This is procexec/child_status.c (Listing 26-3, page 548), 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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/* child_status.c

   Demonstrate the use of wait() and the W* macros for analyzing the child
   status returned by wait()

   Usage: child_status [exit-status]

   If "exit-status" is supplied, then the child immediately exits with this
   status. If no command-line argument is supplied then the child loops waiting
   for signals that either cause it to stop or to terminate - both conditions
   can be detected and differentiated by the parent. The parent process
   repeatedly waits on the child until it detects that the child either exited
   normally or was killed by a signal.
#include <sys/wait.h>
#include "print_wait_status.h"          /* Declares printWaitStatus() */
#include "tlpi_hdr.h"
main(int argc, char *argv[])
    int status;
    pid_t childPid;

    if (argc > 1 && strcmp(argv[1], "--help") == 0)
        usageErr("%s [exit-status]\n", argv[0]);

    switch (fork()) {
    case -1: errExit("fork");

    case 0:             /* Child: either exits immediately with given
                           status or loops waiting for signals */
        printf("Child started with PID = %ld\n", (long) getpid());
        if (argc > 1)                   /* Status supplied on command line? */
            exit(getInt(argv[1], 0, "exit-status"));
        else                            /* Otherwise, wait for signals */
            for (;;)
        exit(EXIT_FAILURE);             /* Not reached, but good practice */

    default:            /* Parent: repeatedly wait on child until it
                           either exits or is terminated by a signal */
        for (;;) {
            childPid = waitpid(-1, &status, WUNTRACED
#ifdef WCONTINUED       /* Not present on older versions of Linux */
                                                | WCONTINUED
            if (childPid == -1)

            /* Print status in hex, and as separate decimal bytes */

            printf("waitpid() returned: PID=%ld; status=0x%04x (%d,%d)\n",
                    (long) childPid,
                    (unsigned int) status, status >> 8, status & 0xff);
            printWaitStatus(NULL, status);

            if (WIFEXITED(status) || WIFSIGNALED(status))


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