\" $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 .

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