Rearrange, reword, clarify. Remove duplicate paragraphs and recurring
sentences. Update the ERRORS to reflect reality in NetBSD.
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@ -1,4 +1,4 @@
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.\" $NetBSD: pthread_cond.3,v 1.3 2010/07/09 10:55:11 wiz Exp $
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.\" $NetBSD: pthread_cond.3,v 1.4 2010/07/09 18:01:53 jruoho Exp $
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.\"
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.\" Copyright (c) 2002, 2008 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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@ -66,8 +66,6 @@
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.Ft int
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.Fn pthread_cond_init "pthread_cond_t * restrict cond" \
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"const pthread_condattr_t * restrict attr"
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.Pp
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.Va pthread_cond_t cond = Dv PTHREAD_COND_INITIALIZER;
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.Ft int
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.Fn pthread_cond_destroy "pthread_cond_t *cond"
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.Ft int
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@ -80,8 +78,19 @@
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.Ft int
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.Fn pthread_cond_timedwait "pthread_cond_t * restrict cond" \
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"pthread_mutex_t * restrict mutex" "const struct timespec * restrict abstime"
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.Pp
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.Va pthread_cond_t cond = Dv PTHREAD_COND_INITIALIZER;
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.\" ----------------------------------------------------------------------------
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.Sh DESCRIPTION
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Condition variables are intended to be used to communicate changes in
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the state of data shared between threads.
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Condition variables are always associated with a mutex to provide
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synchronized access to the shared data.
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A single predicate should always be associated with a
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condition variable.
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The predicate should identify a state of the
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shared data that must be true before the thread proceeds.
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.Pp
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The
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.Fn pthread_cond_init
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function creates a new condition variable, with attributes specified with
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@ -91,15 +100,10 @@ If
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is
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.Dv NULL
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the default attributes are used.
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.Pp
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Condition variables are intended to be used to communicate changes in
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the state of data shared between threads.
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Condition variables are always associated with a mutex to provide
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synchronized access to the shared data.
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A single predicate should always be associated with a
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condition variable.
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The predicate should identify a state of the
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shared data that must be true before the thread proceeds.
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The
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.Fn pthread_cond_destroy
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function frees the resources allocated by the condition variable
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.Fa cond .
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.Pp
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The macro
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.Dv PTHREAD_COND_INITIALIZER
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@ -114,46 +118,20 @@ specified as
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except that no error checking is done.
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.Pp
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.\" -----
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The
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.Fn pthread_cond_destroy
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function frees the resources allocated by the condition variable
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.Fa cond .
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.Pp
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.\" -----
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The
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.Fn pthread_cond_broadcast
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function unblocks all threads waiting for the condition variable
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.Fa cond .
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If no threads are waiting on
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.Fa cond ,
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the
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.Fn pthread_cond_broadcast
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function has no effect.
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.Pp
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The
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.Fn pthread_cond_signal
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function unblocks one thread waiting for the condition variable
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.Fa cond .
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If no threads are waiting on
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.Fa cond ,
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the
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.Fn pthread_cond_signal
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function has no effect.
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.Pp
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When calling
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.Fn pthread_cond_wait
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and/or
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.Fn pthread_cond_timedwait ,
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a temporary binding is established between the condition variable
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.Fa cond
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and a caller-supplied mutex.
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.Pp
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The same mutex must be held while calling
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The difference between
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.Fn pthread_cond_broadcast
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and
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.Fn pthread_cond_signal .
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Neither function enforces this requirement, but if the mutex is not
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held the resulting behaviour is undefined.
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.Fn pthread_cond_signal
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is that the former unblocks all threads waiting for the condition variable,
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whereas the latter blocks only one waiting thread.
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If no threads are waiting on
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.Fa cond ,
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neihter function has any effect.
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If more than one thread is blocked on a condition variable,
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the used scheduling policy determines the order in which threads are unblocked.
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The same mutex used for waiting must be held while calling either function.
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Although neither function strictly enforces this requirement,
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undefined behavior may follow if the mutex is not held.
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.Pp
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.\" -----
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The
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@ -163,64 +141,49 @@ variable specified by
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.Fa cond ,
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and unlocks the mutex specified by
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.Fa mutex .
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The waiting thread unblocks after another thread calls
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.Xr pthread_cond_signal 3 ,
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or
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.Xr pthread_cond_broadcast 3
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with the same condition variable.
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The current thread holds the lock on
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.Fa mutex
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upon return from the
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.Fn pthread_cond_wait
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call.
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.Pp
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The
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.Fn pthread_cond_timedwait
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function atomically blocks the current thread waiting on the condition
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variable specified by
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.Fa cond ,
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and unlocks the mutex specified by
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.Fa mutex .
