Producer/Consumer Problem Example

Example 8–2 shows the producer/consumer problem with the producer and consumer in separate processes. The main routine maps zero-filled memory (that it shares with its child process) into its address space. Note that mutex_init() and cond_init() must be called because the type of the synchronization variables is USYNC_PROCESS.

A child process is created that runs the consumer. The parent runs the producer.

This example also shows the drivers for the producer and consumer. The producer_driver() simply reads characters from stdin and calls producer(). The consumer_driver() gets characters by calling consumer() and writes them to stdout.

The data structure for Example 8–2 is the same as that used for the solution with condition variables (see Nested Locking With a Singly Linked List).

Example 8–2 The Producer/Consumer Problem, Using USYNC_PROCESS

main() {
    int zfd;
    buffer_t *buffer;

    zfd = open(“/dev/zero”, O_RDWR);
    buffer = (buffer_t *)mmap(NULL, sizeof(buffer_t),
        PROT_READ|PROT_WRITE, MAP_SHARED, zfd, 0);
    buffer->occupied = buffer->nextin = buffer->nextout = 0;

    mutex_init(&buffer->lock, USYNC_PROCESS, 0);
    cond_init(&buffer->less, USYNC_PROCESS, 0);
    cond_init(&buffer->more, USYNC_PROCESS, 0);
    if (fork() == 0)
        consumer_driver(buffer);
    else
        producer_driver(buffer);
}

void producer_driver(buffer_t *b) {
    int item;

    while (1) {
        item = getchar();
        if (item == EOF) {
            producer(b, `\0');
            break;
        } else
            producer(b, (char)item);
    }
}

void consumer_driver(buffer_t *b) {
    char item;

    while (1) {
        if ((item = consumer(b)) == '\0')
            break;
        putchar(item);
    }
}

A child process is created to run the consumer; the parent runs the producer.