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35.1.5 Condition Variables

Mutexes are not the only synchronization mechanisms available. For some more complex tasks, the GNU C Library also implements condition variables, which allow the programmer to think at a higher level when solving complex synchronization problems. They are used to synchronize threads waiting on a certain condition to happen.

The fundamental data type for condition variables is the cnd_t:

Data Type: cnd_t

The cnd_t uniquely identifies a condition variable object.

The following functions are used for working with condition variables:

Function: int cnd_init (cnd_t *cond)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

cnd_init initializes a new condition variable, identified by cond.

This function may return thrd_success, thrd_nomem, or thrd_error.

Function: int cnd_signal (cnd_t *cond)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

cnd_signal unblocks one thread that is currently waiting on the condition variable pointed to by cond. If a thread is successfully unblocked, this function returns thrd_success. If no threads are blocked, this function does nothing and returns thrd_success. Otherwise, this function returns thrd_error.

Function: int cnd_broadcast (cnd_t *cond)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

cnd_broadcast unblocks all the threads that are currently waiting on the condition variable pointed to by cond. This function returns thrd_success on success. If no threads are blocked, this function does nothing and returns thrd_success. Otherwise, this function returns thrd_error.

Function: int cnd_wait (cnd_t *cond, mtx_t *mutex)

Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock | See POSIX Safety Concepts.

cnd_wait atomically unlocks the mutex pointed to by mutex and blocks on the condition variable pointed to by cond until the thread is signaled by cnd_signal or cnd_broadcast. The mutex is locked again before the function returns.

This function returns either thrd_success or thrd_error.

Function: int cnd_timedwait (cnd_t *restrict cond, mtx_t *restrict mutex, const struct timespec *restrict time_point)

Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock | See POSIX Safety Concepts.

cnd_timedwait atomically unlocks the mutex pointed to by mutex and blocks on the condition variable pointed to by cond until the thread is signaled by cnd_signal or cnd_broadcast, or until the calendar time pointed to by time_point has been reached. The mutex is locked again before the function returns.

As for mtx_timedlock, since this function takes an absolute time, if a duration is required, the calendar time must be calculated manually. See Time Basics, and Calendar Time.

This function may return thrd_success, thrd_nomem, or thrd_error.

Function: void cnd_destroy (cnd_t *cond)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

cnd_destroy destroys the condition variable pointed to by cond. If there are threads waiting on cond, the behavior is undefined.


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