pthread_cond_signal(3T)pthread_cond_signal(3T)NAMEpthread_cond_signal(), pthread_cond_broadcast() - unblock one or all
threads waiting on a condition variable
SYNOPSISPARAMETERS
cond Pointer to the condition variable to be signaled or
broadcast.
DESCRIPTION
The 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 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 ini‐
tialized by a call to or statically initialized with the macro
An unblocked thread will reacquire the mutex it held when it started
the condition wait before returning from or The threads that are
unblocked contend for the mutex according to their scheduling policy
and priority.
The or functions can be called by a thread whether or not it currently
owns the condition variable's associated mutex. For predictable sched‐
uling 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 and 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, and return zero. Otherwise, an error number
is returned to indicate the error (the variable is not set).
ERRORS
For each of the following conditions, if the condition is detected, the
and functions return the corresponding error number:
[EINVAL] cond is not a valid condition variable.
[EFAULT] cond parameter points to an illegal address.
AUTHOR
and were derived from the IEEE POSIX P1003.1c standard.
SEE ALSOpthread_cond_init(3T), pthread_cond_wait(3T).
STANDARDS CONFORMANCE
Pthread Library pthread_cond_signal(3T)
