Producer and Consumer Problem Using Semaphores

The data structure in Example 4–14 is similar to the structure used for the condition variables example, shown in Example 4–11. Two semaphores represent the number of full and empty buffers. The semaphores ensure that producers wait until buffers are empty and that consumers wait until buffers are full.

Example 4–14 Producer and Consumer Problem With Semaphores

typedef struct {
    char buf[BSIZE];
    sem_t occupied;
    sem_t empty;
    int nextin;
    int nextout;
    sem_t pmut;
    sem_t cmut;
} buffer_t;

buffer_t buffer;

sem_init(&buffer.occupied, 0, 0);
sem_init(&buffer.empty,0, BSIZE);
sem_init(&buffer.pmut, 0, 1);
sem_init(&buffer.cmut, 0, 1);
buffer.nextin = buffer.nextout = 0;

Another pair of binary semaphores plays the same role as mutexes. The semaphores control access to the buffer when multiple producers use multiple empty buffer slots, and when multiple consumers use multiple full buffer slots. Mutexes would work better here, but would not provide as good an example of semaphore use.

Example 4–15 Producer and Consumer Problem: the Producer

void producer(buffer_t *b, char item) {
    sem_wait(&b->empty);
    sem_wait(&b->pmut);

    b->buf[b->nextin] = item;
    b->nextin++;
    b->nextin %= BSIZE;

    sem_post(&b->pmut);
    sem_post(&b->occupied);
}

Example 4–16 Producer and Consumer Problem: the Consumer

char consumer(buffer_t *b) {
    char item;

    sem_wait(&b->occupied);
   
    sem_wait(&b->cmut);

    item = b->buf[b->nextout];
    b->nextout++;
    b->nextout %= BSIZE;

    sem_post(&b->cmut);

    sem_post(&b->empty);

    return(item);
}