pipes/pipe_sync.c

/* pipe_sync.c

   Show how pipes can be used for synchronizing the actions of a parent and
   multiple child processes.

   Usage: pipe_sync sleep-time...

   After creating a pipe, the program creates one child for each command-line
   argument. Each child simulates doing some work by sleeping for the number of
   seconds specified in the corresponding command-line argument. When it has
   finished doing its "work", each child closes its file descriptor for the
   write end of the pipe; the parent can see that all children have finished
   their work when it sees end-of-file on the read end of the pipe.
*/
#include "curr_time.h"                      /* Declaration of currTime() */
#include "tlpi_hdr.h"

int
main(int argc, char *argv[])
{
    int pfd[2];                             /* Process synchronization pipe */
    int j, dummy;

    if (argc < 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s sleep-time...\n", argv[0]);

    setbuf(stdout, NULL);                   /* Make stdout unbuffered, since we
                                               terminate child with _exit() */
    printf("%s  Parent started\n", currTime("%T"));

    if (pipe(pfd) == -1)
        errExit("pipe");

    for (j = 1; j < argc; j++) {
        switch (fork()) {
        case -1:
            errExit("fork %d", j);

        case 0: /* Child */
            if (close(pfd[0]) == -1)        /* Read end is unused */
                errExit("close");

            /* Child does some work, and lets parent know it's done */

            sleep(getInt(argv[j], GN_NONNEG, "sleep-time"));
                                            /* Simulate processing */
            printf("%s  Child %d (PID=%ld) closing pipe\n",
                    currTime("%T"), j, (long) getpid());
            if (close(pfd[1]) == -1)
                errExit("close");

            /* Child now carries on to do other things... */

            _exit(EXIT_SUCCESS);

        default: /* Parent loops to create next child */
            break;
        }
    }

    /* Parent comes here; close write end of pipe so we can see EOF */

    if (close(pfd[1]) == -1)                /* Write end is unused */
        errExit("close");

    /* Parent may do other work, then synchronizes with children */

    if (read(pfd[0], &dummy, 1) != 0)
        fatal("parent didn't get EOF");
    printf("%s  Parent ready to go\n", currTime("%T"));

    /* Parent can now carry on to do other things... */

    exit(EXIT_SUCCESS);
}