Question

I’m trying to create a child process in another process. I am writing both the programs in C language. First I write a dummy process which will be the child process. What it is doing is only to write a string on the screen. It works well on its own. Then I write another program which will be the parent process. However, I can’t make it happen. I’m trying to use fork and execl functions together, but I fail. I also want the child process does not terminate until the parent process terminates.

How should I write the parent process?

Thanks.

Here is the code for the child process:

#include <stdio.h>

int main(void) {
  while(1) {
    printf("*");
    sleep(1);
  }
}

And here is the parent process:

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

int main(void) {
  if (fork()) {
    while(1) {
      printf("-\n");
      sleep(5);
    }
  } else {
    execl("./", "dummy", (char *)0);
  }
}

Answer 1

The fork() system call may return three different statuses: failure (<0), parent process (>0) or child process (==0). You must test the return value properly.

int pid = fork();

if (pid < 0) {
  /* handle error */
  perror("fork");
  exit(1);
} else if (pid > 0) {
  /* parent code */
} else {
  /* child code */
}

Your execl() system call is wrong. The first argument is the path to the program you want to execute, “./” is not valid, it should be something like “./dummy” at least. The next argument is by convention the command name (argv[0] in the executed program), which may be a repetition of the first argument. So:

execl("./dummy", "dummy", NULL);

Also, note that the printf("*") statement in the child program will probably buffer and you won’t see anything on the terminal. You must either add a “\n” to the end or call fflush(stdout) to flush the standard output.