this conversion specification never produces any output. + +@item @samp{%m} +Print the string corresponding to the value of @code{errno}. +(This is a GNU extension.) +@xref{Other Output Conversions}. + +@item @samp{%%} +Print a literal @samp{%} character. @xref{Other Output Conversions}. +@end table + +If the syntax of a conversion specification is invalid, unpredictable +things will happen, so don't do this. If there aren't enough function +arguments provided to supply values for all the conversion +specifications in the template string, or if the arguments are not of +the correct types, the results are unpredictable. If you supply more +arguments than conversion specifications, the extra argument values are +simply ignored; this is sometimes useful. + +@node Integer Conversions +@subsection Integer Conversions + +This section describes the options for the @samp{%d}, @samp{%i}, +@samp{%o}, @samp{%u}, @samp{%x}, and @samp{%X} conversion +specifications. These conversions print integers in various formats. + +The @samp{%d} and @samp{%i} conversion specifications both print an +@code{int} argument as a signed decimal number; while @samp{%o}, +@samp{%u}, and @samp{%x} print the argument as an unsigned octal, +decimal, or hexadecimal number (respectively). The @samp{%X} conversion +specification is just like @samp{%x} except that it uses the characters +@samp{ABCDEF} as digits instead of @samp{abcdef}. + +The following flags are meaningful: + +@table @asis +@item @samp{-} +Left-justify the result in the field (instead of the normal +right-justification). + +@item @samp{+} +For the signed @samp{%d} and @samp{%i} conversions, print a +plus sign if the value is positive. + +@item @samp{ } +For the signed @samp{%d} and @samp{%i} conversions, if the result +doesn't start with a plus or minus sign, prefix it with a space +character instead. Since the @samp{+} flag ensures that the result +includes a sign, this flag is ignored if you supply both of them. + +@item @samp{#} +For the @samp{%o} conversion, this forces the leading digit to be +@samp{0}, as if by increasing the precision. For @samp{%x} or +@samp{%X}, this prefixes a leading @samp{0x} or @samp{0X} (respectively) +to the result. This doesn't do anything useful for the @samp{%d}, +@samp{%i}, or @samp{%u} conversions. Using this flag produces output +which can be parsed by the @code{strtoul} function (@pxref{Parsing of +Integers}) and @code{scanf} with the @samp{%i} conversion +(@pxref{Numeric Input Conversions}). + +@item @samp{'} +Separate the digits into groups as specified by the locale specified for +the @code{LC_NUMERIC} category; @pxref{General Numeric}. This flag is a +GNU extension. + +@item @samp{0} +Pad the field with zeros instead of spaces. The zeros are placed after +any indication of sign or base. This flag is ignored if the @samp{-} +flag is also specified, or if a precision is specified. +@end table + +If a precision is supplied, it specifies the minimum number of digits to +appear; leading zeros are produced if necessary. If you don't specify a +precision, the number is printed with as many digits as it needs. If +you convert a value of zero with an explicit precision of zero, then no +characters at all are produced. + +Without a type modifier, the corresponding argument is treated as an +@code{int} (for the signed conversions @samp{%i} and @samp{%d}) or +@code{unsigned int} (for the unsigned conversions @samp{%o}, @samp{%u}, +@samp{%x}, and @samp{%X}). Recall that since @code{printf} and friends +are variadic, any @code{char} and @code{short} arguments are +automatically converted to @code{int} by the default argument +promotions. For arguments of other integer types, you can use these +modifiers: + +@table @samp +@item hh +Specifies that the argument is a @code{signed char} or @code{unsigned +char}, as appropriate. A @code{char} argument is converted to an +@code{int} or @code{unsigned int} by the default argument promotions +anyway, but the @samp{hh} modifier says to convert it back to a +@code{char} again. + +This modifier was introduced in @w{ISO C99}. + +@item h +Specifies that the argument is a @code{short int} or @code{unsigned +short int}, as appropriate. A @code{short} argument is converted to an +@code{int} or @code{unsigned int} by the default argument promotions +anyway, but the @samp{h} modifier says to convert it back to a +@code{short} again. + +@item j +Specifies that the argument is a @code{intmax_t} or @code{uintmax_t}, as +appropriate. + +This modifier was introduced in @w{ISO C99}. + +@item l +Specifies that the argument is a @code{long int} or @code{unsigned long +int}, as appropriate. Two @samp{l} characters are like the @samp{L} +modifier, below. + +If used with @samp{%c} or @samp{%s} the corresponding parameter is +considered as a wide character or wide character string respectively. +This use of @samp{l} was introduced in @w{Amendment 1} to @w{ISO C90}. + +@item L +@itemx ll +@itemx q +Specifies that the argument is a @code{long long int}. (This type is +an extension supported by the GNU C compiler. On systems that don't +support extra-long integers, this is the same as @code{long int}.) + +The @samp{q} modifier is another name for the same thing, which comes +from 4.4 BSD; a @w{@code{long long int}} is sometimes called a ``quad'' +@code{int}. + +@item t +Specifies that the argument is a @code{ptrdiff_t}. + +This modifier was introduced in @w{ISO C99}. + +@item z +@itemx Z +Specifies that the argument is a @code{size_t}. + +@samp{z} was introduced in @w{ISO C99}. @samp{Z} is a GNU extension +predating this addition and should not be used in new code. +@end table + +Here is an example. Using the template string: + +@smallexample +"|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n" +@end smallexample + +@noindent +to print numbers using the different options for the @samp{%d} +conversion gives results like: + +@smallexample +| 0|0 | +0|+0 | 0|00000| | 00|0| +| 1|1 | +1|+1 | 1|00001| 1| 01|1| +| -1|-1 | -1|-1 | -1|-0001| -1| -01|-1| +|100000|100000|+100000|+100000| 100000|100000|100000|100000|100000| +@end smallexample + +In particular, notice what happens in the last case where the number +is too large to fit in the minimum field width specified. + +Here are some more examples showing how unsigned integers print under +various format options, using the template string: + +@smallexample +"|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n" +@end smallexample + +@smallexample +| 0| 0| 0| 0| 0| 0| 0| 00000000| +| 1| 1| 1| 1| 01| 0x1| 0X1|0x00000001| +|100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0| +@end smallexample + + +@node Floating-Point Conversions +@subsection Floating-Point Conversions + +This section discusses the conversion specifications for floating-point +numbers: the @samp{%f}, @samp{%e}, @samp{%E}, @samp{%g}, and @samp{%G} +conversions. + +The @samp{%f} conversion prints its argument in fixed-point notation, +producing output of the form +@w{[@code{-}]@var{ddd}@code{.}@var{ddd}}, +where the number of digits following the decimal point is controlled +by the precision you specify. + +The @samp{%e} conversion prints its argument in exponential notation, +producing output of the form +@w{[@code{-}]@var{d}@code{.}@var{ddd}@code{e}[@code{+}|@code{-}]@var{dd}}. +Again, the number of digits following the decimal point is controlled by +the precision. The exponent always contains at least two digits. The +@samp{%E} conversion is similar but the exponent is marked with the letter +@samp{E} instead of @samp{e}. + +The @samp{%g} and @samp{%G} conversions print the argument in the style +of @samp{%e} or @samp{%E} (respectively) if the exponent would be less +than -4 or greater than or equal to the precision; otherwise they use +the @samp{%f} style. A precision of @code{0}, is taken as 1. +Trailing zeros are removed from the fractional portion of the result and +a decimal-point character appears only if it is followed by a digit. + +The @samp{%a} and @samp{%A} conversions are meant for representing +floating-point numbers exactly in textual form so that they can be +exchanged as texts between different programs and/or machines. The +numbers are represented in the form +@w{[@code{-}]@code{0x}@var{h}@code{.}@var{hhh}@code{p}[@code{+}|@code{-}]@var{dd}}. +At the left of the decimal-point character exactly one digit is print. +This character is only @code{0} if the number is denormalized. +Otherwise the value is unspecified; it is implementation dependent how many +bits are used. The number of hexadecimal digits on the right side of +the decimal-point character is equal to the precision. If the precision +is zero it is determined to be large enough to provide an exact +representation of the number (or it is large enough to distinguish two +adjacent values if the @code{FLT_RADIX} is not a power of 2, +@pxref{Floating Point Parameters}). For the @samp{%a} conversion +lower-case characters are used to represent the hexadecimal number and +the prefix and exponent sign are printed as @code{0x} and @code{p} +respectively. Otherwise upper-case characters are used and @code{0X} +and @code{P} are used for the representation of prefix and exponent +string. The exponent to the base of two is printed as a decimal number +using at least one digit but at most as many digits as necessary to +represent the value exactly. + +If the value to be printed represents infinity or a NaN, the output is +@w{[@code{-}]@code{inf}} or @code{nan} respectively if the conversion +specifier is @samp{%a}, @samp{%e}, @samp{%f}, or @samp{%g} and it is +@w{[@code{-}]@code{INF}} or @code{NAN} respectively if the conversion is +@samp{%A}, @samp{%E}, or @samp{%G}. + +The following flags can be used to modify the behavior: + +@comment We use @asis instead of @samp so we can have ` ' as an item. +@table @asis +@item @samp{-} +Left-justify the result in the field. Normally the result is +right-justified. + +@item @samp{+} +Always include a plus or minus sign in the result. + +@item @samp{ } +If the result doesn't start with a plus or minus sign, prefix it with a +space instead. Since the @samp{+} flag ensures that the result includes +a sign, this flag is ignored if you supply both of them. + +@item @samp{#} +Specifies that the result should always include a decimal point, even +if no digits follow it. For the @samp{%g} and @samp{%G} conversions, +this also forces trailing zeros after the decimal point to be left +in place where they would otherwise be removed. + +@item @samp{'} +Separate the digits of the integer part of the result into groups as +specified by the locale specified for the @code{LC_NUMERIC} category; +@pxref{General Numeric}. This flag is a GNU extension. + +@item @samp{0} +Pad the field with zeros instead of spaces; the zeros are placed +after any sign. This flag is ignored if the @samp{-} flag is also +specified. +@end table + +The precision specifies how many digits follow the decimal-point +character for the @samp{%f}, @samp{%e}, and @samp{%E} conversions. For +these conversions, the default precision is @code{6}. If the precision +is explicitly @code{0}, this suppresses the decimal point character +entirely. For the @samp{%g} and @samp{%G} conversions, the precision +specifies how many significant digits to print. Significant digits are +the first digit before the decimal point, and all the digits after it. +If the precision is @code{0} or not specified for @samp{%g} or @samp{%G}, +it is treated like a value of @code{1}. If the value being printed +cannot be expressed accurately in the specified number of digits, the +value is rounded to the nearest number that fits. + +Without a type modifier, the floating-point conversions use an argument +of type @code{double}. (By the default argument promotions, any +@code{float} arguments are automatically converted to @code{double}.) +The following type modifier is supported: + +@table @samp +@item L +An uppercase @samp{L} specifies that the argument is a @code{long +double}. +@end table + +Here are some examples showing how numbers print using the various +floating-point conversions. All of the numbers were printed using +this template string: + +@smallexample +"|%13.4a|%13.4f|%13.4e|%13.4g|\n" +@end smallexample + +Here is the output: + +@smallexample +| 0x0.0000p+0| 0.0000| 0.0000e+00| 0| +| 0x1.0000p-1| 0.5000| 5.0000e-01| 0.5| +| 0x1.0000p+0| 1.0000| 1.0000e+00| 1| +| -0x1.0000p+0| -1.0000| -1.0000e+00| -1| +| 0x1.9000p+6| 100.0000| 1.0000e+02| 100| +| 0x1.f400p+9| 1000.0000| 1.0000e+03| 1000| +| 0x1.3880p+13| 10000.0000| 1.0000e+04| 1e+04| +| 0x1.81c8p+13| 12345.0000| 1.2345e+04| 1.234e+04| +| 0x1.86a0p+16| 100000.0000| 1.0000e+05| 1e+05| +| 0x1.e240p+16| 123456.0000| 1.2346e+05| 1.235e+05| +@end smallexample + +Notice how the @samp{%g} conversion drops trailing zeros. + +@node Other Output Conversions +@subsection Other Output Conversions + +This section describes miscellaneous conversions for @code{printf}. + +The @samp{%c} conversion prints a single character. In case there is no +@samp{l} modifier the @code{int} argument is first converted to an +@code{unsigned char}. Then, if used in a wide stream function, the +character is converted into the corresponding wide character. The +@samp{-} flag can be used to specify left-justification in the field, +but no other flags are defined, and no precision or type modifier can be +given. For example: + +@smallexample +printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o'); +@end smallexample + +@noindent +prints @samp{hello}. + +If there is an @samp{l} modifier present the argument is expected to be +of type @code{wint_t}. If used in a multibyte function the wide +character is converted into a multibyte character before being added to +the output. In this case more than one output byte can be produced. + +The @samp{%s} conversion prints a string. If no @samp{l} modifier is +present the corresponding argument must be of type @code{char *} (or +@code{const char *}). If used in a wide stream function the string is +first converted to a wide character string. A precision can be +specified to indicate the maximum number of characters to write; +otherwise characters in the string up to but not including the +terminating null character are written to the output stream. The +@samp{-} flag can be used to specify left-justification in the field, +but no other flags or type modifiers are defined for this conversion. +For example: + +@smallexample +printf ("%3s%-6s", "no", "where"); +@end smallexample + +@noindent +prints @samp{ nowhere }. + +If there is an @samp{l} modifier present, the argument is expected to +be of type @code{wchar_t} (or @code{const wchar_t *}). + +If you accidentally pass a null pointer as the argument for a @samp{%s} +conversion, @theglibc{} prints it as @samp{(null)}. We think this +is more useful than crashing. But it's not good practice to pass a null +argument intentionally. + +The @samp{%m} conversion prints the string corresponding to the error +code in @code{errno}. @xref{Error Messages}. Thus: + +@smallexample +fprintf (stderr, "can't open `%s': %m\n", filename); +@end smallexample + +@noindent +is equivalent to: + +@smallexample +fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno)); +@end smallexample + +@noindent +The @samp{%m} conversion is a @glibcadj{} extension. + +The @samp{%p} conversion prints a pointer value. The corresponding +argument must be of type @code{void *}. In practice, you can use any +type of pointer. + +In @theglibc{}, non-null pointers are printed as unsigned integers, +as if a @samp{%#x} conversion were used. Null pointers print as +@samp{(nil)}. (Pointers might print differently in other systems.) + +For example: + +@smallexample +printf ("%p", "testing"); +@end smallexample + +@noindent +prints @samp{0x} followed by a hexadecimal number---the address of the +string constant @code{"testing"}. It does not print the word +@samp{testing}. + +You can supply the @samp{-} flag with the @samp{%p} conversion to +specify left-justification, but no other flags, precision, or type +modifiers are defined. + +The @samp{%n} conversion is unlike any of the other output conversions. +It uses an argument which must be a pointer to an @code{int}, but +instead of printing anything it stores the number of characters printed +so far by this call at that location. The @samp{h} and @samp{l} type +modifiers are permitted to specify that the argument is of type +@code{short int *} or @code{long int *} instead of @code{int *}, but no +flags, field width, or precision are permitted. + +For example, + +@smallexample +int nchar; +printf ("%d %s%n\n", 3, "bears", &nchar); +@end smallexample + +@noindent +prints: + +@smallexample +3 bears +@end smallexample + +@noindent +and sets @code{nchar} to @code{7}, because @samp{3 bears} is seven +characters. + + +The @samp{%%} conversion prints a literal @samp{%} character. This +conversion doesn't use an argument, and no flags, field width, +precision, or type modifiers are permitted. + + +@node Formatted Output Functions +@subsection Formatted Output Functions + +This section describes how to call @code{printf} and related functions. +Prototypes for these functions are in the header file @file{stdio.h}. +Because these functions take a variable number of arguments, you +@emph{must} declare prototypes for them before using them. Of course, +the easiest way to make sure you have all the right prototypes is to +just include @file{stdio.h}. +@pindex stdio.h + +@comment stdio.h +@comment ISO +@deftypefun int printf (const char *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +The @code{printf} function prints the optional arguments under the +control of the template string @var{template} to the stream +@code{stdout}. It returns the number of characters printed, or a +negative value if there was an output error. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun int wprintf (const wchar_t *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +The @code{wprintf} function prints the optional arguments under the +control of the wide template string @var{template} to the stream +@code{stdout}. It returns the number of wide characters printed, or a +negative value if there was an output error. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int fprintf (FILE *@var{stream}, const char *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +This function is just like @code{printf}, except that the output is +written to the stream @var{stream} instead of @code{stdout}. