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<TITLE>PTHREAD_COND(3) manual page</TITLE>
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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>pthread_cond_init, pthread_cond_destroy, pthread_cond_signal,
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pthread_cond_broadcast, pthread_cond_wait, pthread_cond_timedwait -
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operations on conditions
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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 <pthread.h></B>
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</P>
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<P><B>pthread_cond_t </B><I>cond</I> <B>= PTHREAD_COND_INITIALIZER;</B>
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</P>
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<P><B>int pthread_cond_init(pthread_cond_t *</B><I>cond</I><B>,
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pthread_condattr_t *</B><I>cond_attr</I><B>);</B>
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</P>
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<P><B>int pthread_cond_signal(pthread_cond_t *</B><I>cond</I><B>);</B>
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</P>
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<P><B>int pthread_cond_broadcast(pthread_cond_t *</B><I>cond</I><B>);</B>
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</P>
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<P><B>int pthread_cond_wait(pthread_cond_t *</B><I>cond</I><B>,
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pthread_mutex_t *</B><I>mutex</I><B>);</B>
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</P>
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<P><B>int pthread_cond_timedwait(pthread_cond_t *</B><I>cond</I><B>,
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pthread_mutex_t *</B><I>mutex</I><B>, const struct timespec
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*</B><I>abstime</I><B>);</B>
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</P>
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<P><B>int pthread_cond_destroy(pthread_cond_t *</B><I>cond</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>A condition (short for ‘‘condition variable’’) is a
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synchronization device that allows threads to suspend execution and
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relinquish the processors until some predicate on shared data is
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satisfied. The basic operations on conditions are: signal the
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condition (when the predicate becomes true), and wait for the
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condition, suspending the thread execution until another thread
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signals the condition.
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</P>
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<P>A condition variable must always be associated with a mutex, to
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avoid the race condition where a thread prepares to wait on a
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condition variable and another thread signals the condition just
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before the first thread actually waits on it.
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</P>
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<P><B>pthread_cond_init</B> initializes the condition variable <I>cond</I>,
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using the condition attributes specified in <I>cond_attr</I>, or
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default attributes if <I>cond_attr</I> is <B>NULL</B>.
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</P>
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<P>Variables of type <B>pthread_cond_t</B> can also be initialized
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statically, using the constant <B>PTHREAD_COND_INITIALIZER</B>. In
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the <B>Pthreads-w32</B> implementation, an application should still
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call <B>pthread_cond_destroy</B> at some point to ensure that any
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resources consumed by the condition variable are released.</P>
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<P><B>pthread_cond_signal</B> restarts one of the threads that are
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waiting on the condition variable <I>cond</I>. If no threads are
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waiting on <I>cond</I>, nothing happens. If several threads are
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waiting on <I>cond</I>, exactly one is restarted, but it is not
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specified which.
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</P>
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<P><B>pthread_cond_broadcast</B> restarts all the threads that are
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waiting on the condition variable <I>cond</I>. Nothing happens if no
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threads are waiting on <I>cond</I>.
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</P>
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<P><B>pthread_cond_wait</B> atomically unlocks the <I>mutex</I> (as
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per <B>pthread_unlock_mutex</B>) and waits for the condition variable
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<I>cond</I> to be signalled. The thread execution is suspended and
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does not consume any CPU time until the condition variable is
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signalled. The <I>mutex</I> must be locked by the calling thread on
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entrance to <B>pthread_cond_wait</B>. Before returning to the calling
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thread, <B>pthread_cond_wait</B> re-acquires <I>mutex</I> (as per
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<B>pthread_lock_mutex</B>).
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</P>
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<P>Unlocking the mutex and suspending on the condition variable is
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done atomically. Thus, if all threads always acquire the mutex before
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signalling the condition, this guarantees that the condition cannot
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be signalled (and thus ignored) between the time a thread locks the
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mutex and the time it waits on the condition variable.
