c - Which user can use setpgid() function? -

i worked out function setpgid() when executing function result permission denied. logged in root user time print error message permission denied. user can use function. can explain 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);   }                                         

first of all, need check return value of function calls before call perror(), otherwise don't know of calls failed - might not been recent call before perror() statement failed. code should like:

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

if setpgid() fails, here's docs says:

eperm attempt made move process process group in different session, or change process group id of 1 of children of calling process , child in different session, or change process group id of session leader (setpgid(), setpgrp()).

so sounds hitting first case described since child process calls setsid().

the glibc docs job control has bit of reading material on subject.