#include <errno.h>              /* for errno() */
#include <linux/limits.h>       /* for ARG_MAX */
#include <signal.h>             /* for signal(), SIG* */
#include <stdio.h>              /* for sprintf(), stderr, ... */
#include <stdlib.h>             /* for exit() */
#include <string.h>             /* for strerror() */
#include <sys/types.h>          /* for wait() */
#include <sys/wait.h>           /* for wait() */
#include <unistd.h>             /* for sleep(), fork() and execlp() */
#include "funcs.h"              /* for infomsg(), errormsg() */

/*
 *  Macros
 */

#define CHILD_RUN_TIME          10

/*
 *  Type and struct definitions
 */

/*
 *  Global variables
 */

int child_reaped = FALSE;

/*
 *  Forward declarations
 */

pid_t start_child_sleep(void);
void sigchld_handler(int);

/*
 *  Functions
 */

int main(
int argc,
char *argv[])
{
    pid_t pid;

    /*
     *  Initialise.
     */

    infomsg("parent setting up signal handlers ...");
    signal(SIGCHLD, sigchld_handler);

    /*
     *  Start child.
     */

    infomsg("parent starting one child ...");
    pid = start_child_sleep();

    /*
     *  Main monitoring loop
     */

    infomsg("parent sees child has pid %d and loops checking flag ...", pid);
    while (TRUE) {

        /*
         *  Exit if no running children.
         */

        if (child_reaped) {
            infomsg("parent sees child_reaped flag and so stops looping ...");
            break;
        }

        /*
         *  Let a little time pass before we reinspect the situation.
         */

        sleep(1);
    }

    /*
     *  Clean up and exit.
     */

    infomsg("parent cleaning up and exiting ...");
    signal(SIGCHLD, SIG_DFL);
    return(0);
}

pid_t start_child_sleep(
)
{
    pid_t pid;
    char buf[ARG_MAX];

    if ((pid=fork()) < 0)
        errormsg("fork() failed: %s", strerror(errno));
    else if (pid > 0)
        return(pid);

    /*
     *  Only the child gets here
     */

    sprintf(buf, "echo \"child running ...\";"
                 "sleep %d;"
                 "echo \"child exiting ...\"",
                 CHILD_RUN_TIME);
    execlp("/bin/sh", "sh", "-c", buf, (char *) NULL);
    errormsg("exec() failed: %s", strerror(errno));
}

void sigchld_handler(
int sig)
{
    int wstatus;
    pid_t pid;

    infomsg("parent received SIGCHLD; reaping and setting child_reaped flag ...");
    pid = wait(&wstatus);
    child_reaped = TRUE;
}