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun int fwprintf (FILE *@var{stream}, const wchar_t *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +This function is just like @code{wprintf}, except that the output is +written to the stream @var{stream} instead of @code{stdout}. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int sprintf (char *@var{s}, const char *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} +This is like @code{printf}, except that the output is stored in the character +array @var{s} instead of written to a stream. A null character is written +to mark the end of the string. + +The @code{sprintf} function returns the number of characters stored in +the array @var{s}, not including the terminating null character. + +The behavior of this function is undefined if copying takes place +between objects that overlap---for example, if @var{s} is also given +as an argument to be printed under control of the @samp{%s} conversion. +@xref{Copying Strings and Arrays}. + +@strong{Warning:} The @code{sprintf} function can be @strong{dangerous} +because it can potentially output more characters than can fit in the +allocation size of the string @var{s}. Remember that the field width +given in a conversion specification is only a @emph{minimum} value. + +To avoid this problem, you can use @code{snprintf} or @code{asprintf}, +described below. +@end deftypefun + +@comment wchar.h +@comment GNU +@deftypefun int swprintf (wchar_t *@var{ws}, size_t @var{size}, const wchar_t *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} +This is like @code{wprintf}, except that the output is stored in the +wide character array @var{ws} instead of written to a stream. A null +wide character is written to mark the end of the string. The @var{size} +argument specifies the maximum number of characters to produce. The +trailing null character is counted towards this limit, so you should +allocate at least @var{size} wide characters for the string @var{ws}. + +The return value is the number of characters generated for the given +input, excluding the trailing null. If not all output fits into the +provided buffer a negative value is returned. You should try again with +a bigger output string. @emph{Note:} this is different from how +@code{snprintf} handles this situation. + +Note that the corresponding narrow stream function takes fewer +parameters. @code{swprintf} in fact corresponds to the @code{snprintf} +function. Since the @code{sprintf} function can be dangerous and should +be avoided the @w{ISO C} committee refused to make the same mistake +again and decided to not define a function exactly corresponding to +@code{sprintf}. +@end deftypefun + +@comment stdio.h +@comment GNU +@deftypefun int snprintf (char *@var{s}, size_t @var{size}, const char *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} +The @code{snprintf} function is similar to @code{sprintf}, except that +the @var{size} argument specifies the maximum number of characters to +produce. The trailing null character is counted towards this limit, so +you should allocate at least @var{size} characters for the string @var{s}. +If @var{size} is zero, nothing, not even the null byte, shall be written and +@var{s} may be a null pointer. + +The return value is the number of characters which would be generated +for the given input, excluding the trailing null. If this value is +greater than or equal to @var{size}, not all characters from the result have +been stored in @var{s}. You should try again with a bigger output +string. Here is an example of doing this: + +@smallexample +@group +/* @r{Construct a message describing the value of a variable} + @r{whose name is @var{name} and whose value is @var{value}.} */ +char * +make_message (char *name, char *value) +@{ + /* @r{Guess we need no more than 100 chars of space.} */ + int size = 100; + char *buffer = (char *) xmalloc (size); + int nchars; +@end group +@group + if (buffer == NULL) + return NULL; + + /* @r{Try to print in the allocated space.} */ + nchars = snprintf (buffer, size, "value of %s is %s", + name, value); +@end group +@group + if (nchars >= size) + @{ + /* @r{Reallocate buffer now that we know + how much space is needed.} */ + size = nchars + 1; + buffer = (char *) xrealloc (buffer, size); + + if (buffer != NULL) + /* @r{Try again.} */ + snprintf (buffer, size, "value of %s is %s", + name, value); + @} + /* @r{The last call worked, return the string.} */ + return buffer; +@} +@end group +@end smallexample + +In practice, it is often easier just to use @code{asprintf}, below. + +@strong{Attention:} In versions of @theglibc{} prior to 2.1 the +return value is the number of characters stored, not including the +terminating null; unless there was not enough space in @var{s} to +store the result in which case @code{-1} is returned. This was +changed in order to comply with the @w{ISO C99} standard. +@end deftypefun + +@node Dynamic Output +@subsection Dynamically Allocating Formatted Output + +The functions in this section do formatted output and place the results +in dynamically allocated memory. + +@comment stdio.h +@comment GNU +@deftypefun int asprintf (char **@var{ptr}, const char *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} +This function is similar to @code{sprintf}, except that it dynamically +allocates a string (as with @code{malloc}; @pxref{Unconstrained +Allocation}) to hold the output, instead of putting the output in a +buffer you allocate in advance. The @var{ptr} argument should be the +address of a @code{char *} object, and a successful call to +@code{asprintf} stores a pointer to the newly allocated string at that +location. + +The return value is the number of characters allocated for the buffer, or +less than zero if an error occurred. Usually this means that the buffer +could not be allocated. + +Here is how to use @code{asprintf} to get the same result as the +@code{snprintf} example, but more easily: + +@smallexample +/* @r{Construct a message describing the value of a variable} + @r{whose name is @var{name} and whose value is @var{value}.} */ +char * +make_message (char *name, char *value) +@{ + char *result; + if (asprintf (&result, "value of %s is %s", name, value) < 0) + return NULL; + return result; +@} +@end smallexample +@end deftypefun + +@comment stdio.h +@comment GNU +@deftypefun int obstack_printf (struct obstack *@var{obstack}, const char *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtsrace{:obstack} @mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acucorrupt{} @acsmem{}}} +This function is similar to @code{asprintf}, except that it uses the +obstack @var{obstack} to allocate the space. @xref{Obstacks}. + +The characters are written onto the end of the current object. +To get at them, you must finish the object with @code{obstack_finish} +(@pxref{Growing Objects}).@refill +@end deftypefun + +@node Variable Arguments Output +@subsection Variable Arguments Output Functions + +The functions @code{vprintf} and friends are provided so that you can +define your own variadic @code{printf}-like functions that make use of +the same internals as the built-in formatted output functions. + +The most natural way to define such functions would be to use a language +construct to say, ``Call @code{printf} and pass this template plus all +of my arguments after the first five.'' But there is no way to do this +in C, and it would be hard to provide a way, since at the C language +level there is no way to tell how many arguments your function received. + +Since that method is impossible, we provide alternative functions, the +@code{vprintf} series, which lets you pass a @code{va_list} to describe +``all of my arguments after the first five.'' + +When it is sufficient to define a macro rather than a real function, +the GNU C compiler provides a way to do this much more easily with macros. +For example: + +@smallexample +#define myprintf(a, b, c, d, e, rest...) \ + printf (mytemplate , ## rest) +@end smallexample + +@noindent +@xref{Variadic Macros,,, cpp, The C preprocessor}, for details. +But this is limited to macros, and does not apply to real functions at all. + +Before calling @code{vprintf} or the other functions listed in this +section, you @emph{must} call @code{va_start} (@pxref{Variadic +Functions}) to initialize a pointer to the variable arguments. Then you +can call @code{va_arg} to fetch the arguments that you want to handle +yourself. This advances the pointer past those arguments. + +Once your @code{va_list} pointer is pointing at the argument of your +choice, you are ready to call @code{vprintf}. That argument and all +subsequent arguments that were passed to your function are used by +@code{vprintf} along with the template that you specified separately. + +@strong{Portability Note:} The value of the @code{va_list} pointer is +undetermined after the call to @code{vprintf}, so you must not use +@code{va_arg} after you call @code{vprintf}. Instead, you should call +@code{va_end} to retire the pointer from service. You can call +@code{va_start} again and begin fetching the arguments from the start of +the variable argument list. (Alternatively, you can use @code{va_copy} +to make a copy of the @code{va_list} pointer before calling +@code{vfprintf}.) Calling @code{vprintf} does not destroy the argument +list of your function, merely the particular pointer that you passed to +it. + +Prototypes for these functions are declared in @file{stdio.h}. +@pindex stdio.h + +@comment stdio.h +@comment ISO +@deftypefun int vprintf (const char *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +This function is similar to @code{printf} except that, instead of taking +a variable number of arguments directly, it takes an argument list +pointer @var{ap}. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun int vwprintf (const wchar_t *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +This function is similar to @code{wprintf} except that, instead of taking +a variable number of arguments directly, it takes an argument list +pointer @var{ap}. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int vfprintf (FILE *@var{stream}, const char *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +@c Although vfprintf sets up a cleanup region to release the lock on the +@c output stream, it doesn't use it to release args_value or string in +@c case of cancellation. This doesn't make it unsafe, but cancelling it +@c may leak memory. The unguarded use of __printf_function_table is +@c also of concern for all callers. +@c _itoa ok +@c _udiv_qrnnd_preinv ok +@c group_number ok +@c _i18n_number_rewrite +@c __wctrans ok +@c __towctrans @mtslocale +@c __wcrtomb ok? dup below +@c outdigit_value ok +@c outdigitwc_value ok +@c outchar ok +@c outstring ok +@c PAD ok +@c __printf_fp @mtslocale @ascuheap @acsmem +@c __printf_fphex @mtslocale +@c __readonly_area +@c [GNU/Linux] fopen, strtoul, free +@c __strerror_r ok if no translation, check otherwise +@c __btowc ? gconv-modules +@c __wcrtomb ok (not using internal state) gconv-modules +@c ARGCHECK +@c UNBUFFERED_P (tested before taking the stream lock) +@c buffered_vfprintf ok +@c __find_spec(wc|mb) +@c read_int +@c __libc_use_alloca +@c process_arg +@c process_string_arg +@c extend_alloca +@c __parse_one_spec(wc|mb) +@c *__printf_arginfo_table unguarded +@c __printf_va_arg_table-> unguarded +@c *__printf_function_table unguarded +@c done_add +@c printf_unknown +@c outchar +@c _itoa_word +This is the equivalent of @code{fprintf} with the variable argument list +specified directly as for @code{vprintf}. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun int vfwprintf (FILE *@var{stream}, const wchar_t *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +This is the equivalent of @code{fwprintf} with the variable argument list +specified directly as for @code{vwprintf}. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int vsprintf (char *@var{s}, const char *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} +This is the equivalent of @code{sprintf} with the variable argument list +specified directly as for @code{vprintf}. +@end deftypefun + +@comment wchar.h +@comment GNU +@deftypefun int vswprintf (wchar_t *@var{ws}, size_t @var{size}, const wchar_t *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} +This is the equivalent of @code{swprintf} with the variable argument list +specified directly as for @code{vwprintf}. +@end deftypefun + +@comment stdio.h +@comment GNU +@deftypefun int vsnprintf (char *@var{s}, size_t @var{size}, const char *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} +This is the equivalent of @code{snprintf} with the variable argument list +specified directly as for @code{vprintf}. +@end deftypefun + +@comment stdio.h +@comment GNU +@deftypefun int vasprintf (char **@var{ptr}, const char *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} +The @code{vasprintf} function is the equivalent of @code{asprintf} with the +variable argument list specified directly as for @code{vprintf}. +@end deftypefun + +@comment stdio.h +@comment GNU +@deftypefun int obstack_vprintf (struct obstack *@var{obstack}, const char *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtsrace{:obstack} @mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acucorrupt{} @acsmem{}}} +@c The obstack is not guarded by mutexes, it might be at an inconsistent +@c state within a signal handler, and it could be left at an +@c inconsistent state in case of cancellation. +The @code{obstack_vprintf} function is the equivalent of +@code{obstack_printf} with the variable argument list specified directly +as for @code{vprintf}.@refill +@end deftypefun + +Here's an example showing how you might use @code{vfprintf}. This is a +function that prints error messages to the stream @code{stderr}, along +with a prefix indicating the name of the program +(@pxref{Error Messages}, for a description of +@code{program_invocation_short_name}). + +@smallexample +@group +#include <stdio.h> +#include <stdarg.h> + +void +eprintf (const char *template, ...) +@{ + va_list ap; + extern char *program_invocation_short_name; + + fprintf (stderr, "%s: ", program_invocation_short_name); + va_start (ap, template); + vfprintf (stderr, template, ap); + va_end (ap); +@} +@end group +@end smallexample + +@noindent +You could call @code{eprintf} like this: + +@smallexample +eprintf ("file `%s' does not exist\n", filename); +@end smallexample + +In GNU C, there is a special construct you can use to let the compiler +know that a function uses a @code{printf}-style format string. Then it +can check the number and types of arguments in each call to the +function, and warn you when they do not match the format string. +For example, take this declaration of @code{eprintf}: + +@smallexample +void eprintf (const char *template, ...) + __attribute__ ((format (printf, 1, 2))); +@end smallexample + +@noindent +This tells the compiler that @code{eprintf} uses a format string like +@code{printf} (as opposed to @code{scanf}; @pxref{Formatted Input}); +the format string appears as the first argument; +and the arguments to satisfy the format begin with the second. +@xref{Function Attributes, , Declaring Attributes of Functions, +gcc.info, Using GNU CC}, for more information. + +@node Parsing a Template String +@subsection Parsing a Template String +@cindex parsing a template string + +You can use the function @code{parse_printf_format} to obtain +information about the number and types of arguments that are expected by +a given template string. This function permits interpreters that +provide interfaces to @code{printf} to avoid passing along invalid +arguments from the user's program, which could cause a crash. + +All the symbols described in this section are declared in the header +file @file{printf.h}. + +@comment printf.h +@comment GNU +@deftypefun size_t parse_printf_format (const char *@var{template}, size_t @var{n}, int *@var{argtypes}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@assafe{}@acsafe{}} +This function returns information about the number and types of +arguments expected by the @code{printf} template string @var{template}. +The information is stored in the array @var{argtypes}; each element of +this array describes one argument. This information is encoded using +the various @samp{PA_} macros, listed below. + +The argument @var{n} specifies the number of elements in the array +@var{argtypes}. This is the maximum number of elements that +@code{parse_printf_format} will try to write. + +@code{parse_printf_format} returns the total number of arguments required +by @var{template}. If this number is greater than @var{n}, then the +information returned describes only the first @var{n} arguments. If you +want information about additional arguments, allocate a bigger +array and call @code{parse_printf_format} again. +@end deftypefun + +The argument types are encoded as a combination of a basic type and +modifier flag bits. + +@comment printf.h +@comment GNU +@deftypevr Macro int PA_FLAG_MASK +This macro is a bitmask for the type modifier flag bits. You can write +the expression @code{(argtypes[i] & PA_FLAG_MASK)} to extract just the +flag bits for an argument, or @code{(argtypes[i] & ~PA_FLAG_MASK)} to +extract just the basic type code. +@end deftypevr + +Here are symbolic constants that represent the basic types; they stand +for integer values. + +@vtable @code +@comment printf.h +@comment GNU +@item PA_INT +This specifies that the base type is @code{int}. + +@comment printf.h +@comment GNU +@item PA_CHAR +This specifies that the base type is @code{int}, cast to @code{char}. + +@comment printf.h +@comment GNU +@item PA_STRING +This specifies that the base type is @code{char *}, a null-terminated string. + +@comment printf.h +@comment GNU +@item PA_POINTER +This specifies that the base type is @code{void *}, an arbitrary pointer. + +@comment printf.h +@comment GNU +@item PA_FLOAT +This specifies that the base type is @code{float}. + +@comment printf.h +@comment GNU +@item PA_DOUBLE +This specifies that the base type is @code{double}. + +@comment printf.h +@comment GNU +@item PA_LAST +You can define additional base types for your own programs as offsets +from @code{PA_LAST}. For example, if you have data types @samp{foo} +and @samp{bar} with their own specialized @code{printf} conversions, +you could define encodings for these types as: + +@smallexample +#define PA_FOO PA_LAST +#define PA_BAR (PA_LAST + 1) +@end smallexample +@end vtable + +Here are the flag bits that modify a basic type. They are combined with +the code for the basic type using inclusive-or. + +@vtable @code +@comment printf.h +@comment GNU +@item PA_FLAG_PTR +If this bit is set, it indicates that the encoded type is a pointer to +the base type, rather than an immediate value. +For example, @samp{PA_INT|PA_FLAG_PTR} represents the type @samp{int *}. + +@comment printf.h +@comment GNU +@item PA_FLAG_SHORT +If this bit is set, it indicates that the base type is modified with +@code{short}. (This corresponds to the @samp{h} type modifier.) + +@comment printf.h +@comment GNU +@item PA_FLAG_LONG +If this bit is set, it indicates that the base type is modified with +@code{long}. (This corresponds to the @samp{l} type modifier.) + +@comment printf.h +@comment GNU +@item PA_FLAG_LONG_LONG +If this bit is set, it indicates that the base type is modified with +@code{long long}. (This corresponds to the @samp{L} type modifier.) + +@comment printf.h +@comment GNU +@item PA_FLAG_LONG_DOUBLE +This is a synonym for @code{PA_FLAG_LONG_LONG}, used by convention with +a base type of @code{PA_DOUBLE} to indicate a type of @code{long double}. +@end vtable + +@ifinfo +For an example of using these facilities, see @ref{Example of Parsing}. +@end ifinfo + +@node Example of Parsing +@subsection Example of Parsing a Template String + +Here is an example of decoding argument types for a format string. We +assume this is part of an interpreter which contains arguments of type +@code{NUMBER}, @code{CHAR}, @code{STRING} and @code{STRUCTURE} (and +perhaps others which are not valid here). + +@smallexample +/* @r{Test whether the @var{nargs} specified objects} + @r{in the vector @var{args} are valid} + @r{for the format string @var{format}:} + @r{if so, return 1.