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<META HTTP-EQUIV="CONTENT-TYPE" CONTENT="text/html; charset=utf-8">
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<TITLE>SEMAPHORES(3) manual page</TITLE>
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<BODY LANG="en-GB" BGCOLOR="#ffffff" DIR="LTR">
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<H4>POSIX Threads for Windows – REFERENCE - <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A></H4>
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<P><A HREF="index.html">Reference Index</A></P>
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<P><A HREF="#toc">Table of Contents</A></P>
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<H2><A HREF="#toc0" NAME="sect0">Name</A></H2>
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<P>sem_init, sem_wait, sem_trywait, sem_post, sem_getvalue,
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sem_destroy - operations on semaphores
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</P>
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<H2><A HREF="#toc1" NAME="sect1">Synopsis</A></H2>
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<P><B>#include <semaphore.h></B>
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</P>
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<P><B>int sem_init(sem_t *</B><I>sem</I><B>, int </B><I>pshared</I><B>,
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unsigned int </B><I>value</I><B>);</B>
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</P>
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<P><B>int sem_wait(sem_t * </B><I>sem</I><B>);</B>
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</P>
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<P><B>int sem_timedwait(sem_t * </B><I>sem</I>, <B>const struct
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timespec *</B><I>abstime</I><B>);</B>
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</P>
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<P><B>int sem_trywait(sem_t * </B><I>sem</I><B>);</B>
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</P>
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<P><B>int sem_post(sem_t * </B><I>sem</I><B>);</B>
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</P>
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<P><B>int sem_post_multiple(sem_t * </B><I>sem, </I><B>int</B>
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<I>number</I><B>);</B>
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</P>
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<P><B>int sem_getvalue(sem_t * </B><I>sem</I><B>, int * </B><I>sval</I><B>);</B>
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</P>
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<P><B>int sem_destroy(sem_t * </B><I>sem</I><B>);</B>
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</P>
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<H2><A HREF="#toc2" NAME="sect2">Description</A></H2>
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<P>Semaphores are counters for resources shared between threads. The
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basic operations on semaphores are: increment the counter atomically,
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and wait until the counter is non-null and decrement it atomically.
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</P>
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<P><B>sem_init</B> initializes the semaphore object pointed to by
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<I>sem</I>. The count associated with the semaphore is set initially
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to <I>value</I>. The <I>pshared</I> argument indicates whether the
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semaphore is local to the current process ( <I>pshared</I> is zero)
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or is to be shared between several processes ( <I>pshared</I> is not
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zero).</P>
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<P><B>Pthreads-w32</B> currently does not support process-shared
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semaphores, thus <B>sem_init</B> always returns with error <B>EPERM</B>
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if <I>pshared</I> is not zero.
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</P>
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<P><B>sem_wait</B> atomically decrements <I>sem</I>'s count if it is
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greater than 0 and returns immediately or it suspends the calling
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thread until it can resume following a call to <B>sem_post</B> or
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<B>sem_post_multiple</B>.</P>
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<P><B>sem_timedwait</B> atomically decrements <I>sem</I>'s count if
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it is greater than 0 and returns immediately, or it suspends the
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calling thread. If <I>abstime</I> time arrives before the thread can
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resume following a call to <B>sem_post</B> or <B>sem_post_multiple</B>,
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then <B>sem_timedwait</B> returns with a return code of -1 after
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having set <B>errno</B> to <B>ETIMEDOUT</B>. If the call can return
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without suspending then <I>abstime</I> is not checked.</P>
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<P><B>sem_trywait</B> atomically decrements <I>sem</I>'s count if it
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is greater than 0 and returns immediately, or it returns immediately
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with a return code of -1 after having set <B>errno</B> to <B>EAGAIN</B>.
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<B>sem_trywait</B> never blocks.</P>
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<P><B>sem_post</B> either releases one thread if there are any
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waiting on <I>sem</I>, or it atomically increments <I>sem</I>'s
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count.</P>
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<P><B>sem_post_multiple</B> either releases multiple threads if there
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are any waiting on <I>sem</I> and/or it atomically increases <I>sem</I>'s
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count. If there are currently <I>n</I> waiters, where <I>n</I> the
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largest number less than or equal to <I>number</I>, then <I>n</I>
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waiters are released and <I>sem</I>'s count is incremented by <I>number</I>
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minus <I>n</I>.</P>
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<P><B>sem_getvalue</B> stores in the location pointed to by <I>sval</I>
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the current count of the semaphore <I>sem</I>. In the <B>Pthreads-w32</B>
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implementation: if the value returned in <I>sval</I> is greater than
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or equal to 0 it was the <I>sem</I>'s count at some point during the
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call to <B>sem_getvalue</B>. If the value returned in <I>sval</I> is
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less than 0 then it's absolute value represents the number of threads
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waiting on <I>sem</I> at some point during the call to <B>sem_getvalue.
