SEM_POST(3C) Standard C Library Functions SEM_POST(3C)

NAME

sem_post - increment the count of a semaphore

SYNOPSIS

#include <semaphore.h>

int sem_post(sem_t *sem);

DESCRIPTION

The sem_post() function unlocks the semaphore referenced by sem by
performing a semaphore unlock operation on that semaphore.


If the semaphore value resulting from this operation is positive,
then no threads were blocked waiting for the semaphore to become
unlocked; the semaphore value is simply incremented.


If the value of the semaphore resulting from this operation is 0,
then one of the threads blocked waiting for the semaphore will be
allowed to return successfully from its call to sem_wait(3C). If the
symbol _POSIX_PRIORITY_SCHEDULING is defined, the thread to be
unblocked will be chosen in a manner appropriate to the scheduling
policies and parameters in effect for the blocked threads. In the
case of the schedulers SCHED_FIFO and SCHED_RR, the highest priority
waiting thread will be unblocked, and if there is more than one
highest priority thread blocked waiting for the semaphore, then the
highest priority thread that has been waiting the longest will be
unblocked. If the symbol _POSIX_PRIORITY_SCHEDULING is not defined,
the choice of a thread to unblock is unspecified.

RETURN VALUES

If successful, sem_post() returns 0; otherwise it returns -1 and sets
errno to indicate the error.

ERRORS

The sem_post() function will fail if:

EINVAL
The sem argument does not refer to a valid semaphore.


ENOSYS
The sem_post() function is not supported by the system.


EOVERFLOW
The semaphore value exceeds SEM_VALUE_MAX.

USAGE

The sem_post() function is reentrant with respect to signals and may
be invoked from a signal-catching function. The semaphore
functionality described on this manual page is for the POSIX (see
standards(7)) threads implementation. For the documentation of the
Solaris threads interface, see semaphore(3C)).

EXAMPLES

See sem_wait(3C).

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Committed |
+--------------------+-------------------+
|MT-Level | Async-Signal-Safe |
+--------------------+-------------------+
|Standard | See standards(7). |
+--------------------+-------------------+

SEE ALSO

sched_setscheduler(3C), sem_wait(3C), semaphore(3C), attributes(7),
standards(7)

February 5, 2008 SEM_POST(3C)