.\" $NetBSD: pthread_cond.3,v 1.9 2018/07/28 14:00:19 kre Exp $
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.\" ----------------------------------------------------------------------------
.Dd July 8, 2010
.Dt PTHREAD_COND 3
.Os
.Sh NAME
.Nm pthread_cond ,
.Nm pthread_cond_init ,
.Nm pthread_cond_destroy ,
.Nm pthread_cond_broadcast ,
.Nm pthread_cond_signal ,
.Nm pthread_cond_wait ,
.Nm pthread_cond_timedwait
.Nd condition variable interface
.Sh LIBRARY
.Lb libpthread
.\" ----------------------------------------------------------------------------
.Sh SYNOPSIS
.In pthread.h
.Ft int
.Fn pthread_cond_init "pthread_cond_t * restrict cond" \
"const pthread_condattr_t * restrict attr"
.Vt pthread_cond_t cond No = Dv PTHREAD_COND_INITIALIZER ;
.Ft int
.Fn pthread_cond_destroy "pthread_cond_t *cond"
.Ft int
.Fn pthread_cond_broadcast "pthread_cond_t *cond"
.Ft int
.Fn pthread_cond_signal "pthread_cond_t *cond"
.Ft int
.Fn pthread_cond_wait "pthread_cond_t * restrict cond" \
"pthread_mutex_t * restrict mutex"
.Ft int
.Fn pthread_cond_timedwait "pthread_cond_t * restrict cond" \
"pthread_mutex_t * restrict mutex" "const struct timespec * restrict abstime"
.\" ----------------------------------------------------------------------------
.Sh DESCRIPTION
Condition variables are intended to be used to communicate changes in
the state of data shared between threads.
Condition variables are always associated with a mutex to provide
synchronized access to the shared data.
A single predicate should always be associated with a
condition variable.
The predicate should identify a state of the
shared data that must be true before the thread proceeds.
.Pp
The
.Fn pthread_cond_init
function creates a new condition variable, with attributes specified with
.Fa attr .
If
.Fa attr
is
.Dv NULL
the default attributes are used.
The
.Fn pthread_cond_destroy
function frees the resources allocated by the condition variable
.Fa cond .
.Pp
The macro
.Dv PTHREAD_COND_INITIALIZER
can be used to initialize a condition variable when it can be statically
allocated and the default attributes are appropriate.
The effect is similar to calling
.Fn pthread_cond_init
with
.Fa attr
specified as
.Dv NULL ,
except that no error checking is done.
.Pp
.\" -----
The difference between
.Fn pthread_cond_broadcast
and
.Fn pthread_cond_signal
is that the former unblocks all threads waiting for the condition variable,
whereas the latter unblocks only one waiting thread.
If no threads are waiting on
.Fa cond ,
neither function has any effect.
If more than one thread is blocked on a condition variable,
the used scheduling policy determines the order in which threads are unblocked.
The same mutex used for waiting must be held while calling either function.
Although neither function strictly enforces this requirement,
undefined behavior may follow if the mutex is not held.
.Pp
.\" -----
The
.Fn pthread_cond_wait
function atomically blocks the current thread waiting on the condition
variable specified by
.Fa cond ,
and unlocks the mutex specified by
.Fa mutex .
The
.Fn pthread_cond_timedwait
function behaves similarly, but unblocks also
if the system time reaches the time specified in
.Fa abstime ,
represented as
.Em struct timespec
(see
.Xr timespec 3 ) .
With both functions the waiting thread unblocks after another thread calls
.Fn pthread_cond_signal
or
.Fn pthread_cond_broadcast
with the same condition variable and by holding the same
.Fa mutex
that was associated with
.Fa cond
by either one of the blocking functions.
The current thread holds the lock on
.Fa mutex
upon return from either function.
.\" -----
.Pp
Note that a call to
.Fn pthread_cond_wait
or
.Fn pthread_cond_timedwait
may wake up spontaneously, without a call to
.Fn pthread_cond_signal
or
.Fn pthread_cond_broadcast .
The caller should prepare for this by invoking either function
within a predicate loop that tests whether the thread should proceed.
.Pp
.\" -----
As noted, when calling either function that waits on a condition variable,
a temporary binding is established between the condition variable
.Fa cond
and the mutex
.Fa mutex .
During this time, the effect of an attempt by any thread to wait on
that condition variable using a different mutex is undefined.
The same mutex must be held while broadcasting or signaling on
.Fa cond .
Additionally, the same mutex must be used for concurrent calls to
.Fn pthread_cond_wait
and
.Fn pthread_cond_timedwait .
Only when a condition variable is known to be quiescent may an application
change the mutex associated with it.
In this implementation, none of the functions enforce this requirement, but
if the mutex is not held or independent mutexes are used the resulting
behaviour is undefined.
.\" ----------------------------------------------------------------------------
.Sh RETURN VALUES
If successful, all functions return zero.
Otherwise, an error number will be returned to indicate the error.
.Sh ERRORS
The
.Fn pthread_cond_init
function may fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The value specified by
.Fa attr
is invalid.
.El
.Pp
.\" -----
The
.Fn pthread_cond_destroy
function may fail if:
.Bl -tag -width Er
.It Bq Er EBUSY
The variable
.Fa cond
is locked by another thread.
.It Bq Er EINVAL
The value specified by
.Fa cond
is invalid.
.El
.Pp
.\" -----
Both
.Fn pthread_cond_broadcast
and
.Fn pthread_cond_signal
may fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The value specified by
.Fa cond
is invalid.
.El
.Pp
.\" -----
Both
.Fn pthread_cond_wait
and
.Fn pthread_cond_timedwait
may fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The value specified by
.Fa cond
or the value specified by
.Fa mutex
is invalid.
.It Bq Er EPERM
The value specified by
.Fa mutex
was not locked in the condition wait.
.El
.Pp
The
.Fn pthread_cond_timedwait
function may additionally fail if:
.Bl -tag -width Er
.It Bq Er ETIMEDOUT
The system time has reached or exceeded the time specified in
.Fa abstime .
.El
.Sh SEE ALSO
.Xr pthread 3 ,
.Xr pthread_barrier 3 ,
.Xr pthread_condattr 3 ,
.Xr pthread_mutex 3 ,
.Xr pthread_rwlock 3 ,
.Xr pthread_spin 3
.Sh STANDARDS
These functions conform to
.St -p1003.1-2001 .