mirror of https://github.com/mkerrisk/man-pages
190 lines
7.3 KiB
Plaintext
190 lines
7.3 KiB
Plaintext
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.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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.TH "PTHREAD_MUTEX_LOCK" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
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.\" pthread_mutex_lock
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.SH NAME
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pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock \-
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lock and unlock a mutex
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.SH SYNOPSIS
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.LP
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\fB#include <pthread.h>
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.br
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.sp
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int pthread_mutex_lock(pthread_mutex_t *\fP\fImutex\fP\fB);
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.br
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int pthread_mutex_trylock(pthread_mutex_t *\fP\fImutex\fP\fB);
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.br
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int pthread_mutex_unlock(pthread_mutex_t *\fP\fImutex\fP\fB); \fP
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\fB
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.br
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\fP
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.SH DESCRIPTION
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.LP
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The mutex object referenced by \fImutex\fP shall be locked by calling
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\fIpthread_mutex_lock\fP(). If the mutex is already
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locked, the calling thread shall block until the mutex becomes available.
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This operation shall return with the mutex object
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referenced by \fImutex\fP in the locked state with the calling thread
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as its owner.
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.LP
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If the mutex type is PTHREAD_MUTEX_NORMAL, deadlock detection shall
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not be provided. Attempting to relock the mutex causes
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deadlock. If a thread attempts to unlock a mutex that it has not locked
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or a mutex which is unlocked, undefined behavior
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results.
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.LP
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If the mutex type is PTHREAD_MUTEX_ERRORCHECK, then error checking
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shall be provided. If a thread attempts to relock a mutex
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that it has already locked, an error shall be returned. If a thread
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attempts to unlock a mutex that it has not locked or a mutex
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which is unlocked, an error shall be returned.
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.LP
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If the mutex type is PTHREAD_MUTEX_RECURSIVE, then the mutex shall
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maintain the concept of a lock count. When a thread
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successfully acquires a mutex for the first time, the lock count shall
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be set to one. Every time a thread relocks this mutex, the
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lock count shall be incremented by one. Each time the thread unlocks
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the mutex, the lock count shall be decremented by one. When
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the lock count reaches zero, the mutex shall become available for
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other threads to acquire. If a thread attempts to unlock a mutex
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that it has not locked or a mutex which is unlocked, an error shall
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be returned.
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.LP
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If the mutex type is PTHREAD_MUTEX_DEFAULT, attempting to recursively
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lock the mutex results in undefined behavior. Attempting
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to unlock the mutex if it was not locked by the calling thread results
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in undefined behavior. Attempting to unlock the mutex if it
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is not locked results in undefined behavior.
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.LP
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The \fIpthread_mutex_trylock\fP() function shall be equivalent to
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\fIpthread_mutex_lock\fP(), except that if the mutex object
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referenced by \fImutex\fP is currently locked (by any thread, including
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the current thread), the call shall return immediately. If
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the mutex type is PTHREAD_MUTEX_RECURSIVE and the mutex is currently
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owned by the calling thread, the mutex lock count shall be
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incremented by one and the \fIpthread_mutex_trylock\fP() function
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shall immediately return success.
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.LP
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The \fIpthread_mutex_unlock\fP() function shall release the mutex
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object referenced by \fImutex\fP. \ The manner
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in which a mutex is released is dependent upon the mutex's type attribute.
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If there are threads blocked on the mutex object referenced by \fImutex\fP
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when \fIpthread_mutex_unlock\fP() is
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called, resulting in the mutex becoming available, the scheduling
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policy shall determine which thread shall acquire the mutex.
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.LP
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(In the case of PTHREAD_MUTEX_RECURSIVE mutexes, the mutex shall become
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available when the count reaches zero and the calling
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thread no longer has any locks on this mutex.)
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.LP
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If a signal is delivered to a thread waiting for a mutex, upon return
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from the signal handler the thread shall resume waiting
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for the mutex as if it was not interrupted.
