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 29, Producer and Consumer Problem Using USYNC_PROCESS 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 29, Producer and Consumer Problem Using USYNC_PROCESS is the same as that used for the solution with condition variables. See Examples of Using Nested Locking With a Singly-Linked List.
Example 29 Producer and Consumer Problem Using USYNC_PROCESSmain() { 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.