Question

In bash when I run a command like wc & or cat & that wants standard in right away, it returns immediately with

[1]+ Stopped cat

How is this accomplished? How do I stop a program that I started with exec, and how do I know to stop these programs in the first place? Is there some way to tell that these programs want stdin?

Thanks!

PS also, what is the + about? I’ve always wondered, but that’s really hard to google…

Answer 1

The setpgid() manual page explains how this works:

A session can have a controlling terminal. At any time, one (and only
one) of the process groups in the session can be the foreground
process group for the terminal; the remaining process groups are in
the background. If a signal is generated from the terminal (e.g.,
typing the interrupt key to generate SIGINT), that signal is sent to
the foreground process group. (See termios(3) for a description of
the characters that generate signals.) Only the foreground process
group may read(2) from the terminal; if a background process group
tries to read(2) from the terminal, then the group is sent a
SIGTSTP signal, which suspends it. The tcgetpgrp(3) and
tcsetpgrp(3) functions are used to get/set the foreground process
group of the controlling terminal.

So what you want to do is this:

1. When you create a new pipeline, call setpgid() to put all the members of the pipeline in a new process group (with the PID of the first process in the pipeline as the PGID).

2. Use tcsetpgrp() to manage which process group is in the foreground – if you put a pipeline in the background with &, you should make the shell’s own process group the foreground process group again.

3. Call waitpid() with the WNOHANG and WUNTRACED flags to check on the status of child processes – this will inform you when they are stopped by SIGTSTP, which will allow you to print a message like bash does.