setpgid race condition

infy.c

#include <unistd.h>

int main()
{
  while (1) 
  {
    sleep(1);
  }
}

test.c

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

char* args[] = {
  "./infy",
  NULL
};

int main(int argc, char* argv[])
{
  if (argc != 2)
  {
    fprintf(stderr, "Usage: %s [num]\n", argv[0]);
    return 1;
  }
  int num = strtol(argv[1], NULL, 10);
  if (num < 2)
  {
    fprintf(stderr, "Invalid number of processes\n");
    return 1;
  }

  pid_t pid = fork();
  if (pid > 0)
  {
    int s;
    waitpid(pid, &s, 0);
    fprintf(stderr, "Children done\n");
  }
  else
  {
    pid_t pgid = -1;
    int i;
    for (i = 1; i < num; i++)
    {
      pid_t pid2 = fork();
      if (pid2 > 0)
      {
        if (pgid == -1)
        {
          pgid = pid2;
        }
      }
      else
      {
        if (setpgid(0, pgid == -1 ? 0 : pgid) != 0)
        {
          perror("setpgid failed in non-last process");
        }
        execve(args[0], args, NULL);
        perror("exec failed");
        exit(1);
      }
    }

    // uncomment me to fix
    //fprintf(stderr, "pgid %d\n", pgid);
    if (setpgid(0, pgid) != 0)
    {
      perror("setpgid failed in last process");
    }
    execve(args[0], args, NULL);
    perror("exec failed");
    exit(1);
  }
}

Compile each file as a separate program. After executing ./test 3 (or any number > 2), ps jx will show that all of the infy processes are in the same group. ./test 2 will present an error that setpgid has failed. Uncommenting line 58 will cause ./test 2 to work as expected.