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)