Can I set the process group of an existing process?

One thing you could try is to do setid () in mini-servers. This will make them session and process group leaders.

Also, keep in mind that you cannot change the process group ID to one from another session and that you must make a call to change the process group either from the process you want to change, the group, or from the parent of the process.

I recently wrote some test code to periodically change a process group for a set of very similar tasks. You don't need to change the group id periodically, I just thought I might dodge a certain script that periodically checks for a group that has been running for longer than a certain amount of time. It can also help you track down the error you are getting with setpgid ():

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>

void err(const char *msg);
void prn(const char *msg);
void mydaemon();

int main(int arc, char *argv[]) {

    mydaemon();
    if (setsid() < 0)
        err("setsid");

    int secs = 5*60;

    /* creating a pipe for the group leader to send changed
       group ids to the child */
    int pidx[2];
    if (pipe(pidx))
        err("pipe");

    fcntl(pidx[0], F_SETFL, O_NONBLOCK);
    fcntl(pidx[1], F_SETFL, O_NONBLOCK);

    prn("begin");

    /* here the child forks, it a stand in for the set of
       processes that need to have their group ids changed */
    int child = fork();
    switch (child) {
    case -1: err("fork3");
    case  0:
        close(pidx[1]);

        while(1) {
            sleep(7);
            secs -= 7;
            if (secs <= 0) { prn("end child"); exit(0); }

            int pid;

            /* read new pid if available */
            if (read(pidx[0], &pid, sizeof pid) != sizeof pid) continue;

            /* set new process group id */
            if (setpgid(getpid(), pid)) err("setpgid2");

            prn("child group changed");
        }
    default: break;
    }

    close(pidx[0]);

    /* here the group leader is forked every 20 seconds so that
       a new process group can be sent to the child via the pipe */
    while (1) {
        sleep(20);

        secs -= 20;

        int pid = fork();
        switch (pid) {
        case -1: err("fork2");
        case  0:
            pid = getpid();

            /* set process group leader for this process */
            if (setpgid(pid, pid)) err("setpgid1");

            /* inform child of change */
            if (write(pidx[1], &pid, sizeof pid) != sizeof pid) err("write");

            prn("group leader changed");
            break;
        default:
            close(pidx[1]);
            _exit(0);
        }

        if (secs <= 0) { prn("end leader"); exit(0); }
    }
}

void prn(const char *msg) {
    char buf[256];
    strcpy(buf, msg);
    strcat(buf, "\n");
    write(2, buf, strlen(buf));
}

void err(const char *msg) {
    char buf[256];
    strcpy(buf, msg);
    strcat(buf, ": ");
    strcat(buf, strerror(errno));
    prn(buf);
    exit(1);
}

void mydaemon() {
    int pid = fork();
    switch (pid) {
      case -1: err("fork");
      case  0: break;
      default: _exit(0);
    }

    close(0);
    close(1);
    /* close(2); let keep stderr */
}