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</B>POSIX does not require an implementation of <B>sem_getvalue</B>
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to return a value in <I>sval</I> that is less than 0, but if it does
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then it's absolute value must represent the number of waiters.</P>
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<P><B>sem_destroy</B> destroys a semaphore object, freeing the
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resources it might hold. No threads should be waiting on the
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semaphore at the time <B>sem_destroy</B> is called.</P>
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<H2><A HREF="#toc3" NAME="sect3">Cancellation</A></H2>
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<P><B>sem_wait</B> and <B>sem_timedwait</B> are cancellation points.
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</P>
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<H2><A HREF="#toc4" NAME="sect4">Async-signal Safety</A></H2>
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<P>These routines are not async-cancel safe.</P>
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<H2><A HREF="#toc5" NAME="sect5">Return Value</A></H2>
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<P>All semaphore functions return 0 on success, or -1 on error in
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which case they write an error code in <B>errno</B>.
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</P>
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<H2><A HREF="#toc6" NAME="sect6">Errors</A></H2>
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<P>The <B>sem_init</B> function sets <B>errno</B> to the following
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codes on error:
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</P>
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<DL>
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<DL>
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<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>EINVAL</B>
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</DT><DD STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
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<I>value</I> exceeds the maximal counter value <B>SEM_VALUE_MAX</B>
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</DD><DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
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<B>ENOSYS</B>
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</DT></DL>
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</DL>
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<BLOCKQUOTE STYLE="margin-left: 3cm">
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<I>pshared</I> is not zero
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</BLOCKQUOTE>
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<P>The <B>sem_timedwait</B> function sets <B>errno</B> to the
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following error code on error:
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</P>
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<DL>
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<DL>
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<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>ETIMEDOUT</B>
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</DT></DL>
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</DL>
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<BLOCKQUOTE STYLE="margin-left: 3cm">
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if <I>abstime</I> arrives before the waiting thread can resume
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following a call to <B>sem_post</B> or <B>sem_post_multiple</B>.
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</BLOCKQUOTE>
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<P>The <B>sem_trywait</B> function sets <B>errno</B> to the following
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error code on error:
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</P>
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<DL>
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<DL>
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<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>EAGAIN</B>
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</DT></DL>
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</DL>
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<BLOCKQUOTE STYLE="margin-left: 3cm">
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if the semaphore count is currently 0
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</BLOCKQUOTE>
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<P>The <B>sem_post</B> and <B>sem_post_multiple</B> functions set
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<B>errno</B> to the following error code on error:
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</P>
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<DL>
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<DL>
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<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>ERANGE</B>
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</DT><DD STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
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if after incrementing, the semaphore count would exceed
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<B>SEM_VALUE_MAX</B> (the semaphore count is left unchanged in this
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case)
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</DD></DL>
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</DL>
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<P>
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The <B>sem_destroy</B> function sets <B>errno</B> to the following
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error code on error:
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</P>
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<DL>
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<DL>
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<DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm"><B>EBUSY</B>
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</DT><DD STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
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if some threads are currently blocked waiting on the semaphore.
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</DD></DL>
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</DL>
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<H2>
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<A HREF="#toc7" NAME="sect7">Author</A></H2>
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<P>Xavier Leroy <Xavier.Leroy@inria.fr>
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</P>
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<P>Modified by Ross Johnson for use with <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A>.</P>
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<H2><A HREF="#toc8" NAME="sect8">See Also</A></H2>
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<P><A HREF="pthread_mutex_init.html"><B>pthread_mutex_init</B>(3)</A>
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, <A HREF="pthread_cond_init.html"><B>pthread_cond_init</B>(3)</A> ,
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<A HREF="pthread_cancel.html"><B>pthread_cancel</B>(3)</A> .
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</P>
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<HR>
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<P><A NAME="toc"></A><B>Table of Contents</B></P>
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<UL>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect0" NAME="toc0">Name</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect1" NAME="toc1">Synopsis</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect2" NAME="toc2">Description</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect3" NAME="toc3">Cancellation</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect4" NAME="toc4">Async-signal
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Safety</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect5" NAME="toc5">Return
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Value</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect6" NAME="toc6">Errors</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect7" NAME="toc7">Author</A>
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</P>
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<LI><P><A HREF="#sect8" NAME="toc8">See Also</A>
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</P>
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</UL>
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</BODY>
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