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</P>
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<P><B>pthread_cond_timedwait</B> atomically unlocks <I>mutex</I> and
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waits on <I>cond</I>, as <B>pthread_cond_wait</B> does, but it also
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bounds the duration of the wait. If <I>cond</I> has not been
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signalled within the amount of time specified by <I>abstime</I>, the
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mutex <I>mutex</I> is re-acquired and <B>pthread_cond_timedwait</B>
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returns the error <B>ETIMEDOUT</B>. The <I>abstime</I> parameter
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specifies an absolute time, with the same origin as <A HREF="time.html"><B>time</B>(2)</A>
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and <A HREF="gettimeofday.html"><B>gettimeofday</B>(2)</A>.
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</P>
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<P><B>pthread_cond_destroy</B> destroys a condition variable, freeing
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the resources it might hold. No threads must be waiting on the
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condition variable on entrance to <B>pthread_cond_destroy</B>.</P>
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<H2><A HREF="#toc3" NAME="sect3">Cancellation</A></H2>
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<P><B>pthread_cond_wait</B> and <B>pthread_cond_timedwait</B> are
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cancellation points. If a thread is cancelled while suspended in one
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of these functions, the thread immediately resumes execution, then
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locks again the <I>mutex</I> argument to <B>pthread_cond_wait</B> and
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<B>pthread_cond_timedwait</B>, and finally executes the cancellation.
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Consequently, cleanup handlers are assured that <I>mutex</I> is
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locked when they are called.
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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>The condition functions are not async-signal safe, and should not
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be called from a signal handler. In particular, calling
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<B>pthread_cond_signal</B> or <B>pthread_cond_broadcast</B> from a
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signal handler may deadlock the calling thread.
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</P>
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<H2><A HREF="#toc5" NAME="sect5">Return Value</A></H2>
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<P>All condition variable functions return 0 on success and a
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non-zero error code on error.
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</P>
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<H2><A HREF="#toc6" NAME="sect6">Errors</A></H2>
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<P><B>pthread_cond_init</B>, <B>pthread_cond_signal</B>,
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<B>pthread_cond_broadcast</B>, and <B>pthread_cond_wait</B> never
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return an error code.
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</P>
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<P>The <B>pthread_cond_init</B> function returns the following error
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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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The <I>cond</I> argument is invalid.
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</DD><DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
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<B>ENOMEM</B>
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</DT></DL>
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</DL>
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<BLOCKQUOTE STYLE="margin-left: 4cm">
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There was not enough memory to allocate the condition variable.
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</BLOCKQUOTE>
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<P>The <B>pthread_cond_signal</B> function returns the following
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error 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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The <I>cond</I> argument is invalid.
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</DD></DL>
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</DL>
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<P>
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The <B>pthread_cond_broadcast</B> function returns the following
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error 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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The <I>cond</I> argument is invalid.
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</DD></DL>
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</DL>
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<P>
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The <B>pthread_cond_wait</B> function returns the following error
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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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The <I>cond</I> argument is invalid.
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</DD><DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
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<B>ENOMEM</B>
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</DT></DL>
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</DL>
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<BLOCKQUOTE STYLE="margin-left: 4cm">
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There was not enough memory to allocate the statically initialised
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condition variable. Statically initialised condition variables are
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dynamically allocated by the first thread to wait on them.</BLOCKQUOTE>
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<P>The <B>pthread_cond_timedwait</B> function returns the following
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error 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></DL>
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</DL>
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<P STYLE="margin-left: 2cm">
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The <I>cond</I> argument is invalid.
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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><DD STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
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The condition variable was not signalled before the timeout
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specified by <I>abstime</I>
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</DD><DT STYLE="margin-right: 1cm; margin-bottom: 0.5cm">
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<B>ENOMEM</B>
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</DT></DL>
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</DL>
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<BLOCKQUOTE STYLE="margin-left: 4cm">
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There was not enough memory to allocate the statically initialised
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condition variable. Statically initialised condition variables are
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dynamically allocated by the first thread to wait on them.
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</BLOCKQUOTE>
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<P>The <B>pthread_cond_destroy</B> function returns 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>EINVAL</B>
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</DT></DL>
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</DL>
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<P STYLE="margin-left: 2cm; margin-right: 1cm">
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The <I>cond</I> argument is invalid.
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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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Some threads are currently waiting on <I>cond</I>.
