Why are sem_init(), sem_getvalue(), sem_destroy() deprecated on Mac OS X — and what replaces them?

Question

When I compile a program using the POSIX sem_init() function, I get a compilation warning (error because I normally use -Werror) that the function has been deprecat

Answer 1

I ran into this problem myself when trying to port a library I was working on to OS X. I searched for a while without finding a great answer. When I did find the answer, I was a bit perturbed: the answer is effectively "if Apple implemented POSIX unnamed semaphores, how many X Serves would you buy?".

To summarize the points of why they are deprecated and why some of the functionality remains unimplemented:

• Appendix 9 of the Single UNIX Specification states they are not mandatory interfaces
• "Most portable code" uses SYSV semaphores
• Backwards compatibility with POSIX named semaphores, which share the sem_t type is difficult

As for what to do instead, I went with GCD semaphores. As to why the replacement is preferred: it's the only native unnamed semaphore interface available on vanilla OS X. Apparently GCD helped them sell more X Serves. I fear there's not a better answer.

However, hopefully some code will be helpful. The upshot of all of this is that you effectively have to implement your own portable semaphore interface:

#ifdef __APPLE__
#include <dispatch/dispatch.h>
#else
#include <semaphore.h>
#endif

struct rk_sema {
#ifdef __APPLE__
    dispatch_semaphore_t    sem;
#else
    sem_t                   sem;
#endif
};


static inline void
rk_sema_init(struct rk_sema *s, uint32_t value)
{
#ifdef __APPLE__
    dispatch_semaphore_t *sem = &s->sem;

    *sem = dispatch_semaphore_create(value);
#else
    sem_init(&s->sem, 0, value);
#endif
}

static inline void
rk_sema_wait(struct rk_sema *s)
{

#ifdef __APPLE__
    dispatch_semaphore_wait(s->sem, DISPATCH_TIME_FOREVER);
#else
    int r;

    do {
            r = sem_wait(&s->sem);
    } while (r == -1 && errno == EINTR);
#endif
}

static inline void
rk_sema_post(struct rk_sema *s)
{

#ifdef __APPLE__
    dispatch_semaphore_signal(s->sem);
#else
    sem_post(&s->sem);
#endif
}

This was the minimal set of functionality I cared about; your needs may vary. Hopefully this is helpful.