This is procexec/fork_sig_sync.c (Listing 24-6, page 528), 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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/* fork_sig_sync.c

   Demonstrate how signals can be used to synchronize the actions
   of a parent and child process.
#include <signal.h>
#include "curr_time.h"                  /* Declaration of currTime() */
#include "tlpi_hdr.h"

#define SYNC_SIG SIGUSR1                /* Synchronization signal */
static void             /* Signal handler - does nothing but return */
handler(int sig)
main(int argc, char *argv[])
    pid_t childPid;
    sigset_t blockMask, origMask, emptyMask;
    struct sigaction sa;

    setbuf(stdout, NULL);               /* Disable buffering of stdout */

    sigaddset(&blockMask, SYNC_SIG);    /* Block signal */
    if (sigprocmask(SIG_BLOCK, &blockMask, &origMask) == -1)

    sa.sa_flags = SA_RESTART;
    sa.sa_handler = handler;
    if (sigaction(SYNC_SIG, &sa, NULL) == -1)

    switch (childPid = fork()) {
    case -1:

    case 0: /* Child */

        /* Child does some required action here... */

        printf("[%s %ld] Child started - doing some work\n",
                currTime("%T"), (long) getpid());
        sleep(2);               /* Simulate time spent doing some work */

        /* And then signals parent that it's done */

        printf("[%s %ld] Child about to signal parent\n",
                currTime("%T"), (long) getpid());
        if (kill(getppid(), SYNC_SIG) == -1)

        /* Now child can do other things... */


    default: /* Parent */

        /* Parent may do some work here, and then waits for child to
           complete the required action */

        printf("[%s %ld] Parent about to wait for signal\n",
                currTime("%T"), (long) getpid());
        if (sigsuspend(&emptyMask) == -1 && errno != EINTR)
        printf("[%s %ld] Parent got signal\n", currTime("%T"), (long) getpid());

        /* If required, return signal mask to its original state */

        if (sigprocmask(SIG_SETMASK, &origMask, NULL) == -1)

        /* Parent carries on to do other things... */



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