This is procexec/multi_SIGCHLD.c (Listing 26-5, page 557), 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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/* multi_SIGCHLD.c

   Demonstrate the use of a handler for the SIGCHLD signal, and that multiple
   SIGCHLD signals are not queued while the signal is blocked during the
   execution of the handler.
#include <signal.h>
#include <sys/wait.h>
#include "print_wait_status.h"
#include "curr_time.h"
#include "tlpi_hdr.h"

static volatile int numLiveChildren = 0;
                /* Number of children started but not yet waited on */
static void
sigchldHandler(int sig)
    int status, savedErrno;
    pid_t childPid;

    /* UNSAFE: This handler uses non-async-signal-safe functions
       (printf(), printWaitStatus(), currTime(); see Section 21.1.2) */

    savedErrno = errno;         /* In case we modify 'errno' */

    printf("%s handler: Caught SIGCHLD\n", currTime("%T"));

    /* Do nonblocking waits until no more dead children are found */

    while ((childPid = waitpid(-1, &status, WNOHANG)) > 0) {
        printf("%s handler: Reaped child %ld - ", currTime("%T"),
                (long) childPid);
        printWaitStatus(NULL, status);

    if (childPid == -1 && errno != ECHILD)

    sleep(5);           /* Artificially lengthen execution of handler */
    printf("%s handler: returning\n", currTime("%T"));

    errno = savedErrno;
main(int argc, char *argv[])
    int j, sigCnt;
    sigset_t blockMask, emptyMask;
    struct sigaction sa;

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

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

    sigCnt = 0;
    numLiveChildren = argc - 1;

    sa.sa_flags = 0;
    sa.sa_handler = sigchldHandler;
    if (sigaction(SIGCHLD, &sa, NULL) == -1)

    /* Block SIGCHLD to prevent its delivery if a child terminates
       before the parent commences the sigsuspend() loop below */

    sigaddset(&blockMask, SIGCHLD);
    if (sigprocmask(SIG_SETMASK, &blockMask, NULL) == -1)

    /* Create one child process for each command-line argument */

    for (j = 1; j < argc; j++) {
        switch (fork()) {
        case -1:

        case 0:         /* Child - sleeps and then exits */
            sleep(getInt(argv[j], GN_NONNEG, "child-sleep-time"));
            printf("%s Child %d (PID=%ld) exiting\n", currTime("%T"),
                    j, (long) getpid());

        default:        /* Parent - loops to create next child */

    /* Parent comes here: wait for SIGCHLD until all children are dead */

    while (numLiveChildren > 0) {
        if (sigsuspend(&emptyMask) == -1 && errno != EINTR)

    printf("%s All %d children have terminated; SIGCHLD was caught "
            "%d times\n", currTime("%T"), argc - 1, sigCnt);



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