How will a parent process know that a child is dead?

Question

When a process is created using vfork() child process is created and starts running in the parent process address space, until exit() or exec() is called.

What happens to the parent process, if some one kills the child process before calling the exit()/exec()?

Answer 1

Fork returns the child pid (process identifier) in the parent process, using the waitpid parent can not if child is alive or not

Something like

#include <sys/wait.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>


int  main(int argc, char *argv[])
{
    pid_t cpid, w;
    int status;


    cpid = fork();
    if (cpid == -1) { perror("fork"); exit(EXIT_FAILURE); }


    if (cpid == 0) {            /* Code executed by child */
        printf("Child PID is %ld\n", (long) getpid());
        if (argc == 1)
            pause();                    /* Wait for signals */
        _exit(atoi(argv[1]));
    } else {                    /* Code executed by parent */
        do {
            w = waitpid(cpid, &status, WUNTRACED | WCONTINUED);
            if (w == -1) { perror("waitpid"); exit(EXIT_FAILURE); }


            if (WIFEXITED(status)) {
                printf("exited, status=%d\n", WEXITSTATUS(status));
            } else if (WIFSIGNALED(status)) {
                printf("killed by signal %d\n", WTERMSIG(status));
            } else if (WIFSTOPPED(status)) {
                printf("stopped by signal %d\n", WSTOPSIG(status));
            } else if (WIFCONTINUED(status)) {
                printf("continued\n");
            }
        } while (!WIFEXITED(status) && !WIFSIGNALED(status));
        exit(EXIT_SUCCESS);
    }
}

Output

$ ./a.out &
Child PID is 12345
[1] 12344
$ kill -STOP 12345
stopped by signal 19
$ kill -CONT 12345
continued
$ kill -TERM 12345
killed by signal 15
[1]+  Done                    ./a.out