Multithreaded Programming Guide

Decrementing a Semaphore Count

Use sem_trywait(3RT) to try to atomically decrement the count in the semaphore pointed to by sem when the count is greater than zero.

sem_trywait Syntax

int sem_trywait(sem_t *sem);
#include <semaphore.h>

sem_t sem;
int ret;

ret = sem_trywait(&sem); /* try to wait for semaphore*/

This function is a nonblocking version of sem_wait(). sem_trywait() returns immediately if unsuccessful.

sem_trywait Return Values

sem_trywait() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.



sem points to an illegal address.



A signal interrupted this function.



The semaphore was already locked, so the semaphore cannot be immediately locked by the sem_trywait() operation.