Find out if pipe's read end is currently blocking

Question

I'm trying to find out if a child process is waiting for user input (without parsing its output). Is it possible, in C on Unix, to determine if a pipe's read end currently has a read() call blocking?

Answer 1

If I recall correctly, you can not have a pipe with no reader which means that you have either a read(2) or a select(2) syscal pending at all time.

Answer 2

I don't think that's true: For example, right before calling read() on the reader side, the pipe would have a reader that isn't actually reading.

Answer 3

You would typically just write to the pipe, or use select or poll. If you need a handshake mechanism you can do that out of band various ways or come up with and in-band protocol.

I don't know if there is a built-in way to know if a reader on the other end is blocking. Why do you need to know this?

Answer 4

The thing is, I have no control over the programs exec'd in the child processes. They print all kinds of verbose garbage which I would usually want to redirect to /dev/null. Occasionally though one will prompt the user for something. (With the prompt having no reliable format.) So my idea was:

• In a loop:
  • Drain child's stdout, append it to a temporary buffer.
  • Check (no idea how) if the child is asking for user input, in which case the buffer is printed to stdout.
• When the child exits, throw away the buffer.

Answer 5

You have these options:

• if you know that the child will need certain input (such as shell that will read a command), just write to a pipe
• if you assume the child won't read anything usually, but may do it sometimes, you probably need something like job control in the shell (use a terminal for communication with the child, use process groups and TIOCSPGRP ioctl on the terminal to get the child to the background; the child will get SIGTTIN when it tries to read from the terminal, and you can wait() for that). This is how bash handles things like "(sleep 10; read a;)&"
• if you don't know what to write, or you have more possibilities, you will have to parse the output

Answer 6

That sounds as if you were trying to supervise dpkg where occasionally some post-inst script queries the admin whether it may override some config file.

Anyway, you may want to look at how strace works:

strace -f -etrace=read your.program

Of course you need to keep track of which fds are the pipes you write about, but you probably need only stdin, anyway.