How to check which process is using a given file descriptor?

Question

Somewhere in the middle of my application, the framework I'm using (ROOT) gives me the following error:

 *** Break *** write on a pipe with no one to read it
SysError in <TUnixSystem::UnixSend>: send (Broken pipe)
SysError in <TUnixSystem::DispatchOneEvent>: select: read error on 24
 (Bad file descriptor)

How can I check which process is using this file descriptor, preferably without sudo?

Answer 1

I am afraid that is impossible afterwards. After all "broken pipe" means that the other process is gone (or has at least closed its file descriptor).

While everything is still OK you can do this:

$ sleep 1000 | sleep 1000 & ps
[1] 848156
   PID TTY         TIME CMD
819441 pts/0   00:00:00 bash
848155 pts/0   00:00:00 sleep
848156 pts/0   00:00:00 sleep
848157 pts/0   00:00:00 ps
$ PID=848156 # PID of one of the sleep processes
$ ls -l /proc/$PID/fd
... # Skipped output lines
l-wx------ 1 hl hauke 64 15. Apr 19:11 1 -> pipe:[108237859]
... # Skipped output lines
$ lsof -n | grep 108237859 # gives you all processes which have access to this pipe
sleep     848155     hl    1w     FIFO     0,8     0t0  108237859 pipe
sleep     848156     hl    0r     FIFO     0,8     0t0  108237859 pipe

Edit 1

If lsof is not available:

for dir in /proc/[1-9]*; do
  test -r "$dir"/fd || continue
  if ls -ln "$dir"/fd | grep -q 108237859; then
    echo "PID: ${dir#/proc/}"
  fi
done

Answer 2

The lsof command will show you opened descriptors, then just grep it.

Answer 3

Without lsof you could try:

find /proc -name <descriptor> | grep fdinfo

It should return some results of the form ./<pid>/fdinfo/<fd>