} + @r{If not, return 0 after printing an error message.} */ + +int +validate_args (char *format, int nargs, OBJECT *args) +@{ + int *argtypes; + int nwanted; + + /* @r{Get the information about the arguments.} + @r{Each conversion specification must be at least two characters} + @r{long, so there cannot be more specifications than half the} + @r{length of the string.} */ + + argtypes = (int *) alloca (strlen (format) / 2 * sizeof (int)); + nwanted = parse_printf_format (string, nelts, argtypes); + + /* @r{Check the number of arguments.} */ + if (nwanted > nargs) + @{ + error ("too few arguments (at least %d required)", nwanted); + return 0; + @} + + /* @r{Check the C type wanted for each argument} + @r{and see if the object given is suitable.} */ + for (i = 0; i < nwanted; i++) + @{ + int wanted; + + if (argtypes[i] & PA_FLAG_PTR) + wanted = STRUCTURE; + else + switch (argtypes[i] & ~PA_FLAG_MASK) + @{ + case PA_INT: + case PA_FLOAT: + case PA_DOUBLE: + wanted = NUMBER; + break; + case PA_CHAR: + wanted = CHAR; + break; + case PA_STRING: + wanted = STRING; + break; + case PA_POINTER: + wanted = STRUCTURE; + break; + @} + if (TYPE (args[i]) != wanted) + @{ + error ("type mismatch for arg number %d", i); + return 0; + @} + @} + return 1; +@} +@end smallexample + +@node Customizing Printf +@section Customizing @code{printf} +@cindex customizing @code{printf} +@cindex defining new @code{printf} conversions +@cindex extending @code{printf} + +@Theglibc{} lets you define your own custom conversion specifiers +for @code{printf} template strings, to teach @code{printf} clever ways +to print the important data structures of your program. + +The way you do this is by registering the conversion with the function +@code{register_printf_function}; see @ref{Registering New Conversions}. +One of the arguments you pass to this function is a pointer to a handler +function that produces the actual output; see @ref{Defining the Output +Handler}, for information on how to write this function. + +You can also install a function that just returns information about the +number and type of arguments expected by the conversion specifier. +@xref{Parsing a Template String}, for information about this. + +The facilities of this section are declared in the header file +@file{printf.h}. + +@menu +* Registering New Conversions:: Using @code{register_printf_function} + to register a new output conversion. +* Conversion Specifier Options:: The handler must be able to get + the options specified in the + template when it is called. +* Defining the Output Handler:: Defining the handler and arginfo + functions that are passed as arguments + to @code{register_printf_function}. +* Printf Extension Example:: How to define a @code{printf} + handler function. +* Predefined Printf Handlers:: Predefined @code{printf} handlers. +@end menu + +@strong{Portability Note:} The ability to extend the syntax of +@code{printf} template strings is a GNU extension. ISO standard C has +nothing similar. + +@node Registering New Conversions +@subsection Registering New Conversions + +The function to register a new output conversion is +@code{register_printf_function}, declared in @file{printf.h}. +@pindex printf.h + +@comment printf.h +@comment GNU +@deftypefun int register_printf_function (int @var{spec}, printf_function @var{handler-function}, printf_arginfo_function @var{arginfo-function}) +@safety{@prelim{}@mtunsafe{@mtasuconst{:printfext}}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@acsmem{} @aculock{}}} +@c This function is guarded by the global non-recursive libc lock, but +@c users of the variables it sets aren't, and those should be MT-Safe, +@c so we're ruling out the use of this extension with threads. Calling +@c it from a signal handler may self-deadlock, and cancellation may +@c leave the lock held, besides leaking allocated memory. +This function defines the conversion specifier character @var{spec}. +Thus, if @var{spec} is @code{'Y'}, it defines the conversion @samp{%Y}. +You can redefine the built-in conversions like @samp{%s}, but flag +characters like @samp{#} and type modifiers like @samp{l} can never be +used as conversions; calling @code{register_printf_function} for those +characters has no effect. It is advisable not to use lowercase letters, +since the ISO C standard warns that additional lowercase letters may be +standardized in future editions of the standard. + +The @var{handler-function} is the function called by @code{printf} and +friends when this conversion appears in a template string. +@xref{Defining the Output Handler}, for information about how to define +a function to pass as this argument. If you specify a null pointer, any +existing handler function for @var{spec} is removed. + +The @var{arginfo-function} is the function called by +@code{parse_printf_format} when this conversion appears in a +template string. @xref{Parsing a Template String}, for information +about this. + +@c The following is not true anymore. The `parse_printf_format' function +@c is now also called from `vfprintf' via `parse_one_spec'. +@c --drepper@gnu, 1996/11/14 +@c +@c Normally, you install both functions for a conversion at the same time, +@c but if you are never going to call @code{parse_printf_format}, you do +@c not need to define an arginfo function. + +@strong{Attention:} In @theglibc{} versions before 2.0 the +@var{arginfo-function} function did not need to be installed unless +the user used the @code{parse_printf_format} function. This has changed. +Now a call to any of the @code{printf} functions will call this +function when this format specifier appears in the format string. + +The return value is @code{0} on success, and @code{-1} on failure +(which occurs if @var{spec} is out of range). + +You can redefine the standard output conversions, but this is probably +not a good idea because of the potential for confusion. Library routines +written by other people could break if you do this. +@end deftypefun + +@node Conversion Specifier Options +@subsection Conversion Specifier Options + +If you define a meaning for @samp{%A}, what if the template contains +@samp{%+23A} or @samp{%-#A}? To implement a sensible meaning for these, +the handler when called needs to be able to get the options specified in +the template. + +Both the @var{handler-function} and @var{arginfo-function} accept an +argument that points to a @code{struct printf_info}, which contains +information about the options appearing in an instance of the conversion +specifier. This data type is declared in the header file +@file{printf.h}. +@pindex printf.h + +@comment printf.h +@comment GNU +@deftp {Type} {struct printf_info} +This structure is used to pass information about the options appearing +in an instance of a conversion specifier in a @code{printf} template +string to the handler and arginfo functions for that specifier. It +contains the following members: + +@table @code +@item int prec +This is the precision specified. The value is @code{-1} if no precision +was specified. If the precision was given as @samp{*}, the +@code{printf_info} structure passed to the handler function contains the +actual value retrieved from the argument list. But the structure passed +to the arginfo function contains a value of @code{INT_MIN}, since the +actual value is not known. + +@item int width +This is the minimum field width specified. The value is @code{0} if no +width was specified. If the field width was given as @samp{*}, the +@code{printf_info} structure passed to the handler function contains the +actual value retrieved from the argument list. But the structure passed +to the arginfo function contains a value of @code{INT_MIN}, since the +actual value is not known. + +@item wchar_t spec +This is the conversion specifier character specified. It's stored in +the structure so that you can register the same handler function for +multiple characters, but still have a way to tell them apart when the +handler function is called. + +@item unsigned int is_long_double +This is a boolean that is true if the @samp{L}, @samp{ll}, or @samp{q} +type modifier was specified. For integer conversions, this indicates +@code{long long int}, as opposed to @code{long double} for floating +point conversions. + +@item unsigned int is_char +This is a boolean that is true if the @samp{hh} type modifier was specified. + +@item unsigned int is_short +This is a boolean that is true if the @samp{h} type modifier was specified. + +@item unsigned int is_long +This is a boolean that is true if the @samp{l} type modifier was specified. + +@item unsigned int alt +This is a boolean that is true if the @samp{#} flag was specified. + +@item unsigned int space +This is a boolean that is true if the @samp{ } flag was specified. + +@item unsigned int left +This is a boolean that is true if the @samp{-} flag was specified. + +@item unsigned int showsign +This is a boolean that is true if the @samp{+} flag was specified. + +@item unsigned int group +This is a boolean that is true if the @samp{'} flag was specified. + +@item unsigned int extra +This flag has a special meaning depending on the context. It could +be used freely by the user-defined handlers but when called from +the @code{printf} function this variable always contains the value +@code{0}. + +@item unsigned int wide +This flag is set if the stream is wide oriented. + +@item wchar_t pad +This is the character to use for padding the output to the minimum field +width. The value is @code{'0'} if the @samp{0} flag was specified, and +@code{' '} otherwise. +@end table +@end deftp + + +@node Defining the Output Handler +@subsection Defining the Output Handler + +Now let's look at how to define the handler and arginfo functions +which are passed as arguments to @code{register_printf_function}. + +@strong{Compatibility Note:} The interface changed in @theglibc{} +version 2.0. Previously the third argument was of type +@code{va_list *}. + +You should define your handler functions with a prototype like: + +@smallexample +int @var{function} (FILE *stream, const struct printf_info *info, + const void *const *args) +@end smallexample + +The @var{stream} argument passed to the handler function is the stream to +which it should write output. + +The @var{info} argument is a pointer to a structure that contains +information about the various options that were included with the +conversion in the template string. You should not modify this structure +inside your handler function. @xref{Conversion Specifier Options}, for +a description of this data structure. + +@c The following changes some time back. --drepper@gnu, 1996/11/14 +@c +@c The @code{ap_pointer} argument is used to pass the tail of the variable +@c argument list containing the values to be printed to your handler. +@c Unlike most other functions that can be passed an explicit variable +@c argument list, this is a @emph{pointer} to a @code{va_list}, rather than +@c the @code{va_list} itself. Thus, you should fetch arguments by +@c means of @code{va_arg (*ap_pointer, @var{type})}. +@c +@c (Passing a pointer here allows the function that calls your handler +@c function to update its own @code{va_list} variable to account for the +@c arguments that your handler processes. @xref{Variadic Functions}.) + +The @var{args} is a vector of pointers to the arguments data. +The number of arguments was determined by calling the argument +information function provided by the user. + +Your handler function should return a value just like @code{printf} +does: it should return the number of characters it has written, or a +negative value to indicate an error. + +@comment printf.h +@comment GNU +@deftp {Data Type} printf_function +This is the data type that a handler function should have. +@end deftp + +If you are going to use @w{@code{parse_printf_format}} in your +application, you must also define a function to pass as the +@var{arginfo-function} argument for each new conversion you install with +@code{register_printf_function}. + +You have to define these functions with a prototype like: + +@smallexample +int @var{function} (const struct printf_info *info, + size_t n, int *argtypes) +@end smallexample + +The return value from the function should be the number of arguments the +conversion expects. The function should also fill in no more than +@var{n} elements of the @var{argtypes} array with information about the +types of each of these arguments. This information is encoded using the +various @samp{PA_} macros. (You will notice that this is the same +calling convention @code{parse_printf_format} itself uses.) + +@comment printf.h +@comment GNU +@deftp {Data Type} printf_arginfo_function +This type is used to describe functions that return information about +the number and type of arguments used by a conversion specifier. +@end deftp + +@node Printf Extension Example +@subsection @code{printf} Extension Example + +Here is an example showing how to define a @code{printf} handler function. +This program defines a data structure called a @code{Widget} and +defines the @samp{%W} conversion to print information about @w{@code{Widget *}} +arguments, including the pointer value and the name stored in the data +structure. The @samp{%W} conversion supports the minimum field width and +left-justification options, but ignores everything else. + +@smallexample +@include rprintf.c.texi +@end smallexample + +The output produced by this program looks like: + +@smallexample +|<Widget 0xffeffb7c: mywidget>| +| <Widget 0xffeffb7c: mywidget>| +|<Widget 0xffeffb7c: mywidget> | +@end smallexample + +@node Predefined Printf Handlers +@subsection Predefined @code{printf} Handlers + +@Theglibc{} also contains a concrete and useful application of the +@code{printf} handler extension. There are two functions available +which implement a special way to print floating-point numbers. + +@comment printf.h +@comment GNU +@deftypefun int printf_size (FILE *@var{fp}, const struct printf_info *@var{info}, const void *const *@var{args}) +@safety{@prelim{}@mtsafe{@mtsrace{:fp} @mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @acucorrupt{}}} +@c This is meant to be called by vfprintf, that should hold the lock on +@c the stream, but if this function is called directly, output will be +@c racy, besides the uses of the global locale object while other +@c threads may be changing it and the possbility of leaving the stream +@c object in an inconsistent state in case of cancellation. +Print a given floating point number as for the format @code{%f} except +that there is a postfix character indicating the divisor for the +number to make this less than 1000. There are two possible divisors: +powers of 1024 or powers of 1000. Which one is used depends on the +format character specified while registered this handler. If the +character is of lower case, 1024 is used. For upper case characters, +1000 is used. + +The postfix tag corresponds to bytes, kilobytes, megabytes, gigabytes, +etc. The full table is: + +@ifinfo +@multitable {' '} {2^10 (1024)} {zetta} {Upper} {10^24 (1000)} +@item low @tab Multiplier @tab From @tab Upper @tab Multiplier +@item ' ' @tab 1 @tab @tab ' ' @tab 1 +@item k @tab 2^10 (1024) @tab kilo @tab K @tab 10^3 (1000) +@item m @tab 2^20 @tab mega @tab M @tab 10^6 +@item g @tab 2^30 @tab giga @tab G @tab 10^9 +@item t @tab 2^40 @tab tera @tab T @tab 10^12 +@item p @tab 2^50 @tab peta @tab P @tab 10^15 +@item e @tab 2^60 @tab exa @tab E @tab 10^18 +@item z @tab 2^70 @tab zetta @tab Z @tab 10^21 +@item y @tab 2^80 @tab yotta @tab Y @tab 10^24 +@end multitable +@end ifinfo +@iftex +@tex +\hbox to\hsize{\hfil\vbox{\offinterlineskip +\hrule +\halign{\strut#& \vrule#\tabskip=1em plus2em& {\tt#}\hfil& \vrule#& #\hfil& \vrule#& #\hfil& \vrule#& {\tt#}\hfil& \vrule#& #\hfil& \vrule#\tabskip=0pt\cr +\noalign{\hrule} +\omit&height2pt&\omit&&\omit&&\omit&&\omit&&\omit&\cr +&& \omit low && Multiplier && From && \omit Upper && Multiplier &\cr +\omit&height2pt&\omit&&\omit&&\omit&&\omit&&\omit&\cr +\noalign{\hrule} +&& {\tt\char32} && 1 && && {\tt\char32} && 1 &\cr +&& k && $2^{10} = 1024$ && kilo && K && $10^3 = 1000$ &\cr +&& m && $2^{20}$ && mega && M && $10^6$ &\cr +&& g && $2^{30}$ && giga && G && $10^9$ &\cr +&& t && $2^{40}$ && tera && T && $10^{12}$ &\cr +&& p && $2^{50}$ && peta && P && $10^{15}$ &\cr +&& e && $2^{60}$ && exa && E && $10^{18}$ &\cr +&& z && $2^{70}$ && zetta && Z && $10^{21}$ &\cr +&& y && $2^{80}$ && yotta && Y && $10^{24}$ &\cr +\noalign{\hrule}}}\hfil} +@end tex +@end iftex + +The default