Multithreaded Programming Guide

Synchronizing Across Process Boundaries

Each of the synchronization primitives can be set up to be used across process boundaries. This cross-boundary setup is done by ensuring that the synchronization variable is located in a shared memory segment and by calling the appropriate init routine with type set to USYNC_PROCESS.

If type is set to USYNC_PROCESS, then the operations on the synchronization variables work just as the variables do when type is USYNC_THREAD.

mutex_init(&m, USYNC_PROCESS, 0);
rwlock_init(&rw, USYNC_PROCESS, 0);
cond_init(&cv, USYNC_PROCESS, 0);
sema_init(&s, count, USYNC_PROCESS, 0);

Example of Producer and Consumer Problem

Example 6–2 shows the producer and consumer problem with the producer and consumer in separate processes. The main routine maps zero-filled memory that main 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 to run the consumer. The parent runs the producer.

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

The data structure for Example 6–2 is the same as that used for the solution with condition variables. See Examples of Using Nested Locking With a Singly-Linked List .

Example 6–2 Producer and 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),
    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)

void producer_driver(buffer_t *b) {
    int item;

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

void consumer_driver(buffer_t *b) {
    char item;

    while (1) {
        if ((item = consumer(b)) == '\0')

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