This program demonstrates how to run another process using fork() and wait its termination using waitpid():

• fork() creates an identical copy of the current process. The original process is the parent process, while the newly created one is the child process. Both processes continue exactly after the fork().

• waitpid() blocks until child process exits or terminates and returns its exit code and termination reason.

#include <unistd.h>     /* for fork(), getpid() */
#include <sys/types.h>  /* for waitpid() */
#include <sys/wait.h>   /* for waitpid() */
#include <stdlib.h>     /* for exit() */
#include <stdio.h>      /* for printf(), perror() */

int
main(int argc, char *argv[])
{
    /* Create child process.
     *
     * On success, fork() returns the process ID of the child (> 0) to the
     * parent and 0 to the child. On error, -1 is returned.
     */
    pid_t child_pid = fork();

    if (child_pid < 0) {
        perror("fork() failed");
        exit(EXIT_FAILURE);
    } else if (child_pid == 0) {
        /* Print message from child process.
         *
         * getpid() returns the PID (process identifier) of current process,
         * which is typically int but doesn't have to be, so we cast it.
         *
         * getppid() returns the PID of the parent process.
         */
        printf("from child: pid=%d, parent_pid=%d\n",
                 (int)getpid(), (int)getppid());

        /* We can do something here, e.g. load another program using exec().
         */
        exit(33);
    } else if (child_pid > 0) {
        /* Print message from parent process.
         */
        printf("from parent: pid=%d child_pid=%d\n",
                  (int)getpid(), (int)child_pid); 
        
        /* Wait until child process exits or terminates.
         *
         * The return value of waitpid() is PID of the child process, while
         * its argument is filled with exit code and termination reason.
         */
        int status;
        pid_t waited_pid = waitpid(child_pid, &status, 0);

        if (waited_pid < 0) {
            perror("waitpid() failed");
            exit(EXIT_FAILURE);
        } else if (waited_pid == child_pid) {
            if (WIFEXITED(status)) {
                /* WIFEXITED(status) returns true if the child has terminated 
                 * normally. In this case WEXITSTATUS(status) returns child's
                 * exit code.
                 */
                printf("from parent: child exited with code %d\n",
                          WEXITSTATUS(status));
            }
        }
    }

    exit(EXIT_SUCCESS);
}

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Example output:

from parent: pid=2486 child_pid=2487
from child: pid=2487, parent_pid=2486
from parent: child exited with code 33

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Copied from here, originally created by M.Geiger.