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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_condattr_init.html"><B>pthread_condattr_init</B>(3)</A>
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, <A HREF="pthread_mutex_lock.html"><B>pthread_mutex_lock</B>(3)</A>
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, <A HREF="pthread_mutex_unlock.html"><B>pthread_mutex_unlock</B>(3)</A>
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, <A HREF="pthread_cancel.html"><B>pthread_cancel(3)</B></A>.
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</P>
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<H2><A HREF="#toc9" NAME="sect9">Example</A></H2>
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<P>Consider two shared variables <I>x</I> and <I>y</I>, protected by
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the mutex <I>mut</I>, and a condition variable <I>cond</I> that is to
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be signaled whenever <I>x</I> becomes greater than <I>y</I>.
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</P>
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<PRE STYLE="margin-left: 1cm; margin-right: 1cm">int x,y;
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pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;
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pthread_cond_t cond = PTHREAD_COND_INITIALIZER;</PRE><BLOCKQUOTE>
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Waiting until <I>x</I> is greater than <I>y</I> is performed as
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follows:
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</BLOCKQUOTE>
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<PRE STYLE="margin-left: 1.01cm">pthread_mutex_lock(&mut);
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while (x <= y) {
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pthread_cond_wait(&cond, &mut);
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}
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/* operate on x and y */
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pthread_mutex_unlock(&mut);</PRE><BLOCKQUOTE STYLE="margin-left: 3.01cm">
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Modifications on <I>x</I> and <I>y</I> that may cause <I>x</I> to
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become greater than <I>y</I> should signal the condition if needed:
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</BLOCKQUOTE>
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<PRE STYLE="margin-left: 1.01cm">pthread_mutex_lock(&mut);
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/* modify x and y */
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if (x > y) pthread_cond_broadcast(&cond);
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pthread_mutex_unlock(&mut);</PRE><BLOCKQUOTE STYLE="margin-left: 3.01cm">
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If it can be proved that at most one waiting thread needs to be waken
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up (for instance, if there are only two threads communicating through
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<I>x</I> and <I>y</I>), <B>pthread_cond_signal</B> can be used as a
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slightly more efficient alternative to <B>pthread_cond_broadcast</B>.
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If in doubt, use <B>pthread_cond_broadcast</B>.
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</BLOCKQUOTE>
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<BLOCKQUOTE STYLE="margin-left: 3.01cm">To wait for <I>x</I> to
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become greater than <I>y</I> with a timeout of 5 seconds, do:
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</BLOCKQUOTE>
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<PRE STYLE="margin-left: 1.01cm">struct timeval now;
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struct timespec timeout;
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int retcode;
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pthread_mutex_lock(&mut);
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gettimeofday(&now);
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timeout.tv_sec = now.tv_sec + 5;
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timeout.tv_nsec = now.tv_usec * 1000;
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retcode = 0;
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while (x <= y && retcode != ETIMEDOUT) {
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retcode = pthread_cond_timedwait(&cond, &mut, &timeout);
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}
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if (retcode == ETIMEDOUT) {
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/* timeout occurred */
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} else {
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/* operate on x and y */
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}
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pthread_mutex_unlock(&mut);</PRE>
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<HR>
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<BLOCKQUOTE STYLE="margin-left: 0cm; margin-right: 0cm"><A NAME="toc"></A>
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<B>Table of Contents</B></BLOCKQUOTE>
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<UL>
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<LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect0" NAME="toc0">Name</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect1" NAME="toc1">Synopsis</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect2" NAME="toc2">Description</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect3" NAME="toc3">Cancellation</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect4" NAME="toc4">Async-signal
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Safety</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect5" NAME="toc5">Return
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Value</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect6" NAME="toc6">Errors</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect7" NAME="toc7">Author</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-right: 0cm; margin-bottom: 0cm"><A HREF="#sect8" NAME="toc8">See
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Also</A>
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</BLOCKQUOTE>
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<LI><BLOCKQUOTE STYLE="margin-right: 0cm"><A HREF="#sect9" NAME="toc9">Example</A>
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</BLOCKQUOTE>
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</UL>
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