sem_post(3)sem_post(3)NAMEsem_post - Unlocks a semaphore (P1003.1b)
SYNOPSIS
#include <semaphore.h>
int sem_post ( sem_t *sem);
LIBRARY
Realtime Library (librt.so, librt.a)
PARAMETERS
Pointer to the semaphore to be unlocked.
DESCRIPTION
The sem_post function unlocks the specified semaphore by performing the
semaphore unlock operation on that semaphore. The appropriate function
(sem_open for named semaphores or sem_init for unnamed semaphores) must
be called for a semaphore before you can call the locking and unlocking
functions, sem_wait, sem_trywait, and sem_post.
If the semaphore value after a sem_post function is positive, no pro‐
cesses were blocked waiting for the semaphore to be unlocked; the sema‐
phore value is incremented. If the semaphore value after a sem_post
function is zero, one of the processes blocked waiting for the sema‐
phore is allowed to return successfully from its call to sem_wait.
If more than one process is blocked while waiting for the semaphore,
only one process is unblocked and the state of the semaphore remains
unchanged when the sem_post function returns. The process to be
unblocked is selected according to the scheduling policies and priori‐
ties of all blocked processes. If the scheduling policy is SCHED_FIFO
or SCHED_RR, the highest-priority waiting process is unblocked. If more
than one process of that priority is blocked, then the process that has
waited the longest is unblocked.
The sem_post function can be called from a signal-catching function.
RETURN VALUES
On successful completion, the sem_post function returns the value 0
(zero) and performs a semaphore unlock operation, unblocking a process.
Otherwise, the function returns the value -1 and sets errno to indicate
the error. The state of the semaphore remains unchanged.
ERRORS
The sem_post function fails under the following condition: The sem does
not refer to a valid semaphore.
SEE ALSO
Functions: sem_trywait(3), sem_wait(3)
Guide to Realtime Programming
sem_post(3)