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authorJakub Jelinek <jakub@redhat.com>2007-07-12 18:26:36 +0000
committerJakub Jelinek <jakub@redhat.com>2007-07-12 18:26:36 +0000
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- Linuxthreads - POSIX 1003.1c kernel threads for Linux
-
- Copyright 1996, 1997 Xavier Leroy (Xavier.Leroy@inria.fr)
-
-
-DESCRIPTION:
-
-This is release 0.7 (late beta) of LinuxThreads, a BiCapitalized
-implementation of the Posix 1003.1c "pthread" interface for Linux.
-
-LinuxThreads provides kernel-level threads: each thread is a separate
-Unix process, sharing its address space with the other threads through
-the new system call clone(). Scheduling between threads is handled by
-the kernel scheduler, just like scheduling between Unix processes.
-
-
-REQUIREMENTS:
-
-- Linux version 2.0 and up (requires the new clone() system call
- and the new realtime scheduler).
-
-- For Intel platforms: libc 5.2.18 or later is required.
- 5.2.18 or 5.4.12 or later are recommended;
- 5.3.12 and 5.4.7 have problems (see the FAQ.html file for more info).
-
-- Also supports glibc 2 (a.k.a. libc 6), which actually comes with
- a specially-adapted version of this library.
-
-- Currently supports Intel, Alpha, Sparc, Motorola 68k, ARM and MIPS
- platforms.
-
-- Multiprocessors are supported.
-
-
-INSTALLATION:
-
-- Edit the Makefile, set the variables in the "Configuration" section.
-
-- Do "make".
-
-- Do "make install".
-
-
-USING LINUXTHREADS:
-
- gcc -D_REENTRANT ... -lpthread
-
-A complete set of manual pages is included. Also see the subdirectory
-Examples/ for some sample programs.
-
-
-STATUS:
-
-- All functions in the Posix 1003.1c base interface implemented.
- Also supports priority scheduling.
-
-- For users of libc 5 (H.J.Lu's libc), a number of C library functions
- are reimplemented or wrapped to make them thread-safe, including:
- * malloc functions
- * stdio functions (define _REENTRANT before including <stdio.h>)
- * per-thread errno variable (define _REENTRANT before including <errno.h>)
- * directory reading functions (opendir(), etc)
- * sleep()
- * gmtime(), localtime()
-
- New library functions provided:
- * flockfile(), funlockfile(), ftrylockfile()
- * reentrant versions of network database functions (gethostbyname_r(), etc)
- and password functions (getpwnam_r(), etc).
-
-- libc 6 (glibc 2) provides much better thread support than libc 5,
- and comes with a specially-adapted version of LinuxThreads.
- For serious multithreaded programming, you should consider switching
- to glibc 2. It is available from ftp.gnu.org:/pub/gnu and its mirrors.
-
-
-WARNING:
-
-Many existing libraries are not compatible with LinuxThreads,
-either because they are not inherently thread-safe, or because they
-have not been compiled with the -D_REENTRANT. For more info, see the
-FAQ.html file in this directory.
-
-A prime example of the latter is Xlib. If you link it with
-LinuxThreads, you'll probably get an "unknown 0 error" very
-early. This is just a consequence of the Xlib binaries using the
-global variable "errno" to fetch error codes, while LinuxThreads and
-the C library use the per-thread "errno" location.
-
-See the file README.Xfree3.3 for info on how to compile the Xfree 3.3
-libraries to make them compatible with LinuxThreads.
-
-
-KNOWN BUGS AND LIMITATIONS:
-
-- Threads share pretty much everything they should share according
- to the standard: memory space, file descriptors, signal handlers,
- current working directory, etc. One thing that they do not share
- is their pid's and parent pid's. According to the standard, they
- should have the same, but that's one thing we cannot achieve
- in this implementation (until the CLONE_PID flag to clone() becomes
- usable).
-
-- The current implementation uses the two signals SIGUSR1 and SIGUSR2,
- so user-level code cannot employ them. Ideally, there should be two
- signals reserved for this library. One signal is used for restarting
- threads blocked on mutexes or conditions; the other is for thread
- cancellation.
-
- *** This is not anymore true when the application runs on a kernel
- newer than approximately 2.1.60.
-
-- The stacks for the threads are allocated high in the memory space,
- below the stack of the initial process, and spaced 2M apart.
- Stacks are allocated with the "grow on demand" flag, so they don't
- use much virtual space initially (4k, currently), but can grow
- up to 2M if needed.
-
- Reserving such a large address space for each thread means that,
- on a 32-bit architecture, no more than about 1000 threads can
- coexist (assuming a 2Gb address space for user processes),
- but this is reasonable, since each thread uses up one entry in the
- kernel's process table, which is usually limited to 512 processes.
-
- Another potential problem of the "grow on demand" scheme is that
- nothing prevents the user from mmap'ing something in the 2M address
- window reserved for a thread stack, possibly causing later extensions of
- that stack to fail. Mapping at fixed addresses should be avoided
- when using this library.
-
-- Signal handling does not fully conform to the Posix standard,
- due to the fact that threads are here distinct processes that can be
- sent signals individually, so there's no notion of sending a signal
- to "the" process (the collection of all threads).
- More precisely, here is a summary of the standard requirements
- and how they are met by the implementation:
-
- 1- Synchronous signals (generated by the thread execution, e.g. SIGFPE)
- are delivered to the thread that raised them.
- (OK.)
-
- 2- A fatal asynchronous signal terminates all threads in the process.
- (OK. The thread manager notices when a thread dies on a signal
- and kills all other threads with the same signal.)
-
- 3- An asynchronous signal will be delivered to one of the threads
- of the program which does not block the signal (it is unspecified
- which).
- (No, the signal is delivered to the thread it's been sent to,
- based on the pid of the thread. If that thread is currently
- blocking the signal, the signal remains pending.)
-
- 4- The signal will be delivered to at most one thread.
- (OK, except for signals generated from the terminal or sent to
- the process group, which will be delivered to all threads.)
-
-- The current implementation of the MIPS support assumes a MIPS ISA II
- processor or better. These processors support atomic operations by
- ll/sc instructions. Older R2000/R3000 series processors are not
- supported yet; support for these will have higher overhead.
-
-- The current implementation of the ARM support assumes that the SWP
- (atomic swap register with memory) instruction is available. This is
- the case for all processors except for the ARM1 and ARM2. On StrongARM,
- the SWP instruction does not bypass the cache, so multi-processor support
- will be more troublesome.