c - Why sigchld_handler will get a SIGTSTP too? -

i making small shell program.i want suspend foreground program in sigtstp_handler.and sigchld_handler reap zombie.but when type ctrl z,even if not have foreground program,the sigchld_handler still sigtstp.

    signal(sigint,  sigint_handler);   /* ctrl-c */     signal(sigtstp, sigtstp_handler);  /* ctrl-z */     signal(sigchld, sigchld_handler);  /* terminated or stopped child */ 

handler

void sigchld_handler(int sig)  {     int status;     pid_t pid;      while ((pid = waitpid(-1, &status, wnohang | wuntraced)) > 0 ) {          if (wifexited(status)) {   /*checks if child terminated */             deletejob(jobs, pid);         }          if (wifsignaled(status)) {  /*checks if child terminated signal not caught */             printf("job [%d] (%d) terminated signal %d\n", pid2jid(pid), pid, wtermsig(status));             deletejob(jobs,pid);         }          if (wifstopped(status)) {     /*checks if child process caused return stopped */             getjobpid(jobs, pid)->state = st;             printf("job [%d] (%d) stopped signal %d\n", pid2jid(pid), pid, wtermsig(status));             //printf("[%d] stopped %s\n", pid2jid(pid), jobs->cmdline);         }      }     if (pid < 0 && errno != echild) {         printf("waitpid error: %s\n", strerror(errno));     }      return; }   void sigtstp_handler(int sig)  {        pid_t pid = fgpid(jobs);//return pid of current foreground job, 0 if no such job       if (pid!=0){         struct job_t *p = getjobpid(jobs, pid);         printf("kill %d %s %d\n",p->pid,p->cmdline,p->state);          kill(pid,sigtstp);     }     return; }  void sigint_handler(int sig)  {      pid_t pid = fgpid(jobs);     if (pid!=0){         struct job_t *p = getjobpid(jobs, pid);         printf("kill %d %s %d\n",p->pid,p->cmdline,p->state);         kill(pid,sigint);     }     return; } 

i setpgid(0,0)，after fork child process,then solved problem.but not know going on...

you have foreground process (if use terminal). if didn't launched subprocess shell in foreground state, receiver of tstp. foreground/background management needs more work signal handling. need set processes groups , declare group foreground group.

the basic tasks are:

1. create group when launching commands (setpgid)
2. declare group foreground group if needed (tcsetpgrp)
3. make shell aware of children state changes (handling of sigchld , status wifstopped/wstopped , manage job list accordingly
4. make interactive shells insensible sigtstp (ignore signal in shell)

job controlling rough task complete... read documentation carefully.