precision is 3, i.e., 1024 is printed with a lower-case +format character as if it were @code{%.3fk} and will yield @code{1.000k}. +@end deftypefun + +Due to the requirements of @code{register_printf_function} we must also +provide the function which returns information about the arguments. + +@comment printf.h +@comment GNU +@deftypefun int printf_size_info (const struct printf_info *@var{info}, size_t @var{n}, int *@var{argtypes}) +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} +This function will return in @var{argtypes} the information about the +used parameters in the way the @code{vfprintf} implementation expects +it. The format always takes one argument. +@end deftypefun + +To use these functions both functions must be registered with a call like + +@smallexample +register_printf_function ('B', printf_size, printf_size_info); +@end smallexample + +Here we register the functions to print numbers as powers of 1000 since +the format character @code{'B'} is an upper-case character. If we +would additionally use @code{'b'} in a line like + +@smallexample +register_printf_function ('b', printf_size, printf_size_info); +@end smallexample + +@noindent +we could also print using a power of 1024. Please note that all that is +different in these two lines is the format specifier. The +@code{printf_size} function knows about the difference between lower and upper +case format specifiers. + +The use of @code{'B'} and @code{'b'} is no coincidence. Rather it is +the preferred way to use this functionality since it is available on +some other systems which also use format specifiers. + +@node Formatted Input +@section Formatted Input + +@cindex formatted input from a stream +@cindex reading from a stream, formatted +@cindex format string, for @code{scanf} +@cindex template, for @code{scanf} +The functions described in this section (@code{scanf} and related +functions) provide facilities for formatted input analogous to the +formatted output facilities. These functions provide a mechanism for +reading arbitrary values under the control of a @dfn{format string} or +@dfn{template string}. + +@menu +* Formatted Input Basics:: Some basics to get you started. +* Input Conversion Syntax:: Syntax of conversion specifications. +* Table of Input Conversions:: Summary of input conversions and what they do. +* Numeric Input Conversions:: Details of conversions for reading numbers. +* String Input Conversions:: Details of conversions for reading strings. +* Dynamic String Input:: String conversions that @code{malloc} the buffer. +* Other Input Conversions:: Details of miscellaneous other conversions. +* Formatted Input Functions:: Descriptions of the actual functions. +* Variable Arguments Input:: @code{vscanf} and friends. +@end menu + +@node Formatted Input Basics +@subsection Formatted Input Basics + +Calls to @code{scanf} are superficially similar to calls to +@code{printf} in that arbitrary arguments are read under the control of +a template string. While the syntax of the conversion specifications in +the template is very similar to that for @code{printf}, the +interpretation of the template is oriented more towards free-format +input and simple pattern matching, rather than fixed-field formatting. +For example, most @code{scanf} conversions skip over any amount of +``white space'' (including spaces, tabs, and newlines) in the input +file, and there is no concept of precision for the numeric input +conversions as there is for the corresponding output conversions. +Ordinarily, non-whitespace characters in the template are expected to +match characters in the input stream exactly, but a matching failure is +distinct from an input error on the stream. +@cindex conversion specifications (@code{scanf}) + +Another area of difference between @code{scanf} and @code{printf} is +that you must remember to supply pointers rather than immediate values +as the optional arguments to @code{scanf}; the values that are read are +stored in the objects that the pointers point to. Even experienced +programmers tend to forget this occasionally, so if your program is +getting strange errors that seem to be related to @code{scanf}, you +might want to double-check this. + +When a @dfn{matching failure} occurs, @code{scanf} returns immediately, +leaving the first non-matching character as the next character to be +read from the stream. The normal return value from @code{scanf} is the +number of values that were assigned, so you can use this to determine if +a matching error happened before all the expected values were read. +@cindex matching failure, in @code{scanf} + +The @code{scanf} function is typically used for things like reading in +the contents of tables. For example, here is a function that uses +@code{scanf} to initialize an array of @code{double}: + +@smallexample +void +readarray (double *array, int n) +@{ + int i; + for (i=0; i<n; i++) + if (scanf (" %lf", &(array[i])) != 1) + invalid_input_error (); +@} +@end smallexample + +The formatted input functions are not used as frequently as the +formatted output functions. Partly, this is because it takes some care +to use them properly. Another reason is that it is difficult to recover +from a matching error. + +If you are trying to read input that doesn't match a single, fixed +pattern, you may be better off using a tool such as Flex to generate a +lexical scanner, or Bison to generate a parser, rather than using +@code{scanf}. For more information about these tools, see @ref{Top, , , +flex.info, Flex: The Lexical Scanner Generator}, and @ref{Top, , , +bison.info, The Bison Reference Manual}. + +@node Input Conversion Syntax +@subsection Input Conversion Syntax + +A @code{scanf} template string is a string that contains ordinary +multibyte characters interspersed with conversion specifications that +start with @samp{%}. + +Any whitespace character (as defined by the @code{isspace} function; +@pxref{Classification of Characters}) in the template causes any number +of whitespace characters in the input stream to be read and discarded. +The whitespace characters that are matched need not be exactly the same +whitespace characters that appear in the template string. For example, +write @samp{ , } in the template to recognize a comma with optional +whitespace before and after. + +Other characters in the template string that are not part of conversion +specifications must match characters in the input stream exactly; if +this is not the case, a matching failure occurs. + +The conversion specifications in a @code{scanf} template string +have the general form: + +@smallexample +% @var{flags} @var{width} @var{type} @var{conversion} +@end smallexample + +In more detail, an input conversion specification consists of an initial +@samp{%} character followed in sequence by: + +@itemize @bullet +@item +An optional @dfn{flag character} @samp{*}, which says to ignore the text +read for this specification. When @code{scanf} finds a conversion +specification that uses this flag, it reads input as directed by the +rest of the conversion specification, but it discards this input, does +not use a pointer argument, and does not increment the count of +successful assignments. +@cindex flag character (@code{scanf}) + +@item +An optional flag character @samp{a} (valid with string conversions only) +which requests allocation of a buffer long enough to store the string in. +(This is a GNU extension.) +@xref{Dynamic String Input}. + +@item +An optional decimal integer that specifies the @dfn{maximum field +width}. Reading of characters from the input stream stops either when +this maximum is reached or when a non-matching character is found, +whichever happens first. Most conversions discard initial whitespace +characters (those that don't are explicitly documented), and these +discarded characters don't count towards the maximum field width. +String input conversions store a null character to mark the end of the +input; the maximum field width does not include this terminator. +@cindex maximum field width (@code{scanf}) + +@item +An optional @dfn{type modifier character}. For example, you can +specify a type modifier of @samp{l} with integer conversions such as +@samp{%d} to specify that the argument is a pointer to a @code{long int} +rather than a pointer to an @code{int}. +@cindex type modifier character (@code{scanf}) + +@item +A character that specifies the conversion to be applied. +@end itemize + +The exact options that are permitted and how they are interpreted vary +between the different conversion specifiers. See the descriptions of the +individual conversions for information about the particular options that +they allow. + +With the @samp{-Wformat} option, the GNU C compiler checks calls to +@code{scanf} and related functions. It examines the format string and +verifies that the correct number and types of arguments are supplied. +There is also a GNU C syntax to tell the compiler that a function you +write uses a @code{scanf}-style format string. +@xref{Function Attributes, , Declaring Attributes of Functions, +gcc.info, Using GNU CC}, for more information. + +@node Table of Input Conversions +@subsection Table of Input Conversions +@cindex input conversions, for @code{scanf} + +Here is a table that summarizes the various conversion specifications: + +@table @asis +@item @samp{%d} +Matches an optionally signed integer written in decimal. @xref{Numeric +Input Conversions}. + +@item @samp{%i} +Matches an optionally signed integer in any of the formats that the C +language defines for specifying an integer constant. @xref{Numeric +Input Conversions}. + +@item @samp{%o} +Matches an unsigned integer written in octal radix. +@xref{Numeric Input Conversions}. + +@item @samp{%u} +Matches an unsigned integer written in decimal radix. +@xref{Numeric Input Conversions}. + +@item @samp{%x}, @samp{%X} +Matches an unsigned integer written in hexadecimal radix. +@xref{Numeric Input Conversions}. + +@item @samp{%e}, @samp{%f}, @samp{%g}, @samp{%E}, @samp{%G} +Matches an optionally signed floating-point number. @xref{Numeric Input +Conversions}. + +@item @samp{%s} + +Matches a string containing only non-whitespace characters. +@xref{String Input Conversions}. The presence of the @samp{l} modifier +determines whether the output is stored as a wide character string or a +multibyte string. If @samp{%s} is used in a wide character function the +string is converted as with multiple calls to @code{wcrtomb} into a +multibyte string. This means that the buffer must provide room for +@code{MB_CUR_MAX} bytes for each wide character read. In case +@samp{%ls} is used in a multibyte function the result is converted into +wide characters as with multiple calls of @code{mbrtowc} before being +stored in the user provided buffer. + +@item @samp{%S} +This is an alias for @samp{%ls} which is supported for compatibility +with the Unix standard. + +@item @samp{%[} +Matches a string of characters that belong to a specified set. +@xref{String Input Conversions}. The presence of the @samp{l} modifier +determines whether the output is stored as a wide character string or a +multibyte string. If @samp{%[} is used in a wide character function the +string is converted as with multiple calls to @code{wcrtomb} into a +multibyte string. This means that the buffer must provide room for +@code{MB_CUR_MAX} bytes for each wide character read. In case +@samp{%l[} is used in a multibyte function the result is converted into +wide characters as with multiple calls of @code{mbrtowc} before being +stored in the user provided buffer. + +@item @samp{%c} +Matches a string of one or more characters; the number of characters +read is controlled by the maximum field width given for the conversion. +@xref{String Input Conversions}. + +If @samp{%c} is used in a wide stream function the read value is +converted from a wide character to the corresponding multibyte character +before storing it. Note that this conversion can produce more than one +byte of output and therefore the provided buffer must be large enough for up +to @code{MB_CUR_MAX} bytes for each character. If @samp{%lc} is used in +a multibyte function the input is treated as a multibyte sequence (and +not bytes) and the result is converted as with calls to @code{mbrtowc}. + +@item @samp{%C} +This is an alias for @samp{%lc} which is supported for compatibility +with the Unix standard. + +@item @samp{%p} +Matches a pointer value in the same implementation-defined format used +by the @samp{%p} output conversion for @code{printf}. @xref{Other Input +Conversions}. + +@item @samp{%n} +This conversion doesn't read any characters; it records the number of +characters read so far by this call. @xref{Other Input Conversions}. + +@item @samp{%%} +This matches a literal @samp{%} character in the input stream. No +corresponding argument is used. @xref{Other Input Conversions}. +@end table + +If the syntax of a conversion specification is invalid, the behavior is +undefined. If there aren't enough function arguments provided to supply +addresses for all the conversion specifications in the template strings +that perform assignments, or if the arguments are not of the correct +types, the behavior is also undefined. On the other hand, extra +arguments are simply ignored. + +@node Numeric Input Conversions +@subsection Numeric Input Conversions + +This section describes the @code{scanf} conversions for reading numeric +values. + +The @samp{%d} conversion matches an optionally signed integer in decimal +radix. The syntax that is recognized is the same as that for the +@code{strtol} function (@pxref{Parsing of Integers}) with the value +@code{10} for the @var{base} argument. + +The @samp{%i} conversion matches an optionally signed integer in any of +the formats that the C language defines for specifying an integer +constant. The syntax that is recognized is the same as that for the +@code{strtol} function (@pxref{Parsing of Integers}) with the value +@code{0} for the @var{base} argument. (You can print integers in this +syntax with @code{printf} by using the @samp{#} flag character with the +@samp{%x}, @samp{%o}, or @samp{%d} conversion. @xref{Integer Conversions}.) + +For example, any of the strings @samp{10}, @samp{0xa}, or @samp{012} +could be read in as integers under the @samp{%i} conversion. Each of +these specifies a number with decimal value @code{10}. + +The @samp{%o}, @samp{%u}, and @samp{%x} conversions match unsigned +integers in octal, decimal, and hexadecimal radices, respectively. The +syntax that is recognized is the same as that for the @code{strtoul} +function (@pxref{Parsing of Integers}) with the appropriate value +(@code{8}, @code{10}, or @code{16}) for the @var{base} argument. + +The @samp{%X} conversion is identical to the @samp{%x} conversion. They +both permit either uppercase or lowercase letters to be used as digits. + +The default type of the corresponding argument for the @code{%d} and +@code{%i} conversions is @code{int *}, and @code{unsigned int *} for the +other integer conversions. You can use the following type modifiers to +specify other sizes of integer: + +@table @samp +@item hh +Specifies that the argument is a @code{signed char *} or @code{unsigned +char *}. + +This modifier was introduced in @w{ISO C99}. + +@item h +Specifies that the argument is a @code{short int *} or @code{unsigned +short int *}. + +@item j +Specifies that the argument is a @code{intmax_t *} or @code{uintmax_t *}. + +This modifier was introduced in @w{ISO C99}. + +@item l +Specifies that the argument is a @code{long int *} or @code{unsigned +long int *}. Two @samp{l} characters is like the @samp{L} modifier, below. + +If used with @samp{%c} or @samp{%s} the corresponding parameter is +considered as a pointer to a wide character or wide character string +respectively. This use of @samp{l} was introduced in @w{Amendment 1} to +@w{ISO C90}. + +@need 100 +@item ll +@itemx L +@itemx q +Specifies that the argument is a @code{long long int *} or @code{unsigned long long int *}. (The @code{long long} type is an extension supported by the +GNU C compiler. For systems that don't provide extra-long integers, this +is the same as @code{long int}.) + +The @samp{q} modifier is another name for the same thing, which comes +from 4.4 BSD; a @w{@code{long long int}} is sometimes called a ``quad'' +@code{int}. + +@item t +Specifies that the argument is a @code{ptrdiff_t *}. + +This modifier was introduced in @w{ISO C99}. + +@item z +Specifies that the argument is a @code{size_t *}. + +This modifier was introduced in @w{ISO C99}. +@end table + +All of the @samp{%e}, @samp{%f}, @samp{%g}, @samp{%E}, and @samp{%G} +input conversions are interchangeable. They all match an optionally +signed floating point number, in the same syntax as for the +@code{strtod} function (@pxref{Parsing of Floats}). + +For the floating-point input conversions, the default argument type is +@code{float *}. (This is different from the corresponding output +conversions, where the default type is @code{double}; remember that +@code{float} arguments to @code{printf} are converted to @code{double} +by the default argument promotions, but @code{float *} arguments are +not promoted to @code{double *}.) You can specify other sizes of float +using these type modifiers: + +@table @samp +@item l +Specifies that the argument is of type @code{double *}. + +@item L +Specifies that the argument is of type @code{long double *}. +@end table + +For all the above number parsing formats there is an additional optional +flag @samp{'}. When this flag is given the @code{scanf} function +expects the number represented in the input string to be formatted +according to the grouping rules of the currently selected locale +(@pxref{General Numeric}). + +If the @code{"C"} or @code{"POSIX"} locale is selected there is no +difference. But for a locale which specifies values for the appropriate +fields in the locale the input must have the correct form in the input. +Otherwise the longest prefix with a correct form is processed. + +@node String Input Conversions +@subsection String Input Conversions + +This section describes the @code{scanf} input conversions for reading +string and character values: @samp{%s}, @samp{%S}, @samp{%[}, @samp{%c}, +and @samp{%C}. + +You have two options for how to receive the input from these +conversions: + +@itemize @bullet +@item +Provide a buffer to store it in. This is the default. You should +provide an argument of type @code{char *} or @code{wchar_t *} (the +latter if the @samp{l} modifier is present). + +@strong{Warning:} To make a robust program, you must make sure that the +input (plus its terminating null) cannot possibly exceed the size of the +buffer you provide. In general, the only way to do this is to specify a +maximum field width one less than the buffer size. @strong{If you +provide the buffer, always specify a maximum field width to prevent +overflow.