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author | Roland McGrath <roland@gnu.org> | 1995-02-18 01:27:10 +0000 |
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committer | Roland McGrath <roland@gnu.org> | 1995-02-18 01:27:10 +0000 |
commit | 28f540f45bbacd939bfd07f213bcad2bf730b1bf (patch) | |
tree | 15f07c4c43d635959c6afee96bde71fb1b3614ee /manual/setjmp.texi | |
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initial import
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-rw-r--r-- | manual/setjmp.texi | 213 |
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diff --git a/manual/setjmp.texi b/manual/setjmp.texi new file mode 100644 index 0000000000..dfdac1c4cd --- /dev/null +++ b/manual/setjmp.texi @@ -0,0 +1,213 @@ +@node Non-Local Exits, Signal Handling, Date and Time, Top +@chapter Non-Local Exits +@cindex non-local exits +@cindex long jumps + +Sometimes when your program detects an unusual situation inside a deeply +nested set of function calls, you would like to be able to immediately +return to an outer level of control. This section describes how you can +do such @dfn{non-local exits} using the @code{setjmp} and @code{longjmp} +functions. + +@menu +* Intro: Non-Local Intro. When and how to use these facilities. +* Details: Non-Local Details. Functions for nonlocal exits. +* Non-Local Exits and Signals:: Portability issues. +@end menu + +@node Non-Local Intro, Non-Local Details, , Non-Local Exits +@section Introduction to Non-Local Exits + +As an example of a situation where a non-local exit can be useful, +suppose you have an interactive program that has a ``main loop'' that +prompts for and executes commands. Suppose the ``read'' command reads +input from a file, doing some lexical analysis and parsing of the input +while processing it. If a low-level input error is detected, it would +be useful to be able to return immediately to the ``main loop'' instead +of having to make each of the lexical analysis, parsing, and processing +phases all have to explicitly deal with error situations initially +detected by nested calls. + +(On the other hand, if each of these phases has to do a substantial +amount of cleanup when it exits---such as closing files, deallocating +buffers or other data structures, and the like---then it can be more +appropriate to do a normal return and have each phase do its own +cleanup, because a non-local exit would bypass the intervening phases and +their associated cleanup code entirely. Alternatively, you could use a +non-local exit but do the cleanup explicitly either before or after +returning to the ``main loop''.) + +In some ways, a non-local exit is similar to using the @samp{return} +statement to return from a function. But while @samp{return} abandons +only a single function call, transferring control back to the point at +which it was called, a non-local exit can potentially abandon many +levels of nested function calls. + +You identify return points for non-local exits calling the function +@code{setjmp}. This function saves information about the execution +environment in which the call to @code{setjmp} appears in an object of +type @code{jmp_buf}. Execution of the program continues normally after +the call to @code{setjmp}, but if a exit is later made to this return +point by calling @code{longjmp} with the corresponding @w{@code{jmp_buf}} +object, control is transferred back to the point where @code{setjmp} was +called. The return value from @code{setjmp} is used to distinguish +between an ordinary return and a return made by a call to +@code{longjmp}, so calls to @code{setjmp} usually appear in an @samp{if} +statement. + +Here is how the example program described above might be set up: + +@smallexample +@include setjmp.c.texi +@end smallexample + +The function @code{abort_to_main_loop} causes an immediate transfer of +control back to the main loop of the program, no matter where it is +called from. + +The flow of control inside the @code{main} function may appear a little +mysterious at first, but it is actually a common idiom with +@code{setjmp}. A normal call to @code{setjmp} returns zero, so the +``else'' clause of the conditional is executed. If +@code{abort_to_main_loop} is called somewhere within the execution of +@code{do_command}, then it actually appears as if the @emph{same} call +to @code{setjmp} in @code{main} were returning a second time with a value +of @code{-1}. + +@need 250 +So, the general pattern for using @code{setjmp} looks something like: + +@smallexample +if (setjmp (@var{buffer})) + /* @r{Code to clean up after premature return.} */ + @dots{} +else + /* @r{Code to be executed normally after setting up the return point.