11.1: POSIX Semaphores

A semaphore is a data structure used to help threads work together without interfering with each other.

The POSIX standard specifies an interface for semaphores; it is not part of Pthreads, but most UNIXes that implement Pthreads also provide semaphores.

POSIX semaphores have type sem_t. As usual, I put a wrapper around sem_t to make it easier to use. The interface is defined in sem.h:

typedef sem_t Semaphore;

Semaphore *make_semaphore(int value);
void semaphore_wait(Semaphore *sem);
void semaphore_signal(Semaphore *sem);

Semaphore is a synonym for sem_t, but I find it more readable, and the capital letter reminds me to treat it like an object and pass it by pointer.

The implementation of these functions is in sem.c:

Semaphore *make_semaphore(int value)
{
    Semaphore *sem = check_malloc(sizeof(Semaphore));
    int n = sem_init(sem, 0, value);
    if (n != 0) perror_exit("sem_init failed");
    return sem;
}

make_semaphore takes the initial value of the semaphore as a parameter. It allocates space for a Semaphore, initializes it, and returns a pointer to Semaphore.

sem_init returns 0 if it succeeds and -1 if anything goes wrong. One nice thing about using wrapper functions is that you can encapsulate the error-checking code, which makes the code that uses these functions more readable.

Here is the implementation of semaphore_wait:

void semaphore_wait(Semaphore *sem)
{
    int n = sem_wait(sem);
    if (n != 0) perror_exit("sem_wait failed");
}

And here is semaphore_signal:

void semaphore_signal(Semaphore *sem)
{
    int n = sem_post(sem);
    if (n != 0) perror_exit("sem_post failed");
}

I prefer to call this operation “signal” rather than “post”, although both terms are common.

Here’s an example that shows how to use a semaphore as a mutex:

Semaphore *mutex = make_semaphore(1);

semaphore_wait(mutex);
  // protected code goes here
semaphore_signal(mutex);

When you use a semaphore as a mutex, you usually initialize it to 1 to indicate that the mutex is unlocked; that is, one thread can pass the semaphore without blocking.

Here I am using the variable name mutex to indicate that the semaphore is being used as a mutex. But remember that the behavior of a semaphore is not the same as a Pthread mutex.