Which user can use setpgid() function?

Question

I worked out the function for the setpgid() when i executing this function the result will be permission denied. then i logged in as a root user that time also this will print the error message as permission denied. then which user can use this function. Can anyone explain to me?

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

  main()
   {
    printf("parent pid=%d\tpgid=%d\n",getpid(),getpgid(getpid()));
    pid_t pid,pgid;
    pgid=getpgid(getpid());
    if((pid=fork())==0)
    {
            printf("befor sessionchild pid=%d\tpgid=%d\n",getpid(),getpgid(getpid()));
            sleep(5);
            pid_t p;
            printf("child pid=%d\tpgid=%d\n",getpid(),getpgid(getpid()));
            if((p=fork())==0){
                    sleep(2);
                    setsid();
                    printf("child2 pid=%d\tpgid=%d\n",getpid(),getpgid(getpid()));
                    setpgid(getpid(),pgid);
                    perror("Error");
                    printf("after setting group id child2 pid=%d\tpgid=%d\n",getpid(),getpgid(getpid()));
            }
            wait(0);
            exit(0);
    }
    exit(0);
  }                                        

Answer 1

First of all, you really need to check the return value of your function calls before you call perror(), otherwise you don't know which of your calls that failed - it might not been the most recent call before your perror() statement that failed. The code should be something like:

if (setpgid(getpid(),pgid) != 0) {
    perror("setpgid");
}

If it really is setpgid() that fails, here's what the docs says:

EPERM An attempt was made to move a process into a process group in a different session, or to change the process group ID of one of the children of the calling process and the child was in a different session, or to change the process group ID of a session leader (setpgid(), setpgrp()).

So it sounds like you are hitting this first case described since your child process calls setsid().

The glibc docs about job control has a bit of reading material on this subject.