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The waiting thread unblocks after another thread calls
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.Xr pthread_cond_signal 3 ,
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or
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.Xr pthread_cond_broadcast 3
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with the same condition variable, or if the system time reaches the
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time specified in
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function behaves similarly, but unblocks also
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if the system time reaches the time specified in
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.Fa abstime ,
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represented as
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.Em struct timespec
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(see
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.Xr timespec 3 ) .
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With both functions the waiting thread unblocks after another thread calls
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.Fn pthread_cond_signal
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or
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.Fn pthread_cond_broadcast
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with the same condition variable and by holding the same
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.Fa mutex
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that was associated with
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.Fa cond
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by either one of the blocking functions.
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The current thread holds the lock on
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.Fa mutex
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upon return from either function.
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.\" -----
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.Pp
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Note that a call to
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.Fn pthread_cond_wait
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or
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.Fn pthread_cond_timedwait
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may wake up spontaneously, without a call to
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.Xr pthread_cond_signal 3
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.Fn pthread_cond_signal
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or
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.Xr pthread_cond_broadcast 3 .
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The caller should prepare for this by invoking
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.Fn pthread_cond_wait
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or
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.Fn pthread_cond_timedwait
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.Fn pthread_cond_broadcast .
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The caller should prepare for this by invoking either function
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within a predicate loop that tests whether the thread should proceed.
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.Pp
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When calling
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.Fn pthread_cond_wait
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or
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.Fn pthread_cond_timedwait ,
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.\" -----
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As noted, when calling either function that waits on a condition variable,
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a temporary binding is established between the condition variable
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.Fa cond
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and the mutex
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.Fa mutex .
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.Pp
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The same mutex must be held while calling
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.Fn pthread_cond_broadcast
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and
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.Fn pthread_cond_signal
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on
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During this time, the effect of an attempt by any thread to wait on
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that condition variable using a different mutex is undefined.
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The same mutex must be held while broadcasting or signaling on
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.Fa cond .
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Additionally, the same mutex must be used for concurrent calls to
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.Fn pthread_cond_wait
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@ -233,41 +196,23 @@ if the mutex is not held or independent mutexes are used the resulting
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behaviour is undefined.
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.\" ----------------------------------------------------------------------------
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.Sh RETURN VALUES
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If successful, the
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.Fn pthread_cond_init
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function will return zero and put the new condition variable id into
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.Fa cond ,
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otherwise an error number will be returned to indicate the error.
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.Pp
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.\" -----
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If successful, the
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.Fn pthread_cond_destroy ,
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.Fn pthread_cond_signal ,
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.Fn pthread_cond_broadcast ,
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.Fn pthread_cond_wait
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and
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.Fn pthread_cond_timedwait
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will return zero, otherwise an error number will be returned to
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indicate the error.
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.\" ----------------------------------------------------------------------------
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If successful, all functions return zero.
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Otherwise, an error number will be returned to indicate the error.
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.Sh ERRORS
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The
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.Fn pthread_cond_init
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may fail if:
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function may fail if:
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.Bl -tag -width Er
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.It Bq Er EAGAIN
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The system lacks the resources to initialize another condition variable.
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.It Bq Er EINVAL
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The value specified by
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.Fa attr
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is invalid.
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.It Bq Er ENOMEM
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The process cannot allocate enough memory to initialize another condition
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variable.
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.El
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.Pp
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.\" -----
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The
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.Fn pthread_cond_destroy
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may fail if:
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function may fail if:
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.Bl -tag -width Er
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.It Bq Er EBUSY
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The variable
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@ -280,6 +225,7 @@ is invalid.
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.El
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.Pp
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.\" -----
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Both
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.Fn pthread_cond_broadcast
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and
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.Fn pthread_cond_signal
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@ -292,7 +238,10 @@ is invalid.
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.El
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.Pp
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.\" -----
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Both
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.Fn pthread_cond_wait
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and
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.Fn pthread_cond_timedwait
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may fail if:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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@ -301,34 +250,20 @@ The value specified by
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or the value specified by
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.Fa mutex
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is invalid.
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.It Bq Er EPERM
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The value specified by
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.Fa mutex
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was not locked in the condition wait.
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.El
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.Pp
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The
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.Fn pthread_cond_timedwait
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may fail if:
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function may additionally fail if:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The value specified by
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.Fa cond ,
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.Fa mutex ,
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or
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.Fa abstime
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is invalid.
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.It Bq Er ETIMEDOUT
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The system time has reached or exceeded the time specified in
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.Fa abstime .
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.El
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.Pp
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.Fn pthread_cond_wait
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and
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.Fn pthread_cond_timedwait
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may fail if:
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.Bl -tag -width Er
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.It Bq Er EPERM
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The value specified by
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.Fa mutex
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is not locked in condition wait.
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.El
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.\" ----------------------------------------------------------------------------
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.Sh SEE ALSO
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.Xr pthread 3 ,
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.Xr pthread_barrier 3 ,
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