CND(3C) Standard C Library Functions CND(3C)

cnd, cnd_broadcast, cnd_destroy, cnd_init, cnd_signal, cnd_timedwait, cnd_wait — C11 condition variable functions

#include <threads.h>

int
cnd_init(cnd_t *cnd);

void
cnd_destroy(cnd_t *cnd);

int
cnd_broadcast(cnd_t *cnd);

int
cnd_signal(cnd_t *cnd);

int
cnd_timedwait(cnd_t *restrict cnd, mtx_t *restrict mtx, const struct timespec *abstime);

int
cnd_wait(cnd_t *restrict cnd, mtx_t *restrict mtx);

The family of functions implement condition variables which allow threads within a process to wait until a condition occurs and be signaled when it does. These functions behave similar to both the POSIX threads and illumos threads; however, they have slightly different call signatures and return values. For more information, see threads(7). Importantly, they do not allow for inter-process synchronization.

The function () initializes the condition variable referred to by cnd. The condition variable is suitable for intra-process use. Initializing an already initialized condition variable results in undefined behavior.

The function () destroys an initialized condition variable at which point it is illegal to use it, though it may be initialized again.

The function cond_wait() can be used to wait on a condition variable. A thread that waits on a condition variable blocks until another thread signals that the condition has changed, generally after making the condition that was false, true.

The function () atomically release the mutex pointed to by mtx and blocks on the condition variable cond. When the thread returns, it will once again be holding mtx and must check the current state of the condition. There is no guarantee that another thread has not gotten in and changed the value before being woken. In addition, a thread blocking on a condition variable, may be woken spuriously, such as when a signal is received or () is called .

The function () allows a thread to block in a similar fashion to cond_wait(), except that when the absolute time specified in seconds since the epoch (based on ) in UTC, expires, then the thread will be woken up. The timeout is specified in abstime.

The cnd_signal() and cnd_broadcast() functions can be used to signal threads waiting on the condition variable cnd that they should be woken up and check the variable again. The cnd_signal() function will only wake a single thread that is blocked on the condition variable cnd; while cnd_broadcast() will wake up every thread waiting on the condition variable cnd.

A thread calling either () or () is not required to hold any of the mutexes that are associated with the condition variable.

If there are no threads currently blocked in the condition variable cnd then neither function has an effect.

Upon successful completion, the cond_init() function returns If insufficient memory was available, then is returned; otherwise, if any other error occurred, thrd_error is returned.

Upon successful completion, the cond_broadcast(), cond_signal(), and cond_wait() functions return thrd_success. Otherwise, they return thrd_error to indicate that an error occurred and they were unable to complete.

Upon successful completion, the cond_timedwait() function returns thrd_success. If abstime expires without being signaled, it instead returns . Otherwise, thrd_error is returned to indicate an error.

cond_broadcast(3C), cond_destroy(3C), cond_init(3C), cond_signal(3C), cond_timedwait(3C), cond_wait(3C), threads.h(3HEAD), attributes(7), threads(7)

January 11, 2015 OmniOS