Question

I’m writing a simple Shell here. In brief, I’m doing these stuff (pseudo code):

call fork()
if return value is zero: //child
  setpgid(0,0)
  if (foreground)
    tcsetpgrp() //give this child access to terminal
  reset all signal handlers messed up by parent
  unblock all signals that are blocked by parent
  execve()
else: //parent
  if (foreground)
    tcsetpgrp() //give child access to terminal
    add this job to job list
    unblock all signals
    wait for this job to finish
    tcsetpgrp() //get terminal access back
  if (background)
    add this job to job list
    unblock all signals

What I want to test is /bin/cat. Now, this Shell works fine for /bin/cat if I run it in foreground. However, if I type /bin/cat & to let it run in background, I got an input/output error. According to what I learned, when a background job is trying to read from terminal, SIGTTIN should be sent to it. So ideally, when I type /bin/cat &, it will be stopped by a SIGTTIN. If I use “bg” command to resume it, it will again be stopped by a SIGTTIN.

What I am confused is how could I implement this. Is SIGTTIN automatically sent by the OS, or I have to send it myself? If it is that latter one, how could I tell a child is going to read from the terminal or not?

Thanks!

Answer 1

Finally got it. No response to SIGTTIN is because the parent has ignored this signal. In child, all the signal handlers has to set to default, including SIGTTIN signal.