} + +@item +Ask @code{scanf} to allocate a big enough buffer, by specifying the +@samp{a} flag character. This is a GNU extension. You should provide +an argument of type @code{char **} for the buffer address to be stored +in. @xref{Dynamic String Input}. +@end itemize + +The @samp{%c} conversion is the simplest: it matches a fixed number of +characters, always. The maximum field width says how many characters to +read; if you don't specify the maximum, the default is 1. This +conversion doesn't append a null character to the end of the text it +reads. It also does not skip over initial whitespace characters. It +reads precisely the next @var{n} characters, and fails if it cannot get +that many. Since there is always a maximum field width with @samp{%c} +(whether specified, or 1 by default), you can always prevent overflow by +making the buffer long enough. +@comment Is character == byte here??? --drepper + +If the format is @samp{%lc} or @samp{%C} the function stores wide +characters which are converted using the conversion determined at the +time the stream was opened from the external byte stream. The number of +bytes read from the medium is limited by @code{MB_CUR_LEN * @var{n}} but +at most @var{n} wide characters get stored in the output string. + +The @samp{%s} conversion matches a string of non-whitespace characters. +It skips and discards initial whitespace, but stops when it encounters +more whitespace after having read something. It stores a null character +at the end of the text that it reads. + +For example, reading the input: + +@smallexample + hello, world +@end smallexample + +@noindent +with the conversion @samp{%10c} produces @code{" hello, wo"}, but +reading the same input with the conversion @samp{%10s} produces +@code{"hello,"}. + +@strong{Warning:} If you do not specify a field width for @samp{%s}, +then the number of characters read is limited only by where the next +whitespace character appears. This almost certainly means that invalid +input can make your program crash---which is a bug. + +The @samp{%ls} and @samp{%S} format are handled just like @samp{%s} +except that the external byte sequence is converted using the conversion +associated with the stream to wide characters with their own encoding. +A width or precision specified with the format do not directly determine +how many bytes are read from the stream since they measure wide +characters. But an upper limit can be computed by multiplying the value +of the width or precision by @code{MB_CUR_MAX}. + +To read in characters that belong to an arbitrary set of your choice, +use the @samp{%[} conversion. You specify the set between the @samp{[} +character and a following @samp{]} character, using the same syntax used +in regular expressions for explicit sets of characters. As special cases: + +@itemize @bullet +@item +A literal @samp{]} character can be specified as the first character +of the set. + +@item +An embedded @samp{-} character (that is, one that is not the first or +last character of the set) is used to specify a range of characters. + +@item +If a caret character @samp{^} immediately follows the initial @samp{[}, +then the set of allowed input characters is everything @emph{except} +the characters listed. +@end itemize + +The @samp{%[} conversion does not skip over initial whitespace +characters. + +Note that the @dfn{character class} syntax available in character sets +that appear inside regular expressions (such as @samp{[:alpha:]}) is +@emph{not} available in the @samp{%[} conversion. + +Here are some examples of @samp{%[} conversions and what they mean: + +@table @samp +@item %25[1234567890] +Matches a string of up to 25 digits. + +@item %25[][] +Matches a string of up to 25 square brackets. + +@item %25[^ \f\n\r\t\v] +Matches a string up to 25 characters long that doesn't contain any of +the standard whitespace characters. This is slightly different from +@samp{%s}, because if the input begins with a whitespace character, +@samp{%[} reports a matching failure while @samp{%s} simply discards the +initial whitespace. + +@item %25[a-z] +Matches up to 25 lowercase characters. +@end table + +As for @samp{%c} and @samp{%s} the @samp{%[} format is also modified to +produce wide characters if the @samp{l} modifier is present. All what +is said about @samp{%ls} above is true for @samp{%l[}. + +One more reminder: the @samp{%s} and @samp{%[} conversions are +@strong{dangerous} if you don't specify a maximum width or use the +@samp{a} flag, because input too long would overflow whatever buffer you +have provided for it. No matter how long your buffer is, a user could +supply input that is longer. A well-written program reports invalid +input with a comprehensible error message, not with a crash. + +@node Dynamic String Input +@subsection Dynamically Allocating String Conversions + +A GNU extension to formatted input lets you safely read a string with no +maximum size. Using this feature, you don't supply a buffer; instead, +@code{scanf} allocates a buffer big enough to hold the data and gives +you its address. To use this feature, write @samp{a} as a flag +character, as in @samp{%as} or @samp{%a[0-9a-z]}. + +The pointer argument you supply for where to store the input should have +type @code{char **}. The @code{scanf} function allocates a buffer and +stores its address in the word that the argument points to. You should +free the buffer with @code{free} when you no longer need it. + +Here is an example of using the @samp{a} flag with the @samp{%[@dots{}]} +conversion specification to read a ``variable assignment'' of the form +@samp{@var{variable} = @var{value}}. + +@smallexample +@{ + char *variable, *value; + + if (2 > scanf ("%a[a-zA-Z0-9] = %a[^\n]\n", + &variable, &value)) + @{ + invalid_input_error (); + return 0; + @} + + @dots{} +@} +@end smallexample + +@node Other Input Conversions +@subsection Other Input Conversions + +This section describes the miscellaneous input conversions. + +The @samp{%p} conversion is used to read a pointer value. It recognizes +the same syntax used by the @samp{%p} output conversion for +@code{printf} (@pxref{Other Output Conversions}); that is, a hexadecimal +number just as the @samp{%x} conversion accepts. The corresponding +argument should be of type @code{void **}; that is, the address of a +place to store a pointer. + +The resulting pointer value is not guaranteed to be valid if it was not +originally written during the same program execution that reads it in. + +The @samp{%n} conversion produces the number of characters read so far +by this call. The corresponding argument should be of type @code{int *}. +This conversion works in the same way as the @samp{%n} conversion for +@code{printf}; see @ref{Other Output Conversions}, for an example. + +The @samp{%n} conversion is the only mechanism for determining the +success of literal matches or conversions with suppressed assignments. +If the @samp{%n} follows the locus of a matching failure, then no value +is stored for it since @code{scanf} returns before processing the +@samp{%n}. If you store @code{-1} in that argument slot before calling +@code{scanf}, the presence of @code{-1} after @code{scanf} indicates an +error occurred before the @samp{%n} was reached. + +Finally, the @samp{%%} conversion matches a literal @samp{%} character +in the input stream, without using an argument. This conversion does +not permit any flags, field width, or type modifier to be specified. + +@node Formatted Input Functions +@subsection Formatted Input Functions + +Here are the descriptions of the functions for performing formatted +input. +Prototypes for these functions are in the header file @file{stdio.h}. +@pindex stdio.h + +@comment stdio.h +@comment ISO +@deftypefun int scanf (const char *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +The @code{scanf} function reads formatted input from the stream +@code{stdin} under the control of the template string @var{template}. +The optional arguments are pointers to the places which receive the +resulting values. + +The return value is normally the number of successful assignments. If +an end-of-file condition is detected before any matches are performed, +including matches against whitespace and literal characters in the +template, then @code{EOF} is returned. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun int wscanf (const wchar_t *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +The @code{wscanf} function reads formatted input from the stream +@code{stdin} under the control of the template string @var{template}. +The optional arguments are pointers to the places which receive the +resulting values. + +The return value is normally the number of successful assignments. If +an end-of-file condition is detected before any matches are performed, +including matches against whitespace and literal characters in the +template, then @code{WEOF} is returned. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int fscanf (FILE *@var{stream}, const char *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +This function is just like @code{scanf}, except that the input is read +from the stream @var{stream} instead of @code{stdin}. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun int fwscanf (FILE *@var{stream}, const wchar_t *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +This function is just like @code{wscanf}, except that the input is read +from the stream @var{stream} instead of @code{stdin}. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int sscanf (const char *@var{s}, const char *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} +This is like @code{scanf}, except that the characters are taken from the +null-terminated string @var{s} instead of from a stream. Reaching the +end of the string is treated as an end-of-file condition. + +The behavior of this function is undefined if copying takes place +between objects that overlap---for example, if @var{s} is also given +as an argument to receive a string read under control of the @samp{%s}, +@samp{%S}, or @samp{%[} conversion. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun int swscanf (const wchar_t *@var{ws}, const wchar_t *@var{template}, @dots{}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} +This is like @code{wscanf}, except that the characters are taken from the +null-terminated string @var{ws} instead of from a stream. Reaching the +end of the string is treated as an end-of-file condition. + +The behavior of this function is undefined if copying takes place +between objects that overlap---for example, if @var{ws} is also given as +an argument to receive a string read under control of the @samp{%s}, +@samp{%S}, or @samp{%[} conversion. +@end deftypefun + +@node Variable Arguments Input +@subsection Variable Arguments Input Functions + +The functions @code{vscanf} and friends are provided so that you can +define your own variadic @code{scanf}-like functions that make use of +the same internals as the built-in formatted output functions. +These functions are analogous to the @code{vprintf} series of output +functions. @xref{Variable Arguments Output}, for important +information on how to use them. + +@strong{Portability Note:} The functions listed in this section were +introduced in @w{ISO C99} and were before available as GNU extensions. + +@comment stdio.h +@comment ISO +@deftypefun int vscanf (const char *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +This function is similar to @code{scanf}, but instead of taking +a variable number of arguments directly, it takes an argument list +pointer @var{ap} of type @code{va_list} (@pxref{Variadic Functions}). +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun int vwscanf (const wchar_t *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +This function is similar to @code{wscanf}, but instead of taking +a variable number of arguments directly, it takes an argument list +pointer @var{ap} of type @code{va_list} (@pxref{Variadic Functions}). +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int vfscanf (FILE *@var{stream}, const char *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +This is the equivalent of @code{fscanf} with the variable argument list +specified directly as for @code{vscanf}. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun int vfwscanf (FILE *@var{stream}, const wchar_t *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}} +This is the equivalent of @code{fwscanf} with the variable argument list +specified directly as for @code{vwscanf}. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int vsscanf (const char *@var{s}, const char *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} +This is the equivalent of @code{sscanf} with the variable argument list +specified directly as for @code{vscanf}. +@end deftypefun + +@comment wchar.h +@comment ISO +@deftypefun int vswscanf (const wchar_t *@var{s}, const wchar_t *@var{template}, va_list @var{ap}) +@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} +This is the equivalent of @code{swscanf} with the variable argument list +specified directly as for @code{vwscanf}. +@end deftypefun + +In GNU C, there is a special construct you can use to let the compiler +know that a function uses a @code{scanf}-style format string. Then it +can check the number and types of arguments in each call to the +function, and warn you when they do not match the format string. +For details, see @ref{Function Attributes, , Declaring Attributes of Functions, +gcc.info, Using GNU CC}. + +@node EOF and Errors +@section End-Of-File and Errors + +@cindex end of file, on a stream +Many of the functions described in this chapter return the value of the +macro @code{EOF} to indicate unsuccessful completion of the operation. +Since @code{EOF} is used to report both end of file and random errors, +it's often better to use the @code{feof} function to check explicitly +for end of file and @code{ferror} to check for errors. These functions +check indicators that are part of the internal state of the stream +object, indicators set if the appropriate condition was detected by a +previous I/O operation on that stream. + +@comment stdio.h +@comment ISO +@deftypevr Macro int EOF +This macro is an integer value that is returned by a number of narrow +stream functions to indicate an end-of-file condition, or some other +error situation. With @theglibc{}, @code{EOF} is @code{-1}. In +other libraries, its value may be some other negative number. + +This symbol is declared in @file{stdio.h}. +@end deftypevr + +@comment wchar.h +@comment ISO +@deftypevr Macro int WEOF +This macro is an integer value that is returned by a number of wide +stream functions to indicate an end-of-file condition, or some other +error situation. With @theglibc{}, @code{WEOF} is @code{-1}. In +other libraries, its value may be some other negative number. + +This symbol is declared in @file{wchar.h}. +@end deftypevr + +@comment stdio.h +@comment ISO +@deftypefun int feof (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@assafe{}@acunsafe{@aculock{}}} +The @code{feof} function returns nonzero if and only if the end-of-file +indicator for the stream @var{stream} is set. + +This symbol is declared in @file{stdio.h}. +@end deftypefun + +@comment stdio.h +@comment GNU +@deftypefun int feof_unlocked (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} +@c There isn't much of a thread unsafety risk in reading a flag word and +@c testing a bit in it. +The @code{feof_unlocked} function is equivalent to the @code{feof} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. + +This symbol is declared in @file{stdio.h}. