Find what process is on the other end of a pipe

Question

I'm trying to trace some odd behavior of a few processes and ran into a point I'm not sure how to trace past. The hung process, which I attached to using strace -p showed this:

Process 7926 attached - interrupt to quit
read(3, 

Okay, so it's waiting for input on fd 3, so I went to check what it is:

$ ls -l /proc/7926/fd/3
lr-x------ 1 user grp 64 Mar 15 10:41 /proc/7926/fd/3 -> pipe:[20043922]

Okay, so it's a pipe... now the question -- who is the writer of this pipe? I recall that on Linux there is a special feature for unix domain sockets where you can request a file path that starts with a NUL byte to access the "abstract socket namespace" (mentioned here: http://tkhanson.net/cgit.cgi/misc.git/plain/unixdomain/Unix_domain_sockets.html). I'm not sure if there's something similar for pipes that I could take advantage of, but I haven't found anything.

I was hoping a tool like fuser or lsof might help, but I haven't gotten anywhere.

Any ideas?

Answer 1

The symlink contents "pipe:[20043922]" are a unique ID; the other end of the pipe will have a matching ID.

(find /proc -type l | xargs ls -l | fgrep 'pipe:[20043922]') 2>/dev/null

should show you both ends of the pipe.

Answer 2

You can get the list of processes using the pipe by using lsof command:

lsof | grep 'FIFO.*20043922'

The output will show the readers (in the FD column with entries like 1r) and writers (same columns with entries like 2w).