PTHREAD_COND(3)

PTHREAD_COND(3) | Library Functions Manual | PTHREAD_COND(3)

# NAME

`pthread_cond`, `pthread_cond_init`, `pthread_cond_destroy`, `pthread_cond_broadcast`, `pthread_cond_signal`, `pthread_cond_wait`, `pthread_cond_timedwait` —
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);

pthread_cond_t cond = `PTHREAD_COND_INITIALIZER`;

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);

# 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 allocated 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 variable, whereas the latter unblocks 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 follow 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 specified 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 variable, 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 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.

# 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 specified in abstime.

# SEE ALSO

pthread(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”).

July 8, 2010 | NetBSD 10.1