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int ferror (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@assafe{}@acunsafe{@aculock{}}} +The @code{ferror} function returns nonzero if and only if the error +indicator for the stream @var{stream} is set, indicating that an error +has occurred on a previous operation on the stream. + +This symbol is declared in @file{stdio.h}. +@end deftypefun + +@comment stdio.h +@comment GNU +@deftypefun int ferror_unlocked (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} +The @code{ferror_unlocked} function is equivalent to the @code{ferror} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. + +This symbol is declared in @file{stdio.h}. +@end deftypefun + +In addition to setting the error indicator associated with the stream, +the functions that operate on streams also set @code{errno} in the same +way as the corresponding low-level functions that operate on file +descriptors. For example, all of the functions that perform output to a +stream---such as @code{fputc}, @code{printf}, and @code{fflush}---are +implemented in terms of @code{write}, and all of the @code{errno} error +conditions defined for @code{write} are meaningful for these functions. +For more information about the descriptor-level I/O functions, see +@ref{Low-Level I/O}. + +@node Error Recovery +@section Recovering from errors + +You may explicitly clear the error and EOF flags with the @code{clearerr} +function. + +@comment stdio.h +@comment ISO +@deftypefun void clearerr (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@assafe{}@acunsafe{@aculock{}}} +This function clears the end-of-file and error indicators for the +stream @var{stream}. + +The file positioning functions (@pxref{File Positioning}) also clear the +end-of-file indicator for the stream. +@end deftypefun + +@comment stdio.h +@comment GNU +@deftypefun void clearerr_unlocked (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@assafe{}@acsafe{}} +The @code{clearerr_unlocked} function is equivalent to the @code{clearerr} +function except that it does not implicitly lock the stream. + +This function is a GNU extension. +@end deftypefun + +Note that it is @emph{not} correct to just clear the error flag and retry +a failed stream operation. After a failed write, any number of +characters since the last buffer flush may have been committed to the +file, while some buffered data may have been discarded. Merely retrying +can thus cause lost or repeated data. + +A failed read may leave the file pointer in an inappropriate position for +a second try. In both cases, you should seek to a known position before +retrying. + +Most errors that can happen are not recoverable --- a second try will +always fail again in the same way. So usually it is best to give up and +report the error to the user, rather than install complicated recovery +logic. + +One important exception is @code{EINTR} (@pxref{Interrupted Primitives}). +Many stream I/O implementations will treat it as an ordinary error, which +can be quite inconvenient. You can avoid this hassle by installing all +signals with the @code{SA_RESTART} flag. + +For similar reasons, setting nonblocking I/O on a stream's file +descriptor is not usually advisable. + +@node Binary Streams +@section Text and Binary Streams + +@gnusystems{} and other POSIX-compatible operating systems organize all +files as uniform sequences of characters. However, some other systems +make a distinction between files containing text and files containing +binary data, and the input and output facilities of @w{ISO C} provide for +this distinction. This section tells you how to write programs portable +to such systems. + +@cindex text stream +@cindex binary stream +When you open a stream, you can specify either a @dfn{text stream} or a +@dfn{binary stream}. You indicate that you want a binary stream by +specifying the @samp{b} modifier in the @var{opentype} argument to +@code{fopen}; see @ref{Opening Streams}. Without this +option, @code{fopen} opens the file as a text stream. + +Text and binary streams differ in several ways: + +@itemize @bullet +@item +The data read from a text stream is divided into @dfn{lines} which are +terminated by newline (@code{'\n'}) characters, while a binary stream is +simply a long series of characters. A text stream might on some systems +fail to handle lines more than 254 characters long (including the +terminating newline character). +@cindex lines (in a text file) + +@item +On some systems, text files can contain only printing characters, +horizontal tab characters, and newlines, and so text streams may not +support other characters. However, binary streams can handle any +character value. + +@item +Space characters that are written immediately preceding a newline +character in a text stream may disappear when the file is read in again. + +@item +More generally, there need not be a one-to-one mapping between +characters that are read from or written to a text stream, and the +characters in the actual file. +@end itemize + +Since a binary stream is always more capable and more predictable than a +text stream, you might wonder what purpose text streams serve. Why not +simply always use binary streams? The answer is that on these operating +systems, text and binary streams use different file formats, and the +only way to read or write ``an ordinary file of text'' that can work +with other text-oriented programs is through a text stream. + +In @theglibc{}, and on all POSIX systems, there is no difference +between text streams and binary streams. When you open a stream, you +get the same kind of stream regardless of whether you ask for binary. +This stream can handle any file content, and has none of the +restrictions that text streams sometimes have. + +@node File Positioning +@section File Positioning +@cindex file positioning on a stream +@cindex positioning a stream +@cindex seeking on a stream + +The @dfn{file position} of a stream describes where in the file the +stream is currently reading or writing. I/O on the stream advances the +file position through the file. On @gnusystems{}, the file position is +represented as an integer, which counts the number of bytes from the +beginning of the file. @xref{File Position}. + +During I/O to an ordinary disk file, you can change the file position +whenever you wish, so as to read or write any portion of the file. Some +other kinds of files may also permit this. Files which support changing +the file position are sometimes referred to as @dfn{random-access} +files. + +You can use the functions in this section to examine or modify the file +position indicator associated with a stream. The symbols listed below +are declared in the header file @file{stdio.h}. +@pindex stdio.h + +@comment stdio.h +@comment ISO +@deftypefun {long int} ftell (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function returns the current file position of the stream +@var{stream}. + +This function can fail if the stream doesn't support file positioning, +or if the file position can't be represented in a @code{long int}, and +possibly for other reasons as well. If a failure occurs, a value of +@code{-1} is returned. +@end deftypefun + +@comment stdio.h +@comment Unix98 +@deftypefun off_t ftello (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +The @code{ftello} function is similar to @code{ftell}, except that it +returns a value of type @code{off_t}. Systems which support this type +use it to describe all file positions, unlike the POSIX specification +which uses a long int. The two are not necessarily the same size. +Therefore, using ftell can lead to problems if the implementation is +written on top of a POSIX compliant low-level I/O implementation, and using +@code{ftello} is preferable whenever it is available. + +If this function fails it returns @code{(off_t) -1}. This can happen due +to missing support for file positioning or internal errors. Otherwise +the return value is the current file position. + +The function is an extension defined in the Unix Single Specification +version 2. + +When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a +32 bit system this function is in fact @code{ftello64}. I.e., the +LFS interface transparently replaces the old interface. +@end deftypefun + +@comment stdio.h +@comment Unix98 +@deftypefun off64_t ftello64 (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function is similar to @code{ftello} with the only difference that +the return value is of type @code{off64_t}. This also requires that the +stream @var{stream} was opened using either @code{fopen64}, +@code{freopen64}, or @code{tmpfile64} since otherwise the underlying +file operations to position the file pointer beyond the @twoexp{31} +bytes limit might fail. + +If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 +bits machine this function is available under the name @code{ftello} +and so transparently replaces the old interface. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int fseek (FILE *@var{stream}, long int @var{offset}, int @var{whence}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +The @code{fseek} function is used to change the file position of the +stream @var{stream}. The value of @var{whence} must be one of the +constants @code{SEEK_SET}, @code{SEEK_CUR}, or @code{SEEK_END}, to +indicate whether the @var{offset} is relative to the beginning of the +file, the current file position, or the end of the file, respectively. + +This function returns a value of zero if the operation was successful, +and a nonzero value to indicate failure. A successful call also clears +the end-of-file indicator of @var{stream} and discards any characters +that were ``pushed back'' by the use of @code{ungetc}. + +@code{fseek} either flushes any buffered output before setting the file +position or else remembers it so it will be written later in its proper +place in the file. +@end deftypefun + +@comment stdio.h +@comment Unix98 +@deftypefun int fseeko (FILE *@var{stream}, off_t @var{offset}, int @var{whence}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function is similar to @code{fseek} but it corrects a problem with +@code{fseek} in a system with POSIX types. Using a value of type +@code{long int} for the offset is not compatible with POSIX. +@code{fseeko} uses the correct type @code{off_t} for the @var{offset} +parameter. + +For this reason it is a good idea to prefer @code{ftello} whenever it is +available since its functionality is (if different at all) closer the +underlying definition. + +The functionality and return value are the same as for @code{fseek}. + +The function is an extension defined in the Unix Single Specification +version 2. + +When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a +32 bit system this function is in fact @code{fseeko64}. I.e., the +LFS interface transparently replaces the old interface. +@end deftypefun + +@comment stdio.h +@comment Unix98 +@deftypefun int fseeko64 (FILE *@var{stream}, off64_t @var{offset}, int @var{whence}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function is similar to @code{fseeko} with the only difference that +the @var{offset} parameter is of type @code{off64_t}. This also +requires that the stream @var{stream} was opened using either +@code{fopen64}, @code{freopen64}, or @code{tmpfile64} since otherwise +the underlying file operations to position the file pointer beyond the +@twoexp{31} bytes limit might fail. + +If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 +bits machine this function is available under the name @code{fseeko} +and so transparently replaces the old interface. +@end deftypefun + +@strong{Portability Note:} In non-POSIX systems, @code{ftell}, +@code{ftello}, @code{fseek} and @code{fseeko} might work reliably only +on binary streams. @xref{Binary Streams}. + +The following symbolic constants are defined for use as the @var{whence} +argument to @code{fseek}. They are also used with the @code{lseek} +function (@pxref{I/O Primitives}) and to specify offsets for file locks +(@pxref{Control Operations}). + +@comment stdio.h +@comment ISO +@deftypevr Macro int SEEK_SET +This is an integer constant which, when used as the @var{whence} +argument to the @code{fseek} or @code{fseeko} functions, specifies that +the offset provided is relative to the beginning of the file. +@end deftypevr + +@comment stdio.h +@comment ISO +@deftypevr Macro int SEEK_CUR +This is an integer constant which, when used as the @var{whence} +argument to the @code{fseek} or @code{fseeko} functions, specifies that +the offset provided is relative to the current file position. +@end deftypevr + +@comment stdio.h +@comment ISO +@deftypevr Macro int SEEK_END +This is an integer constant which, when used as the @var{whence} +argument to the @code{fseek} or @code{fseeko} functions, specifies that +the offset provided is relative to the end of the file. +@end deftypevr + +@comment stdio.h +@comment ISO +@deftypefun void rewind (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +The @code{rewind} function positions the stream @var{stream} at the +beginning of the file. It is equivalent to calling @code{fseek} or +@code{fseeko} on the @var{stream} with an @var{offset} argument of +@code{0L} and a @var{whence} argument of @code{SEEK_SET}, except that +the return value is discarded and the error indicator for the stream is +reset. +@end deftypefun + +These three aliases for the @samp{SEEK_@dots{}} constants exist for the +sake of compatibility with older BSD systems. They are defined in two +different header files: @file{fcntl.h} and @file{sys/file.h}. + +@vtable @code +@comment sys/file.h +@comment BSD +@item L_SET +An alias for @code{SEEK_SET}. + +@comment sys/file.h +@comment BSD +@item L_INCR +An alias for @code{SEEK_CUR}. + +@comment sys/file.h +@comment BSD +@item L_XTND +An alias for @code{SEEK_END}. +@end vtable + +@node Portable Positioning +@section Portable File-Position Functions + +On @gnusystems{}, the file position is truly a character count. You +can specify any character count value as an argument to @code{fseek} or +@code{fseeko} and get reliable results for any random access file. +However, some @w{ISO C} systems do not represent file positions in this +way. + +On some systems where text streams truly differ from binary streams, it +is impossible to represent the file position of a text stream as a count +of characters from the beginning of the file. For example, the file +position on some systems must encode both a record offset within the +file, and a character offset within the record. + +As a consequence, if you want your programs to be portable to these +systems, you must observe certain rules: + +@itemize @bullet +@item +The value returned from @code{ftell} on a text stream has no predictable +relationship to the number of characters you have read so far. The only +thing you can rely on is that you can use it subsequently as the +@var{offset} argument to @code{fseek} or @code{fseeko} to move back to +the same file position. + +@item +In a call to @code{fseek} or @code{fseeko} on a text stream, either the +@var{offset} must be zero, or @var{whence} must be @code{SEEK_SET} and +the @var{offset} must be the result of an earlier call to @code{ftell} +on the same stream. + +@item +The value of the file position indicator of a text stream is undefined +while there are characters that have been pushed back with @code{ungetc} +that haven't been read or discarded. @xref{Unreading}. +@end itemize + +But even if you observe these rules, you may still have trouble for long +files, because @code{ftell} and @code{fseek} use a @code{long int} value +to represent the file position. This type may not have room to encode +all the file positions in a large file. Using the @code{ftello} and +@code{fseeko} functions might help here since the @code{off_t} type is +expected to be able to hold all file position values but this still does +not help to handle additional information which must be associated with +a file position. + +So if you do want to support systems with peculiar encodings for the +file positions, it is better to use the functions @code{fgetpos} and +@code{fsetpos} instead. These functions represent the file position +using the data type @code{fpos_t}, whose internal representation varies +from system to system. + +These symbols are declared in the header file @file{stdio.h}. +@pindex stdio.h + +@comment stdio.h +@comment ISO +@deftp {Data Type} fpos_t +This is the type of an object that can encode information about the +file position of a stream, for use by the functions @code{fgetpos} and +@code{fsetpos}. + +In @theglibc{}, @code{fpos_t} is an opaque data structure that +contains internal data to represent file offset and conversion state +information. In other systems, it might have a different internal +representation. + +When compiling with @code{_FILE_OFFSET_BITS == 64} on a 32 bit machine +this type is in fact equivalent to @code{fpos64_t} since the LFS +interface transparently replaces the old interface. +@end deftp + +@comment stdio.h +@comment Unix98 +@deftp {Data Type} fpos64_t +This is the type of an object that can encode information about the +file position of a stream, for use by the functions @code{fgetpos64} and +@code{fsetpos64}. + +In @theglibc{}, @code{fpos64_t} is an opaque data structure that +contains internal data to represent file offset and conversion state +information. In other systems, it might have a different internal +representation. +@end deftp + +@comment stdio.h +@comment ISO +@deftypefun int fgetpos (FILE *@var{stream}, fpos_t *@var{position}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function stores the value of the file position indicator for the +stream @var{stream} in the @code{fpos_t} object pointed to by +@var{position}. If successful, @code{fgetpos} returns zero; otherwise +it returns a nonzero value and stores an implementation-defined positive +value in @code{errno}. + +When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a +32 bit system the function is in fact @code{fgetpos64}. I.e., the LFS +interface transparently replaces the old interface. +@end deftypefun + +@comment stdio.h +@comment Unix98 +@deftypefun int fgetpos64 (FILE *@var{stream}, fpos64_t *@var{position}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function is similar to @code{fgetpos} but the file position is +returned in a variable of type @code{fpos64_t} to which @var{position} +points. + +If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 +bits machine this function is available under the name @code{fgetpos} +and so transparently replaces the old interface. