PTHREAD_COND(3) BSD Library Functions Manual PTHREAD_COND(3)NAME
pthread_cond — condition variable interface
LIBRARY
POSIX Threads Library (libpthread, -lpthread)
SYNOPSIS
#include <pthread.h>
int
pthread_cond_init(pthread_cond_t * restrict cond,
const pthread_condattr_t * restrict attr);
int
pthread_cond_destroy(pthread_cond_t *cond);
int
pthread_cond_broadcast(pthread_cond_t *cond);
int
pthread_cond_signal(pthread_cond_t *cond);
int
pthread_cond_wait(pthread_cond_t * restrict cond,
pthread_mutex_t * restrict mutex);
int
pthread_cond_timedwait(pthread_cond_t * restrict cond,
pthread_mutex_t * restrict mutex,
const struct timespec * restrict abstime);
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
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.
The pthread_cond_init() function creates a new condition variable, with
attributes specified with attr. If attr is NULL the default attributes
are used. The pthread_cond_destroy() function frees the resources allo‐
cated by the condition variable cond.
The macro 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 pthread_cond_init()
with attr specified as NULL, except that no error checking is done.
The difference between pthread_cond_broadcast() and pthread_cond_signal()
is that the former unblocks all threads waiting for the condition vari‐
able, whereas the latter blocks only one waiting thread. If no threads
are waiting on 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 fol‐
low if the mutex is not held.
The pthread_cond_wait() function atomically blocks the current thread
waiting on the condition variable specified by cond, and unlocks the
mutex specified by mutex. The pthread_cond_timedwait() function behaves
similarly, but unblocks also if the system time reaches the time speci‐
fied in abstime, represented as struct timespec (see timespec(3)). With
both functions the waiting thread unblocks after another thread calls
pthread_cond_signal() or pthread_cond_broadcast() with the same condition
variable and by holding the same mutex that was associated with cond by
either one of the blocking functions. The current thread holds the lock
on mutex upon return from either function.
Note that a call to pthread_cond_wait() or pthread_cond_timedwait() may
wake up spontaneously, without a call to pthread_cond_signal() or
pthread_cond_broadcast(). The caller should prepare for this by invoking
either function within a predicate loop that tests whether the thread
should proceed.
As noted, when calling either function that waits on a condition vari‐
able, a temporary binding is established between the condition variable
cond and the mutex 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 cond. Additionally, the same mutex must be used for concurrent calls
to pthread_cond_wait() and pthread_cond_timedwait(). Only when a condi‐
tion 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.
RETURN VALUES
If successful, all functions return zero. Otherwise, an error number
will be returned to indicate the error.
ERRORS
The pthread_cond_init() function may fail if:
[EINVAL] The value specified by attr is invalid.
The pthread_cond_destroy() function may fail if:
[EBUSY] The variable cond is locked by another thread.
[EINVAL] The value specified by cond is invalid.
Both pthread_cond_broadcast() and pthread_cond_signal() may fail if:
[EINVAL] The value specified by cond is invalid.
Both pthread_cond_wait() and pthread_cond_timedwait() may fail if:
[EINVAL] The value specified by cond or the value specified by
mutex is invalid.
[EPERM] The value specified by mutex was not locked in the
condition wait.
The pthread_cond_timedwait() function may additionally fail if:
[ETIMEDOUT] The system time has reached or exceeded the time spec‐
ified in abstime.
SEE ALSOpthread(3), pthread_barrier(3), pthread_condattr(3), pthread_mutex(3),
pthread_rwlock(3), pthread_spin(3)STANDARDS
These functions conform to IEEE Std 1003.1-2001 (“POSIX.1”).
BSD July 8, 2010 BSD
