pthread_cond_signal(3T)

NAME

pthread_cond_signal(), pthread_cond_broadcast() — unblock one or all threads waiting on a condition variable

SYNOPSIS

#include <pthread.h>

int pthread_cond_signal( 
   pthread_cond_t *cond 
); 

int pthread_cond_broadcast( 
   pthread_cond_t *cond 
); 

PARAMETERS

cond: Pointer to the condition variable to be signaled or broadcast.

DESCRIPTION

The pthread_cond_signal() function is used to wake-up one of the threads that are waiting for the occurrence of a condition associated with condition variable cond. If there are no threads blocked on cond, this function has no effect. If more than one thread is blocked on cond, the scheduling policy determines which thread is unblocked. It is possible that more than one thread can be unblocked due to a spurious wakeup.

The pthread_cond_broadcast() function is used to wake-up all threads that are waiting for the occurrence of a condition associated with the condition variable cond. If there are no threads blocked on cond, this function has no effect. If more than one thread is blocked on cond, the scheduling policy determines the order in which threads are unblocked.

The condition variabled denoted by cond must have been dynamically initialized by a call to pthread_cond_init() or statically initialized with the macro PTHREAD_COND_INITIALIZER.

An unblocked thread will reacquire the mutex it held when it started the condition wait before returning from pthread_cond_wait() or pthread_cond_timedwait(). The threads that are unblocked contend for the mutex according to their scheduling policy and priority.

The pthread_cond_signal() or pthread_cond_broadcast() functions can be called by a thread whether or not it currently owns the condition variable's associated mutex. For predictable scheduling behavior and to prevent lost wake-ups, the mutex should be held when signaling a condition variable.

Usage

When using condition variables, there is a boolean predicate associated with each condition wait. If this predicate is false, the thread should do a condition wait. Spurious wakeups may occur when waiting on a condition variable. Because the return values from pthread_cond_wait() and pthread_cond_timedwait() do not imply anything about the value of this predicate, the predicate should always be re-evaluated.

Applications using condition variables typically acquire a mutex and enter a loop which checks the predicate. Depending on the value of the predicate, the thread either breaks out of the loop or waits on the condition. On return from the condition wait, the predicate is re-evaluated.

RETURN VALUE

Upon successful completion, pthread_cond_signal() and pthread_cond_broadcast() return zero. Otherwise, an error number is returned to indicate the error (the errno variable is not set).

ERRORS

For each of the following conditions, if the condition is detected, the pthread_cond_signal() and pthread_cond_broadcast() functions return the corresponding error number:

[EINVAL]: cond is not a valid condition variable.
[EFAULT]: cond parameter points to an illegal address.

AUTHOR

pthread_cond_signal() and pthread_cond_broadcast() were derived from the IEEE POSIX P1003.1c standard.

STANDARDS CONFORMANCE

pthread_cond_signal(): POSIX 1003.1c.
pthread_cond_broadcast(): POSIX 1003.1c.