sem_init on OS X

Question

I am working on some code which uses the pthread and semaphore libraries. The sem_init function works fine on my Ubuntu machine, but on OS X the sem_init function has absolutely no effect. Is there something wrong with the library or is there a different way of doing it? This is the code I am using to test.

sem_t sem1; sem_t sem2; sem_t sem3; sem_t sem4; sem_t sem5; sem_t sem6;  sem_init(&sem1, 1, 1); sem_init(&sem2, 1, 2); sem_init(&sem3, 1, 3); sem_init(&sem4, 1, 4); sem_init(&sem5, 1, 5); sem_init(&sem6, 1, 6); 

The values appear to be random numbers, and they do not change after the sem_init call.

Answer 1

Unnamed semaphores are not supported, you need to use named semaphores.

To use named semaphores instead of unnamed semaphores, use sem_open instead of sem_init, and use sem_close and sem_unlink instead of sem_destroy.

Answer 2

A better solution (these days) than named semaphores on OS X is Grand Central Dispatch's dispatch_semaphore_t. It works very much like the unnamed POSIX semaphores.

Initialize the semaphore:

#include <dispatch/dispatch.h> dispatch_semaphore_t semaphore; semaphore = dispatch_semaphore_create(1); // init with value of 1 

Wait & post (signal):

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER); ... dispatch_semaphore_signal(semaphore); 

Destroy:

dispatch_release(semaphore); 

The header file is well documented and I found it quite easy to use.