Check if piped-in process is blocked on input

Is it possible from within bar to determine [...] the process that's piping into it[?]

This should be possible by searching the /proc directory. Consider the following shell session (^Z means that the user pressed CTRL-Z, all other lines not preceded by a dollar sign are command output.):

$ sleep 100 | cat
^Z
[1]+  Stopped                 sleep 100 | cat
$ ps
    PID TTY          TIME CMD
   5801 pts/1    00:00:00 bash
   5805 pts/1    00:00:00 sleep
   5806 pts/1    00:00:00 cat
   5815 pts/1    00:00:00 ps
$ readlink /proc/5805/fd/0
/dev/pts/1
$ readlink /proc/5805/fd/1
pipe:[59748]
$ readlink /proc/5806/fd/0
pipe:[59748]
$ readlink /proc/5806/fd/1
/dev/pts/1

This suggests that you can use the readlink syscall to get the number of the pipe connected to stdin of bar, and then search for processes whose stdout is connected to the pipe with this number.

Is it possible from within bar to determine [...] whether that process is blocked on input?

I am afraid this is not possible without ptraceing that process.

I want to distinguish situations where interaction is required from situations where foo produces all its output up front and immediately exits.

You can read the whole input from stdin, and once you encounter EOF, you can be sure that foo won't send any more output. However, not receiving an EOF may indicate that foo is still working, not that it awaits input.

posted 6 months ago

CC BY-SA 4.0

samo‭

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