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypefun int fsetpos (FILE *@var{stream}, const fpos_t *@var{position}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function sets the file position indicator for the stream @var{stream} +to the position @var{position}, which must have been set by a previous +call to @code{fgetpos} on the same stream. If successful, @code{fsetpos} +clears the end-of-file indicator on the stream, discards any characters +that were ``pushed back'' by the use of @code{ungetc}, and returns a value +of zero. Otherwise, @code{fsetpos} returns a nonzero value and stores +an implementation-defined positive value in @code{errno}. + +When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a +32 bit system the function is in fact @code{fsetpos64}. I.e., the LFS +interface transparently replaces the old interface. +@end deftypefun + +@comment stdio.h +@comment Unix98 +@deftypefun int fsetpos64 (FILE *@var{stream}, const fpos64_t *@var{position}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function is similar to @code{fsetpos} but the file position used +for positioning is provided in a variable of type @code{fpos64_t} to +which @var{position} points. + +If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 +bits machine this function is available under the name @code{fsetpos} +and so transparently replaces the old interface. +@end deftypefun + +@node Stream Buffering +@section Stream Buffering + +@cindex buffering of streams +Characters that are written to a stream are normally accumulated and +transmitted asynchronously to the file in a block, instead of appearing +as soon as they are output by the application program. Similarly, +streams often retrieve input from the host environment in blocks rather +than on a character-by-character basis. This is called @dfn{buffering}. + +If you are writing programs that do interactive input and output using +streams, you need to understand how buffering works when you design the +user interface to your program. Otherwise, you might find that output +(such as progress or prompt messages) doesn't appear when you intended +it to, or displays some other unexpected behavior. + +This section deals only with controlling when characters are transmitted +between the stream and the file or device, and @emph{not} with how +things like echoing, flow control, and the like are handled on specific +classes of devices. For information on common control operations on +terminal devices, see @ref{Low-Level Terminal Interface}. + +You can bypass the stream buffering facilities altogether by using the +low-level input and output functions that operate on file descriptors +instead. @xref{Low-Level I/O}. + +@menu +* Buffering Concepts:: Terminology is defined here. +* Flushing Buffers:: How to ensure that output buffers are flushed. +* Controlling Buffering:: How to specify what kind of buffering to use. +@end menu + +@node Buffering Concepts +@subsection Buffering Concepts + +There are three different kinds of buffering strategies: + +@itemize @bullet +@item +Characters written to or read from an @dfn{unbuffered} stream are +transmitted individually to or from the file as soon as possible. +@cindex unbuffered stream + +@item +Characters written to a @dfn{line buffered} stream are transmitted to +the file in blocks when a newline character is encountered. +@cindex line buffered stream + +@item +Characters written to or read from a @dfn{fully buffered} stream are +transmitted to or from the file in blocks of arbitrary size. +@cindex fully buffered stream +@end itemize + +Newly opened streams are normally fully buffered, with one exception: a +stream connected to an interactive device such as a terminal is +initially line buffered. @xref{Controlling Buffering}, for information +on how to select a different kind of buffering. Usually the automatic +selection gives you the most convenient kind of buffering for the file +or device you open. + +The use of line buffering for interactive devices implies that output +messages ending in a newline will appear immediately---which is usually +what you want. Output that doesn't end in a newline might or might not +show up immediately, so if you want them to appear immediately, you +should flush buffered output explicitly with @code{fflush}, as described +in @ref{Flushing Buffers}. + +@node Flushing Buffers +@subsection Flushing Buffers + +@cindex flushing a stream +@dfn{Flushing} output on a buffered stream means transmitting all +accumulated characters to the file. There are many circumstances when +buffered output on a stream is flushed automatically: + +@itemize @bullet +@item +When you try to do output and the output buffer is full. + +@item +When the stream is closed. @xref{Closing Streams}. + +@item +When the program terminates by calling @code{exit}. +@xref{Normal Termination}. + +@item +When a newline is written, if the stream is line buffered. + +@item +Whenever an input operation on @emph{any} stream actually reads data +from its file. +@end itemize + +If you want to flush the buffered output at another time, call +@code{fflush}, which is declared in the header file @file{stdio.h}. +@pindex stdio.h + +@comment stdio.h +@comment ISO +@deftypefun int fflush (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function causes any buffered output on @var{stream} to be delivered +to the file. If @var{stream} is a null pointer, then +@code{fflush} causes buffered output on @emph{all} open output streams +to be flushed. + +This function returns @code{EOF} if a write error occurs, or zero +otherwise. +@end deftypefun + +@comment stdio.h +@comment POSIX +@deftypefun int fflush_unlocked (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{fflush_unlocked} function is equivalent to the @code{fflush} +function except that it does not implicitly lock the stream. +@end deftypefun + +The @code{fflush} function can be used to flush all streams currently +opened. While this is useful in some situations it does often more than +necessary since it might be done in situations when terminal input is +required and the program wants to be sure that all output is visible on +the terminal. But this means that only line buffered streams have to be +flushed. Solaris introduced a function especially for this. It was +always available in @theglibc{} in some form but never officially +exported. + +@comment stdio_ext.h +@comment GNU +@deftypefun void _flushlbf (void) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +The @code{_flushlbf} function flushes all line buffered streams +currently opened. + +This function is declared in the @file{stdio_ext.h} header. +@end deftypefun + +@strong{Compatibility Note:} Some brain-damaged operating systems have +been known to be so thoroughly fixated on line-oriented input and output +that flushing a line buffered stream causes a newline to be written! +Fortunately, this ``feature'' seems to be becoming less common. You do +not need to worry about this with @theglibc{}. + +In some situations it might be useful to not flush the output pending +for a stream but instead simply forget it. If transmission is costly +and the output is not needed anymore this is valid reasoning. In this +situation a non-standard function introduced in Solaris and available in +@theglibc{} can be used. + +@comment stdio_ext.h +@comment GNU +@deftypefun void __fpurge (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}} +The @code{__fpurge} function causes the buffer of the stream +@var{stream} to be emptied. If the stream is currently in read mode all +input in the buffer is lost. If the stream is in output mode the +buffered output is not written to the device (or whatever other +underlying storage) and the buffer is cleared. + +This function is declared in @file{stdio_ext.h}. +@end deftypefun + +@node Controlling Buffering +@subsection Controlling Which Kind of Buffering + +After opening a stream (but before any other operations have been +performed on it), you can explicitly specify what kind of buffering you +want it to have using the @code{setvbuf} function. +@cindex buffering, controlling + +The facilities listed in this section are declared in the header +file @file{stdio.h}. +@pindex stdio.h + +@comment stdio.h +@comment ISO +@deftypefun int setvbuf (FILE *@var{stream}, char *@var{buf}, int @var{mode}, size_t @var{size}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function is used to specify that the stream @var{stream} should +have the buffering mode @var{mode}, which can be either @code{_IOFBF} +(for full buffering), @code{_IOLBF} (for line buffering), or +@code{_IONBF} (for unbuffered input/output). + +If you specify a null pointer as the @var{buf} argument, then @code{setvbuf} +allocates a buffer itself using @code{malloc}. This buffer will be freed +when you close the stream. + +Otherwise, @var{buf} should be a character array that can hold at least +@var{size} characters. You should not free the space for this array as +long as the stream remains open and this array remains its buffer. You +should usually either allocate it statically, or @code{malloc} +(@pxref{Unconstrained Allocation}) the buffer. Using an automatic array +is not a good idea unless you close the file before exiting the block +that declares the array. + +While the array remains a stream buffer, the stream I/O functions will +use the buffer for their internal purposes. You shouldn't try to access +the values in the array directly while the stream is using it for +buffering. + +The @code{setvbuf} function returns zero on success, or a nonzero value +if the value of @var{mode} is not valid or if the request could not +be honored. +@end deftypefun + +@comment stdio.h +@comment ISO +@deftypevr Macro int _IOFBF +The value of this macro is an integer constant expression that can be +used as the @var{mode} argument to the @code{setvbuf} function to +specify that the stream should be fully buffered. +@end deftypevr + +@comment stdio.h +@comment ISO +@deftypevr Macro int _IOLBF +The value of this macro is an integer constant expression that can be +used as the @var{mode} argument to the @code{setvbuf} function to +specify that the stream should be line buffered. +@end deftypevr + +@comment stdio.h +@comment ISO +@deftypevr Macro int _IONBF +The value of this macro is an integer constant expression that can be +used as the @var{mode} argument to the @code{setvbuf} function to +specify that the stream should be unbuffered. +@end deftypevr + +@comment stdio.h +@comment ISO +@deftypevr Macro int BUFSIZ +The value of this macro is an integer constant expression that is good +to use for the @var{size} argument to @code{setvbuf}. This value is +guaranteed to be at least @code{256}. + +The value of @code{BUFSIZ} is chosen on each system so as to make stream +I/O efficient. So it is a good idea to use @code{BUFSIZ} as the size +for the buffer when you call @code{setvbuf}. + +Actually, you can get an even better value to use for the buffer size +by means of the @code{fstat} system call: it is found in the +@code{st_blksize} field of the file attributes. @xref{Attribute Meanings}. + +Sometimes people also use @code{BUFSIZ} as the allocation size of +buffers used for related purposes, such as strings used to receive a +line of input with @code{fgets} (@pxref{Character Input}). There is no +particular reason to use @code{BUFSIZ} for this instead of any other +integer, except that it might lead to doing I/O in chunks of an +efficient size. +@end deftypevr + +@comment stdio.h +@comment ISO +@deftypefun void setbuf (FILE *@var{stream}, char *@var{buf}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +If @var{buf} is a null pointer, the effect of this function is +equivalent to calling @code{setvbuf} with a @var{mode} argument of +@code{_IONBF}. Otherwise, it is equivalent to calling @code{setvbuf} +with @var{buf}, and a @var{mode} of @code{_IOFBF} and a @var{size} +argument of @code{BUFSIZ}. + +The @code{setbuf} function is provided for compatibility with old code; +use @code{setvbuf} in all new programs. +@end deftypefun + +@comment stdio.h +@comment BSD +@deftypefun void setbuffer (FILE *@var{stream}, char *@var{buf}, size_t @var{size}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +If @var{buf} is a null pointer, this function makes @var{stream} unbuffered. +Otherwise, it makes @var{stream} fully buffered using @var{buf} as the +buffer. The @var{size} argument specifies the length of @var{buf}. + +This function is provided for compatibility with old BSD code. Use +@code{setvbuf} instead. +@end deftypefun + +@comment stdio.h +@comment BSD +@deftypefun void setlinebuf (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@aculock{} @acucorrupt{}}} +This function makes @var{stream} be line buffered, and allocates the +buffer for you. + +This function is provided for compatibility with old BSD code. Use +@code{setvbuf} instead. +@end deftypefun + +It is possible to query whether a given stream is line buffered or not +using a non-standard function introduced in Solaris and available in +@theglibc{}. + +@comment stdio_ext.h +@comment GNU +@deftypefun int __flbf (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} +The @code{__flbf} function will return a nonzero value in case the +stream @var{stream} is line buffered. Otherwise the return value is +zero. + +This function is declared in the @file{stdio_ext.h} header. +@end deftypefun + +Two more extensions allow to determine the size of the buffer and how +much of it is used. These functions were also introduced in Solaris. + +@comment stdio_ext.h +@comment GNU +@deftypefun size_t __fbufsize (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acsafe{}} +The @code{__fbufsize} function return the size of the buffer in the +stream @var{stream}. This value can be used to optimize the use of the +stream. + +This function is declared in the @file{stdio_ext.h} header. +@end deftypefun + +@comment stdio_ext.h +@comment GNU +@deftypefun size_t __fpending (FILE *@var{stream}) +@safety{@prelim{}@mtsafe{@mtsrace{:stream}}@asunsafe{@asucorrupt{}}@acsafe{}} +The @code{__fpending} +function returns the number of bytes currently in the output buffer. +For wide-oriented streams the measuring unit is wide characters. This +function should not be used on buffers in read mode or opened read-only. + +This function is declared in the @file{stdio_ext.h} header. +@end deftypefun + +@node Other Kinds of Streams +@section Other Kinds of Streams + +@Theglibc{} provides ways for you to define additional kinds of +streams that do not necessarily correspond to an open file. + +One such type of stream takes input from or writes output to a string. +These kinds of streams are used internally to implement the +@code{sprintf} and @code{sscanf} functions. You can also create such a +stream explicitly, using the functions described in @ref{String Streams}. + +More generally, you can define streams that do input/output to arbitrary +objects using functions supplied by your program. This protocol is +discussed in @ref{Custom Streams}. + +@strong{Portability Note:} The facilities described in this section are +specific to GNU. Other systems or C implementations might or might not +provide equivalent functionality. + +@menu +* String Streams:: Streams that get data from or put data in + a string or memory buffer. +* Custom Streams:: Defining your own streams with an arbitrary + input data source and/or output data sink. +@end menu + +@node String Streams +@subsection String Streams + +@cindex stream, for I/O to a string +@cindex string stream +The @code{fmemopen} and @code{open_memstream} functions allow you to do +I/O to a string or memory buffer. These facilities are declared in +@file{stdio.h}. +@pindex stdio.h + +@comment stdio.h +@comment GNU +@deftypefun {FILE *} fmemopen (void *@var{buf}, size_t @var{size}, const char *@var{opentype}) +@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@acsmem{} @aculock{}}} +@c Unlike open_memstream, fmemopen does (indirectly) call _IO_link_in, +@c bringing with it additional potential for async trouble with +@c list_all_lock. +This function opens a stream that allows the access specified by the +@var{opentype} argument, that reads from or writes to the buffer specified +by the argument @var{buf}. This array must be at least @var{size} bytes long. + +If you specify a null pointer as the @var{buf} argument, @code{fmemopen} +dynamically allocates an array @var{size} bytes long (as with @code{malloc}; +@pxref{Unconstrained Allocation}). This is really only useful +if you are going to write things to the buffer and then read them back +in again, because you have no way of actually getting a pointer to the +buffer (for this, try @code{open_memstream}, below). The buffer is +freed when the stream is closed. + +The argument @var{opentype} is the same as in @code{fopen} +(@pxref{Opening Streams}). If the @var{opentype} specifies +append mode, then the initial file position is set to the first null +character in the buffer. Otherwise the initial file position is at the +beginning of the buffer. + +When a stream open for writing is flushed or closed, a null character +(zero byte) is written at the end of the buffer if it fits. You +should add an extra byte to the @var{size} argument to account for this. +Attempts to write more than @var{size} bytes to the buffer result +in an error. + +For a stream open for reading, null characters (zero bytes) in the +buffer do not count as ``end of file''. Read operations indicate end of +file only when the file position advances past @var{size} bytes. So, if +you want to read characters from a null-terminated string, you should +supply the length of the string as the @var{size} argument. +@end deftypefun + +Here is an example of using @code{fmemopen} to create a stream for +reading from a string: + +@smallexample +@include memopen.c.texi +@end smallexample + +This program produces the following output: + +@smallexample +Got f +Got o +Got o +Got b +Got a +Got r +@end smallexample + +@comment stdio.h +@comment GNU +@deftypefun {FILE *} open_memstream (char **@var{ptr}, size_t *@var{sizeloc}) +@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} +This function opens a stream for writing to a buffer. The buffer is +allocated dynamically and grown as necessary, using @code{malloc}. +After you've closed the stream, this buffer is your responsibility to +clean up using @code{free} or @code{realloc}. @xref{Unconstrained Allocation}. + +When the stream is closed with @code{fclose} or flushed with +@code{fflush}, the locations @var{ptr} and @var{sizeloc} are updated to +contain the pointer to the buffer and its size. The values thus stored +remain valid only as long as no further output on the stream takes +place. If you do more output, you must flush the stream again to store +new values before you use them again. + +A null character is written at the end of the buffer. This null character +is @emph{not} included in the size value stored at @var{sizeloc}. + +You can move the stream's file position with @code{fseek} or +@code{fseeko} (@pxref{File Positioning}). Moving the file position past +the end of the data already written fills the intervening space with +zeroes. +@end deftypefun + +Here is an example of using @code{open_memstream}: + +@smallexample +@include memstrm.c.texi +@end smallexample + +This program produces the following output: + +@smallexample +buf = `hello', size = 5 +buf = `hello, world', size = 12 +@end smallexample + +@node Custom Streams +@subsection Programming Your Own Custom Streams +@cindex custom streams +@cindex programming your own streams + +This section describes how you can make a stream that gets input from an +arbitrary data source or writes output to an arbitrary data sink +programmed by you. We call these @dfn{custom streams}. The functions +and types described here are all GNU extensions. + +@c !!! this does not talk at all about the higher-level hooks + +@menu +* Streams and Cookies:: The @dfn{cookie} records where to fetch or + store data that is read or written. +* Hook Functions:: How you should define the four @dfn{hook + functions} that a custom stream needs. +@end menu + +@node Streams and Cookies +@subsubsection Custom Streams and Cookies +@cindex cookie, for custom stream + +Inside every custom stream is a special object called the @dfn{cookie}. +This is an object supplied by you which records where to fetch or store +the data read or written. It is up to you to define a data type to use +for the cookie. The stream functions in the library never