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.Dd "Jan 11, 2015" .Dt CND 3C .Os .Sh NAME .Nm cnd .Nm cnd_broadcast , .Nm cnd_destroy , .Nm cnd_init , .Nm cnd_signal , .Nm cnd_timedwait , .Nm cnd_wait .Nd C11 condition variable functions .Sh SYNOPSIS n threads.h .Ft int .Fo cnd_init .Fa "cnd_t *cnd" .Fc .Ft void .Fo cnd_destroy .Fa "cnd_t *cnd" .Fc .Ft int .Fo cnd_broadcast .Fa "cnd_t *cnd" .Fc .Ft int .Fo cnd_signal .Fa "cnd_t *cnd" .Fc .Ft int .Fo cnd_timedwait .Fa "cnd_t *restrict cnd" .Fa "mtx_t *restrict mtx" .Fa "const struct timespec *abstime" .Fc .Ft int .Fo cnd_wait .Fa "cnd_t *restrict cnd" .Fa "mtx_t *restrict mtx" .Fc .Sh DESCRIPTION The .Sy cnd 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 .Xr threads 5 . Importantly, they do not allow for inter-process synchronization. .Ss Creating and Destroy Condition Variables The function .Fn cnd_init initializes the condition variable referred to by .Fa cnd . The condition variable is suitable for intra-process use. Initializing an already initialized condition variable results in undefined behavior.

p The function .Fn cnd_destroy destroys an initialized condition variable at which point it is illegal to use it, though it may be initialized again. .Ss Condition Waiting The function .Fn 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.

p The function .Fn cond_wait atomically release the mutex pointed to by .Fa mtx and blocks on the condition variable .Fa cond . When the thread returns, it will once again be holding .Fa 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 .Fn fork is called .

p The function .Fn cond_timedwait allows a thread to block in a similar fashion to .Fn cond_wait , except that when the absolute time specified in seconds since the epoch (based on .Sy CLOCK_REALTIME ) in UTC, expires, then the thread will be woken up. The timeout is specified in .Fa abstime . .Ss Conditional Signaling The .Fn cnd_signal and .Fn cnd_broadcast functions can be used to signal threads waiting on the condition variable .Fa cnd that they should be woken up and check the variable again. The .Fn cnd_signal function will only wake a single thread that is blocked on the condition variable .Fa cnd ; while .Fn cnd_broadcast will wake up every thread waiting on the condition variable .Fa cnd .

p A thread calling either .Fn cnd_signal or .Fn cnd_broadcast is not required to hold any of the mutexes that are associated with the condition variable.

p If there are no threads currently blocked in the condition variable .Fa cnd then neither function has an effect. .Sh RETURN VALUES Upon successful completion, the .Fn cond_init function returns .Sy thrd_success. If insufficient memory was available, then .Sy thrd_nomem is returned; otherwise, if any other error occurred, .Sy thrd_error is returned.

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

p Upon successful completion, the .Fn cond_timedwait function returns .Sy thrd_success . If .Fa abstime expires without being signaled, it instead returns .Sy thrd_timedout . Otherwise, .Sy thrd_error is returned to indicate an error. .Sh INTERFACE STABILITY .Sy Standard .Sh MT-LEVEL .Sy MT-Safe .Sh SEE ALSO .Xr cond_braodcast 3C , .Xr cond_destroy 3C , .Xr cond_init 3C , .Xr cond_signal 3C , .Xr cond_timedwait 3C , .Xr cond_wait 3C , .Xr threads 3HEAD , .Xr attributes 5 , .Xr threads 5