pipes/pipe_sync.c

This is pipes/pipe_sync.c (Listing 44-3, page 897), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* 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);
}

 

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