Unable to change gid of a process

Question

I need to change the PGID of my parent process, so I did something like this:

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

#define KIDS 10

int main()
{
    struct timespec a = {10, 0};
    int pid;
    int* pids = (int*) calloc(KIDS, sizeof(int));
    int argument = 0;
    int procNumber;

    for (procNumber = 0; procNumber < KIDS; procNumber++) {
        pid = fork();
        argument = procNumber;
        if (pid == 0)
            break;
        pids[procNumber] = pid;
    }

    if (pid == 0) {  
        // child stuff
    } else {
        printf("My group: %d\n", getpgrp());
        if (setpgid(0, 6654) == -1)
            perror("setpgid() error:");
        printf("My new group: %d\n", getpgrp());
    }

    nanosleep(&a,NULL);
    free(pids);

    return 0;
}

And I get Operation not permitted error.

What should I do to avoid this error and change groupid of the process?

Answer 1

The Operation not permitted error message is associated with the EPERM error code, which according to man 2 setpgid (quoting):

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 seems like there are 3 different explanations for the error you receive:

1. You're trying to move a process into a process group in a different session.

2. You're trying to change the PGID of a child which is in a different session.

3. You're trying to change the PGID of a session leader.

Cases 2 and 3 look irrelevant to your problem, so my guess would be that you're in the first case. If 6654 is randomly selected, it could be that process group 6654 is in a different session.

You can verify this running something like $ ps eajx and checking the SID field of the output for each of the processes involved, including the process group 6654.

Answer 2

if(setpgid(0,6654)==-1)
    perror("setpgid() error:");

I think the error means " An attempt was made to move a process into a process group in a different session"

So:

[root@localhost test_c]# sleep 10000 &
[2] 2922

I start a new process which pid is 2922 in my terminal，which means it's process group id also be 2922.

Then i change you source code if(setpgid(0,6654)==-1) to if(setpgid(0,2922)==-1) , things works.