Understanding of Fork in C [duplicate]

Answer 1

try this:

void exit_message (void)
{
   // use write to have an unbuffered output
   char m[100];
   sprintf (m, "%d exit\n", getpid());
   write (1, m, strlen(m));
}

main(){
  atexit (exit_message);

  printf("%d test 1\n",getpid()); 

  fork();

  printf("%d test 2\n",getpid());
  }

The output will look like this:

14866 exit   // <- parent process flushes its output at exit
14866 test 1 // <- parent process: 1st printf
14866 test 2 // <- parent process: 2nd printf
14867 exit   // <- forked process flushes its output at exit
14866 test 1 // <- forked process: unflushed parent process output buffer
14867 test 2 // <- forked process: 2nd printf

we can see the only printf done by the forked process is the last one, as expected.

The previous line is a ghost of the stdout output buffer, which has been duplicated by fork() before being flushed.

Making stdout unbuffered

main(){
  atexit (exit_message);
  setvbuf(stdout, NULL, _IONBF, 0);
  printf("%d test 1\n",getpid()); 

  fork();

  printf("%d test 2\n",getpid());
}

gets rid of the ghost

14866 test 1 // <- parent process: 1st printf
14866 test 2 // <- parent process: 2nd printf
14866 exit   // <- parent process exits
14867 test 2 // <- forked process: 2nd printf
14867 exit   // <- forked process exits