Is there a way to execute code in the parent process after the child, which is created using fork an-CodePudding

I am having a parent process play that creates a fork and runs foo using execl

Code for play.c

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

int main() {
    if (fork() == 0) {
      execl("./foo", "", NULL);
    } else {
      wait(0);
      write(STDOUT_FILENO, "in parent after waiting", 5);
    }
  printf("outside everything");
  return 0;
}

Code for foo.c

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

void signal_handler() {
  write(STDOUT_FILENO, "\nBye!\n", 6);
  exit(1);
}

int main() {
  struct sigaction sa;
  sa.sa_handler = signal_handler;
  sigaction(SIGINT, &sa, NULL);

  while (1) {
    printf("Wasting time...%d \n", getpid());
    sleep(1);
  }

  return 0;
}

My questions are,

Why aren't the print statements after the wait(0) statement executed?
Why isn't the signal handler in the child process triggered when Ctrl C?

CodePudding user response：

You should ensure that the sa_flags and sa_mask fields of struct sigaction are set. You can initialize them — struct sigaction sa = { 0 }; will probably do the job. Or you can use sigemptyset(&sa.sa_mask); and sa.sa_flags = 0; to assign values. Or you can set them to some non-zero value. Not setting sa_flags means you've no idea what operation you requested. You also need a signal handler in play.c. You need to ignore SIGINT before the fork(), then in the child re-enable the signal before executing foo. The write() in the parent does not print much; it may once have printed "\nBar!\n" or something.

Here's some working code.

`play.c`

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

int main(void)
{
    struct sigaction sa = { 0 };
    sa.sa_handler = SIG_IGN;
    sigaction(SIGINT, &sa, NULL);
    if (fork() == 0)
    {
        sa.sa_handler = SIG_DFL;
        sigaction(SIGINT, &sa, NULL);
        execl("./foo", "", NULL);
        exit(1);
    }
    else
    {
        wait(0);
        printf("in parent after waiting\n");
    }
    printf("outside everything\n");
    return 0;
}

`foo.c`

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

static void signal_handler(int signum)
{
    char message[] = "\nBye (XX)\n";
    message[6] = signum / 10   '0';
    message[7] = signum % 10   '0';
    write(STDOUT_FILENO, message, sizeof(message) - 1);
    exit(1);
}

int main(void)
{
    struct sigaction sa = { 0 };
    sa.sa_handler = signal_handler;
    sigaction(SIGINT, &sa, NULL);

    while (1)
    {
        printf("Wasting time...%d \n", getpid());
        sleep(1);
    }

    return 0;
}

Example output

$ play
Wasting time...11383 
Wasting time...11383 
Wasting time...11383 
Wasting time...11383 
^C
Bye (02)
in parent after waiting
outside everything
$