refer +directly to its contents, and they don't even know what the type is; +they record its address with type @code{void *}. + +To implement a custom stream, you must specify @emph{how} to fetch or +store the data in the specified place. You do this by defining +@dfn{hook functions} to read, write, change ``file position'', and close +the stream. All four of these functions will be passed the stream's +cookie so they can tell where to fetch or store the data. The library +functions don't know what's inside the cookie, but your functions will +know. + +When you create a custom stream, you must specify the cookie pointer, +and also the four hook functions stored in a structure of type +@code{cookie_io_functions_t}. + +These facilities are declared in @file{stdio.h}. +@pindex stdio.h + +@comment stdio.h +@comment GNU +@deftp {Data Type} {cookie_io_functions_t} +This is a structure type that holds the functions that define the +communications protocol between the stream and its cookie. It has +the following members: + +@table @code +@item cookie_read_function_t *read +This is the function that reads data from the cookie. If the value is a +null pointer instead of a function, then read operations on this stream +always return @code{EOF}. + +@item cookie_write_function_t *write +This is the function that writes data to the cookie. If the value is a +null pointer instead of a function, then data written to the stream is +discarded. + +@item cookie_seek_function_t *seek +This is the function that performs the equivalent of file positioning on +the cookie. If the value is a null pointer instead of a function, calls +to @code{fseek} or @code{fseeko} on this stream can only seek to +locations within the buffer; any attempt to seek outside the buffer will +return an @code{ESPIPE} error. + +@item cookie_close_function_t *close +This function performs any appropriate cleanup on the cookie when +closing the stream. If the value is a null pointer instead of a +function, nothing special is done to close the cookie when the stream is +closed. +@end table +@end deftp + +@comment stdio.h +@comment GNU +@deftypefun {FILE *} fopencookie (void *@var{cookie}, const char *@var{opentype}, cookie_io_functions_t @var{io-functions}) +@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@acsmem{} @aculock{}}} +This function actually creates the stream for communicating with the +@var{cookie} using the functions in the @var{io-functions} argument. +The @var{opentype} argument is interpreted as for @code{fopen}; +see @ref{Opening Streams}. (But note that the ``truncate on +open'' option is ignored.) The new stream is fully buffered. + +The @code{fopencookie} function returns the newly created stream, or a null +pointer in case of an error. +@end deftypefun + +@node Hook Functions +@subsubsection Custom Stream Hook Functions +@cindex hook functions (of custom streams) + +Here are more details on how you should define the four hook functions +that a custom stream needs. + +You should define the function to read data from the cookie as: + +@smallexample +ssize_t @var{reader} (void *@var{cookie}, char *@var{buffer}, size_t @var{size}) +@end smallexample + +This is very similar to the @code{read} function; see @ref{I/O +Primitives}. Your function should transfer up to @var{size} bytes into +the @var{buffer}, and return the number of bytes read, or zero to +indicate end-of-file. You can return a value of @code{-1} to indicate +an error. + +You should define the function to write data to the cookie as: + +@smallexample +ssize_t @var{writer} (void *@var{cookie}, const char *@var{buffer}, size_t @var{size}) +@end smallexample + +This is very similar to the @code{write} function; see @ref{I/O +Primitives}. Your function should transfer up to @var{size} bytes from +the buffer, and return the number of bytes written. You can return a +value of @code{0} to indicate an error. You must not return any +negative value. + +You should define the function to perform seek operations on the cookie +as: + +@smallexample +int @var{seeker} (void *@var{cookie}, off64_t *@var{position}, int @var{whence}) +@end smallexample + +For this function, the @var{position} and @var{whence} arguments are +interpreted as for @code{fgetpos}; see @ref{Portable Positioning}. + +After doing the seek operation, your function should store the resulting +file position relative to the beginning of the file in @var{position}. +Your function should return a value of @code{0} on success and @code{-1} +to indicate an error. + +You should define the function to do cleanup operations on the cookie +appropriate for closing the stream as: + +@smallexample +int @var{cleaner} (void *@var{cookie}) +@end smallexample + +Your function should return @code{-1} to indicate an error, and @code{0} +otherwise. + +@comment stdio.h +@comment GNU +@deftp {Data Type} cookie_read_function_t +This is the data type that the read function for a custom stream should have. +If you declare the function as shown above, this is the type it will have. +@end deftp + +@comment stdio.h +@comment GNU +@deftp {Data Type} cookie_write_function_t +The data type of the write function for a custom stream. +@end deftp + +@comment stdio.h +@comment GNU +@deftp {Data Type} cookie_seek_function_t +The data type of the seek function for a custom stream. +@end deftp + +@comment stdio.h +@comment GNU +@deftp {Data Type} cookie_close_function_t +The data type of the close function for a custom stream. +@end deftp + +@ignore +Roland says: + +@quotation +There is another set of functions one can give a stream, the +input-room and output-room functions. These functions must +understand stdio internals. To describe how to use these +functions, you also need to document lots of how stdio works +internally (which isn't relevant for other uses of stdio). +Perhaps I can write an interface spec from which you can write +good documentation. But it's pretty complex and deals with lots +of nitty-gritty details. I think it might be better to let this +wait until the rest of the manual is more done and polished. +@end quotation +@end ignore + +@c ??? This section could use an example. + + +@node Formatted Messages +@section Formatted Messages +@cindex formatted messages + +On systems which are based on System V messages of programs (especially +the system tools) are printed in a strict form using the @code{fmtmsg} +function. The uniformity sometimes helps the user to interpret messages +and the strictness tests of the @code{fmtmsg} function ensure that the +programmer follows some minimal requirements. + +@menu +* Printing Formatted Messages:: The @code{fmtmsg} function. +* Adding Severity Classes:: Add more severity classes. +* Example:: How to use @code{fmtmsg} and @code{addseverity}. +@end menu + + +@node Printing Formatted Messages +@subsection Printing Formatted Messages + +Messages can be printed to standard error and/or to the console. To +select the destination the programmer can use the following two values, +bitwise OR combined if wanted, for the @var{classification} parameter of +@code{fmtmsg}: + +@vtable @code +@item MM_PRINT +Display the message in standard error. +@item MM_CONSOLE +Display the message on the system console. +@end vtable + +The erroneous piece of the system can be signalled by exactly one of the +following values which also is bitwise ORed with the +@var{classification} parameter to @code{fmtmsg}: + +@vtable @code +@item MM_HARD +The source of the condition is some hardware. +@item MM_SOFT +The source of the condition is some software. +@item MM_FIRM +The source of the condition is some firmware. +@end vtable + +A third component of the @var{classification} parameter to @code{fmtmsg} +can describe the part of the system which detects the problem. This is +done by using exactly one of the following values: + +@vtable @code +@item MM_APPL +The erroneous condition is detected by the application. +@item MM_UTIL +The erroneous condition is detected by a utility. +@item MM_OPSYS +The erroneous condition is detected by the operating system. +@end vtable + +A last component of @var{classification} can signal the results of this +message. Exactly one of the following values can be used: + +@vtable @code +@item MM_RECOVER +It is a recoverable error. +@item MM_NRECOV +It is a non-recoverable error. +@end vtable + +@comment fmtmsg.h +@comment XPG +@deftypefun int fmtmsg (long int @var{classification}, const char *@var{label}, int @var{severity}, const char *@var{text}, const char *@var{action}, const char *@var{tag}) +@safety{@prelim{}@mtsafe{}@asunsafe{@asulock{}}@acsafe{}} +Display a message described by its parameters on the device(s) specified +in the @var{classification} parameter. The @var{label} parameter +identifies the source of the message. The string should consist of two +colon separated parts where the first part has not more than 10 and the +second part not more than 14 characters. The @var{text} parameter +describes the condition of the error, the @var{action} parameter possible +steps to recover from the error and the @var{tag} parameter is a +reference to the online documentation where more information can be +found. It should contain the @var{label} value and a unique +identification number. + +Each of the parameters can be a special value which means this value +is to be omitted. The symbolic names for these values are: + +@vtable @code +@item MM_NULLLBL +Ignore @var{label} parameter. +@item MM_NULLSEV +Ignore @var{severity} parameter. +@item MM_NULLMC +Ignore @var{classification} parameter. This implies that nothing is +actually printed. +@item MM_NULLTXT +Ignore @var{text} parameter. +@item MM_NULLACT +Ignore @var{action} parameter. +@item MM_NULLTAG +Ignore @var{tag} parameter. +@end vtable + +There is another way certain fields can be omitted from the output to +standard error. This is described below in the description of +environment variables influencing the behavior. + +The @var{severity} parameter can have one of the values in the following +table: +@cindex severity class + +@vtable @code +@item MM_NOSEV +Nothing is printed, this value is the same as @code{MM_NULLSEV}. +@item MM_HALT +This value is printed as @code{HALT}. +@item MM_ERROR +This value is printed as @code{ERROR}. +@item MM_WARNING +This value is printed as @code{WARNING}. +@item MM_INFO +This value is printed as @code{INFO}. +@end vtable + +The numeric value of these five macros are between @code{0} and +@code{4}. Using the environment variable @code{SEV_LEVEL} or using the +@code{addseverity} function one can add more severity levels with their +corresponding string to print. This is described below +(@pxref{Adding Severity Classes}). + +@noindent +If no parameter is ignored the output looks like this: + +@smallexample +@var{label}: @var{severity-string}: @var{text} +TO FIX: @var{action} @var{tag} +@end smallexample + +The colons, new line characters and the @code{TO FIX} string are +inserted if necessary, i.e., if the corresponding parameter is not +ignored. + +This function is specified in the X/Open Portability Guide. It is also +available on all systems derived from System V. + +The function returns the value @code{MM_OK} if no error occurred. If +only the printing to standard error failed, it returns @code{MM_NOMSG}. +If printing to the console fails, it returns @code{MM_NOCON}. If +nothing is printed @code{MM_NOTOK} is returned. Among situations where +all outputs fail this last value is also returned if a parameter value +is incorrect. +@end deftypefun + +There are two environment variables which influence the behavior of +@code{fmtmsg}. The first is @code{MSGVERB}. It is used to control the +output actually happening on standard error (@emph{not} the console +output). Each of the five fields can explicitly be enabled. To do +this the user has to put the @code{MSGVERB} variable with a format like +the following in the environment before calling the @code{fmtmsg} function +the first time: + +@smallexample +MSGVERB=@var{keyword}[:@var{keyword}[:@dots{}]] +@end smallexample + +Valid @var{keyword}s are @code{label}, @code{severity}, @code{text}, +@code{action}, and @code{tag}. If the environment variable is not given +or is the empty string, a not supported keyword is given or the value is +somehow else invalid, no part of the message is masked out. + +The second environment variable which influences the behavior of +@code{fmtmsg} is @code{SEV_LEVEL}. This variable and the change in the +behavior of @code{fmtmsg} is not specified in the X/Open Portability +Guide. It is available in System V systems, though. It can be used to +introduce new severity levels. By default, only the five severity levels +described above are available. Any other numeric value would make +@code{fmtmsg} print nothing. + +If the user puts @code{SEV_LEVEL} with a format like + +@smallexample +SEV_LEVEL=[@var{description}[:@var{description}[:@dots{}]]] +@end smallexample + +@noindent +in the environment of the process before the first call to +@code{fmtmsg}, where @var{description} has a value of the form + +@smallexample +@var{severity-keyword},@var{level},@var{printstring} +@end smallexample + +The @var{severity-keyword} part is not used by @code{fmtmsg} but it has +to be present. The @var{level} part is a string representation of a +number. The numeric value must be a number greater than 4. This value +must be used in the @var{severity} parameter of @code{fmtmsg} to select +this class. It is not possible to overwrite any of the predefined +classes. The @var{printstring} is the string printed when a message of +this class is processed by @code{fmtmsg} (see above, @code{fmtsmg} does +not print the numeric value but instead the string representation). + + +@node Adding Severity Classes +@subsection Adding Severity Classes +@cindex severity class + +There is another possibility to introduce severity classes besides using +the environment variable @code{SEV_LEVEL}. This simplifies the task of +introducing new classes in a running program. One could use the +@code{setenv} or @code{putenv} function to set the environment variable, +but this is toilsome. + +@deftypefun int addseverity (int @var{severity}, const char *@var{string}) +@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{} @asulock{}}@acunsafe{@aculock{} @acsmem{}}} +This function allows the introduction of new severity classes which can be +addressed by the @var{severity} parameter of the @code{fmtmsg} function. +The @var{severity} parameter of @code{addseverity} must match the value +for the parameter with the same name of @code{fmtmsg}, and @var{string} +is the string printed in the actual messages instead of the numeric +value. + +If @var{string} is @code{NULL} the severity class with the numeric value +according to @var{severity} is removed. + +It is not possible to overwrite or remove one of the default severity +classes. All calls to @code{addseverity} with @var{severity} set to one +of the values for the default classes will fail. + +The return value is @code{MM_OK} if the task was successfully performed. +If the return value is @code{MM_NOTOK} something went wrong. This could +mean that no more memory is available or a class is not available when +it has to be removed. + +This function is not specified in the X/Open Portability Guide although +the @code{fmtsmg} function is. It is available on System V systems. +@end deftypefun + + +@node Example +@subsection How to use @code{fmtmsg} and @code{addseverity} + +Here is a simple example program to illustrate the use of both +functions described in this section. + +@smallexample +@include fmtmsgexpl.c.texi +@end smallexample + +The second call to @code{fmtmsg} illustrates a use of this function as +it usually occurs on System V systems, which heavily use this function. +It seems worthwhile to give a short explanation here of how this system +works on System V. The value of the +@var{label} field (@code{UX:cat}) says that the error occurred in the +Unix program @code{cat}. The explanation of the error follows and the +value for the @var{action} parameter is @code{"refer to manual"}. One +could be more specific here, if necessary. The @var{tag} field contains, +as proposed above, the value of the string given for the @var{label} +parameter, and additionally a unique ID (@code{001} in this case). For +a GNU environment this string could contain a reference to the +corresponding node in the Info page for the program. + +@noindent +Running this program without specifying the @code{MSGVERB} and +@code{SEV_LEVEL} function produces the following output: + +@smallexample +UX:cat: NOTE2: invalid syntax +TO FIX: refer to manual UX:cat:001 +@end smallexample + +We see the different fields of the message and how the extra glue (the +colons and the @code{TO FIX} string) is printed. But only one of the +three calls to @code{fmtmsg} produced output. The first call does not +print anything because the @var{label} parameter is not in the correct +form. The string must contain two fields, separated by a colon +(@pxref{Printing Formatted Messages}). The third @code{fmtmsg} call +produced no output since the class with the numeric value @code{6} is +not defined. Although a class with numeric value @code{5} is also not +defined by default, the call to @code{addseverity} introduces it and +the second call to @code{fmtmsg} produces the above output. + +When we change the environment of the program to contain +@code{SEV_LEVEL=XXX,6,NOTE} when running it we get a different result: + +@smallexample +UX:cat: NOTE2: invalid syntax +TO FIX: refer to manual UX:cat:001 +label:foo: NOTE: text +TO FIX: action tag +@end smallexample + +Now the third call to @code{fmtmsg} produced some output and we see how +the string @code{NOTE} from the environment variable appears in the +message. + +Now we can reduce the output by specifying which fields we are +interested in. If we additionally set the environment variable +@code{MSGVERB} to the value @code{severity:label:action} we get the +following output: + +@smallexample +UX:cat: NOTE2 +TO FIX: refer to manual +label:foo: NOTE +TO FIX: action +@end smallexample + +@noindent +I.e., the output produced by the @var{text} and the @var{tag} parameters +to @code{fmtmsg} vanished. Please also note that now there is no colon +after the @code{NOTE} and @code{NOTE2} strings in the output. This is +not necessary since there is no more output on this line because the text +is missing. |