} */ + @dots{} +@end smallexample + +@node Non-Local Details, Non-Local Exits and Signals, Non-Local Intro, Non-Local Exits +@section Details of Non-Local Exits + +Here are the details on the functions and data structures used for +performing non-local exits. These facilities are declared in +@file{setjmp.h}. +@pindex setjmp.h + +@comment setjmp.h +@comment ANSI +@deftp {Data Type} jmp_buf +Objects of type @code{jmp_buf} hold the state information to +be restored by a non-local exit. The contents of a @code{jmp_buf} +identify a specific place to return to. +@end deftp + +@comment setjmp.h +@comment ANSI +@deftypefn Macro int setjmp (jmp_buf @var{state}) +When called normally, @code{setjmp} stores information about the +execution state of the program in @var{state} and returns zero. If +@code{longjmp} is later used to perform a non-local exit to this +@var{state}, @code{setjmp} returns a nonzero value. +@end deftypefn + +@comment setjmp.h +@comment ANSI +@deftypefun void longjmp (jmp_buf @var{state}, int @var{value}) +This function restores current execution to the state saved in +@var{state}, and continues execution from the call to @code{setjmp} that +established that return point. Returning from @code{setjmp} by means of +@code{longjmp} returns the @var{value} argument that was passed to +@code{longjmp}, rather than @code{0}. (But if @var{value} is given as +@code{0}, @code{setjmp} returns @code{1}).@refill +@end deftypefun + +There are a lot of obscure but important restrictions on the use of +@code{setjmp} and @code{longjmp}. Most of these restrictions are +present because non-local exits require a fair amount of magic on the +part of the C compiler and can interact with other parts of the language +in strange ways. + +The @code{setjmp} function is actually a macro without an actual +function definition, so you shouldn't try to @samp{#undef} it or take +its address. In addition, calls to @code{setjmp} are safe in only the +following contexts: + +@itemize @bullet +@item +As the test expression of a selection or iteration +statement (such as @samp{if}, @samp{switch}, or @samp{while}). + +@item +As one operand of a equality or comparison operator that appears as the +test expression of a selection or iteration statement. The other +operand must be an integer constant expression. + +@item +As the operand of a unary @samp{!} operator, that appears as the +test expression of a selection or iteration statement. + +@item +By itself as an expression statement. +@end itemize + +Return points are valid only during the dynamic extent of the function +that called @code{setjmp} to establish them. If you @code{longjmp} to +a return point that was established in a function that has already +returned, unpredictable and disastrous things are likely to happen. + +You should use a nonzero @var{value} argument to @code{longjmp}. While +@code{longjmp} refuses to pass back a zero argument as the return value +from @code{setjmp}, this is intended as a safety net against accidental +misuse and is not really good programming style. + +When you perform a non-local exit, accessible objects generally retain +whatever values they had at the time @code{longjmp} was called. The +exception is that the values of automatic variables local to the +function containing the @code{setjmp} call that have been changed since +the call to @code{setjmp} are indeterminate, unless you have declared +them @code{volatile}. + +@node Non-Local Exits and Signals,, Non-Local Details, Non-Local Exits +@section Non-Local Exits and Signals + +In BSD Unix systems, @code{setjmp} and @code{longjmp} also save and +restore the set of blocked signals; see @ref{Blocking Signals}. However, +the POSIX.1 standard requires @code{setjmp} and @code{longjmp} not to +change the set of blocked signals, and provides an additional pair of +functions (@code{sigsetjmp} and @code{sigsetjmp}) to get the BSD +behavior. + +The behavior of @code{setjmp} and @code{longjmp} in the GNU library is +controlled by feature test macros; see @ref{Feature Test Macros}. The +default in the GNU system is the POSIX.1 behavior rather than the BSD +behavior. + +The facilities in this section are declared in the header file +@file{setjmp.h}. +@pindex setjmp.h + +@comment setjmp.h +@comment POSIX.1 +@deftp {Data Type} sigjmp_buf +This is similar to @code{jmp_buf}, except that it can also store state +information about the set of blocked signals. +@end deftp + +@comment setjmp.h +@comment POSIX.1 +@deftypefun int sigsetjmp (sigjmp_buf @var{state}, int @var{savesigs}) +This is similar to @code{setjmp}. If @var{savesigs} is nonzero, the set +of blocked signals is saved in @var{state} and will be restored if a +@code{siglongjmp} is later performed with this @var{state}. +@end deftypefun + +@comment setjmp.h +@comment POSIX.1 +@deftypefun void siglongjmp (sigjmp_buf @var{state}, int @var{value}) +This is similar to @code{longjmp} except for the type of its @var{state} +argument. If the @code{sigsetjmp} call that set this @var{state} used a +nonzero @var{savesigs} flag, @code{siglongjmp} also restores the set of +blocked signals. +@end deftypefun + |