pthreads — POSIX threads
POSIX.1 specifies a set of interfaces (functions, header files) for threaded programming commonly known as POSIX threads, or Pthreads. A single process can contain multiple threads, all of which are executing the same program. These threads share the same global memory (data and heap segments), but each thread has its own stack (automatic variables).
POSIX.1 also requires that threads share a range of other attributes (i.e., these attributes are process-wide rather than per-thread):
process ID
parent process ID
process group ID and session ID
controlling terminal
user and group IDs
open file descriptors
record locks (see fcntl(2))
signal dispositions
file mode creation mask (umask(2))
interval timers (setitimer(2)) and POSIX timers (timer_create(3))
nice value (setpriority(2))
resource limits (setrlimit(2))
measurements of the consumption of CPU time (times(2)) and resources (getrusage(2))
As well as the stack, POSIX.1 specifies that various other attributes are distinct for each thread, including:
thread ID (the pthread_t data
type)
signal mask (pthread_sigmask(3))
the errno variable
alternate signal stack (sigaltstack(2))
real-time scheduling policy and priority (sched_setscheduler(2) and sched_setparam(2))
The following Linux-specific features are also per-thread:
capabilities (see capabilities(7))
CPU affinity (sched_setaffinity(2))
On Linux, programs that use the Pthreads API should be compiled using cc −pthread.
Over time, two threading implementations have been provided by the GNU C library on Linux:
This is the original Pthreads implementation. Since glibc 2.4, this implementation is no longer supported.
NPTL (Native POSIX Threads
Library)This is the modern Pthreads implementation. By comparison with LinuxThreads, NPTL provides closer conformance to the requirements of the POSIX.1 specification and better performance when creating large numbers of threads. NPTL is available since glibc 2.3.2, and requires features that are present in the Linux 2.6 kernel.
Both of these are so-called 1:1 implementations, meaning that each thread maps to a kernel scheduling entity. Both threading implementations employ the Linux clone(2) system call. In NPTL, thread synchronization primitives (mutexes, thread joining, etc.) are implemented using the Linux futex(2) system call.
The notable features of this implementation are the following:
In addition to the main (initial) thread, and the threads that the program creates using pthread_create(3), the implementation creates a "manager" thread. This thread handles thread creation and termination. (Problems can result if this thread is inadvertently killed.)
Signals are used internally by the implementation.
On Linux 2.2 and later, the first three real-time
signals are used. On older Linux kernels,
SIGUSR1 and
SIGUSR2 are used.
Applications must avoid the use of whichever set of
signals is employed by the implementation.
Threads do not share process IDs. (In effect, LinuxThreads threads are implemented as processes which share more information than usual, but which do not share a common process ID.) LinuxThreads threads (including the manager thread) are visible as separate processes using ps(1).
The LinuxThreads implementation deviates from the POSIX.1 specification in a number of ways, including the following:
Calls to getpid(2) return a different value in each thread.
Calls to getppid(2) in threads other than the main thread return the process ID of the manager thread; instead getppid(2) in these threads should return the same value as getppid(2) in the main thread.
When one thread creates a new child process using fork(2), any thread should be able to wait(2) on the child. However, the implementation only allows the thread that created the child to wait(2) on it.
When a thread calls execve(2), all other threads are terminated (as required by POSIX.1). However, the resulting process has the same PID as the thread that called execve(2): it should have the same PID as the main thread.
Threads do not share user and group IDs. This can cause complications with set-user-ID programs and can cause failures in Pthreads functions if an application changes its credentials using seteuid(2) or similar.
Threads do not share a common session ID and process group ID.
Threads do not share record locks created using fcntl(2).
The information returned by times(2) and getrusage(2) is per-thread rather than process-wide.
Threads do not share semaphore undo values (see semop(2)).
Threads do not share interval timers.
Threads do not share a common nice value.
POSIX.1 distinguishes the notions of signals that are directed to the process as a whole and signals are directed to individual threads. According to POSIX.1, a process-directed signal (sent using kill(2), for example) should be handled by a single, arbitrarily selected thread within the process. LinuxThreads does not support the notion of process-directed signals: signals may only be sent to specific threads.
Threads have distinct alternate signal stack settings. However, a new thread's alternate signal stack settings are copied from the thread that created it, so that the threads initially share an alternate signal stack. (A new thread should start with no alternate signal stack defined. If two threads handle signals on their shared alternate signal stack at the same time, unpredictable program failures are likely to occur.)
With NPTL, all of the threads in a process are placed in the same thread group; all members of a thread groups share the same PID. NPTL does not employ a manager thread. NPTL makes internal use of the first two real-time signals; these signals cannot be used in applications.
NPTL still has at least one non-conformance with POSIX.1:
Threads do not share a common nice value.
Some NPTL non-conformances only occur with older kernels:
The information returned by times(2) and getrusage(2) is per-thread rather than process-wide (fixed in kernel 2.6.9).
Threads do not share resource limits (fixed in kernel 2.6.10).
Threads do not share interval timers (fixed in kernel 2.6.12).
Only the main thread is permitted to start a new session using setsid(2) (fixed in kernel 2.6.16).
Only the main thread is permitted to make the process into a process group leader using setpgid(2) (fixed in kernel 2.6.16).
Threads have distinct alternate signal stack settings. However, a new thread's alternate signal stack settings are copied from the thread that created it, so that the threads initially share an alternate signal stack (fixed in kernel 2.6.16).
Note the following further points about the NPTL implementation:
If the stack size soft resource limit (see the
description of RLIMIT_STACK in setrlimit(2)) is
set to a value other than unlimited, then this
value defines the default stack size for new threads.
To be effective, this limit must be set before the
program is executed, perhaps using the ulimit -s shell built-in
command (limit
stacksize in the C shell).
Since glibc 2.3.2, the getconf(1) command can be used to determine the system's threading implementation, for example:
bash$ getconf GNU_LIBPTHREAD_VERSION NPTL 2.3.4
With older glibc versions, a command such as the following should be sufficient to determine the default threading implementation:
bash$ $( ldd /bin/ls | grep libc.so | awk '{print $3}' ) | \ egrep −i 'threads|ntpl' Native POSIX Threads Library by Ulrich Drepper et al
On systems with a glibc that supports both LinuxThreads
and NPTL (i.e., glibc 2.3.x), the LD_ASSUME_KERNEL environment variable can
be used to override the dynamic linker's default choice of
threading implementation. This variable tells the dynamic
linker to assume that it is running on top of a particular
kernel version. By specifying a kernel version that does
not provide the support required by NPTL, we can force the
use of LinuxThreads. (The most likely reason for doing this
is to run a (broken) application that depends on some
non-conformant behavior in LinuxThreads.) For example:
bash$ $( LD_ASSUME_KERNEL=2.2.5 ldd /bin/ls | grep libc.so | \ awk '{print $3}' ) | egrep −i 'threads|ntpl' linuxthreads-0.10 by Xavier Leroy
clone(2), futex(2), gettid(2), futex(7), and various Pthreads manual pages, for example: pthread_atfork(3), pthread_cleanup_push(3), pthread_cond_signal(3), pthread_cond_wait(3), pthread_create(3), pthread_detach(3), pthread_equal(3), pthread_exit(3), pthread_key_create(3), pthread_kill(3), pthread_mutex_lock(3), pthread_mutex_unlock(3), pthread_once(3), pthread_setcancelstate(3), pthread_setcanceltype(3), pthread_setspecific(3), pthread_sigmask(3), and pthread_testcancel(3).
This page is part of release 2.79 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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t Copyright (c) 2005 by Michael Kerrisk <mtk.manpagesgmail.com> Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. |