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.SH RETURN VALUE
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.LP
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If successful, the \fIpthread_mutex_lock\fP() and \fIpthread_mutex_unlock\fP()
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functions shall return zero; otherwise, an
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error number shall be returned to indicate the error.
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.LP
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The \fIpthread_mutex_trylock\fP() function shall return zero if a
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lock on the mutex object referenced by \fImutex\fP is
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acquired. Otherwise, an error number is returned to indicate the error.
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.SH ERRORS
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.LP
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The \fIpthread_mutex_lock\fP() and \fIpthread_mutex_trylock\fP() functions
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shall fail if:
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.TP 7
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.B EINVAL
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The \fImutex\fP was created with the protocol attribute having the
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value PTHREAD_PRIO_PROTECT and the calling thread's
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priority is higher than the mutex's current priority ceiling.
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.sp
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.LP
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The \fIpthread_mutex_trylock\fP() function shall fail if:
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.TP 7
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.B EBUSY
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The \fImutex\fP could not be acquired because it was already locked.
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.sp
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.LP
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The \fIpthread_mutex_lock\fP(), \fIpthread_mutex_trylock\fP(), and
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\fIpthread_mutex_unlock\fP() functions may fail if:
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.TP 7
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.B EINVAL
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The value specified by \fImutex\fP does not refer to an initialized
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mutex object.
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.TP 7
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.B EAGAIN
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The mutex could not be acquired because the maximum number of recursive
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locks for \fImutex\fP has been exceeded.
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.sp
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.LP
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The \fIpthread_mutex_lock\fP() function may fail if:
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.TP 7
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.B EDEADLK
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The current thread already owns the mutex.
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.sp
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.LP
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The \fIpthread_mutex_unlock\fP() function may fail if:
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.TP 7
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.B EPERM
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The current thread does not own the mutex.
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.sp
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.LP
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These functions shall not return an error code of [EINTR].
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.LP
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\fIThe following sections are informative.\fP
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.SH EXAMPLES
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.LP
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None.
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.SH APPLICATION USAGE
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.LP
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None.
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.SH RATIONALE
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.LP
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Mutex objects are intended to serve as a low-level primitive from
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which other thread synchronization functions can be built. As
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such, the implementation of mutexes should be as efficient as possible,
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and this has ramifications on the features available at the
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interface.
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.LP
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The mutex functions and the particular default settings of the mutex
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attributes have been motivated by the desire to not
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preclude fast, inlined implementations of mutex locking and unlocking.
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.LP
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For example, deadlocking on a double-lock is explicitly allowed behavior
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in order to avoid requiring more overhead in the basic
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mechanism than is absolutely necessary. (More "friendly" mutexes that
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detect deadlock or that allow multiple locking by the same
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thread are easily constructed by the user via the other mechanisms
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provided. For example, \fIpthread_self\fP() can be used to record
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mutex ownership.) Implementations might also
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choose to provide such extended features as options via special mutex
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attributes.
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.LP
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Since most attributes only need to be checked when a thread is going
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to be blocked, the use of attributes does not slow the
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(common) mutex-locking case.
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.LP
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Likewise, while being able to extract the thread ID of the owner of
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a mutex might be desirable, it would require storing the
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current thread ID when each mutex is locked, and this could incur
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unacceptable levels of overhead. Similar arguments apply to a
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\fImutex_tryunlock\fP operation.
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.SH FUTURE DIRECTIONS
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.LP
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None.
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.SH SEE ALSO
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.LP
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\fIpthread_mutex_destroy\fP() , \fIpthread_mutex_timedlock\fP() ,
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the Base Definitions volume of
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IEEE\ Std\ 1003.1-2001, \fI<pthread.h>\fP
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.SH COPYRIGHT
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Portions of this text are reprinted and reproduced in electronic form
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from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
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-- Portable Operating System Interface (POSIX), The Open Group Base
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Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
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Electrical and Electronics Engineers, Inc and The Open Group. In the
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event of any discrepancy between this version and the original IEEE and
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The Open Group Standard, the original IEEE and The Open Group Standard
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is the referee document. The original Standard can be obtained online at
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http://www.opengroup.org/unix/online.html .
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