sigemptyset과 setpgid()의 중요성이 궁금합니다.

void eval(char *cmdline) 
{
    char *argv[MAXARGS];
    int bg;
    int built_in;
    pid_t pid;
    sigset_t *set;
 
    bg=parseline(cmdline, argv);
 
    if (argv[0]==NULL) return; //blank -> NULL
 
    built_in=builtin_cmd(argv);//quit, fg, bg, jbos ->1, otherwise ->0
    if (built_in==0){//other command
        sigemptyset(set); //set==empty set
        sigaddset(set, SIGCHLD);
        sigprocmask(SIG_BLOCK, set, NULL);
 
 
        if ((pid=fork())==0){            
            sigprocmask(SIG_UNBLOCK, set, NULL);
            setpgid(0, 0);
            if (execve(argv[0],argv, environ)<0){
                printf("%s: Command not found\n", argv[0]);
                exit(1);
            }
 
        }
        else { //parent
            if (bg){ //need to run the command in background
                addjob(jobs, pid, BG, cmdline);
                sigprocmask(SIG_UNBLOCK, set, NULL);
                printf("[%d] (%d) %s", pid2jid(pid), pid, cmdline);
 
            }
            else { //foreground
                //printf("foreground\n");
                addjob(jobs, pid, FG, cmdline);
                sigprocmask(SIG_UNBLOCK, set, NULL);
                waitfg(pid);
            }
        }
    }
    return;
}