How is it that I can attach strace to a process that isn't in the output of ps?

Question

I'm trying to debug why our integration tests are taking so long and it looks like they're hanging part way through.

I logged onto our test service and saw the following behaviour:

root@colossus:~# strace -p 18310
Process 18310 attached - interrupt to quit
futex(0x7f9915c609d0, FUTEX_WAIT, 18313, NULL^C <unfinished ...>
Process 18310 detached

root@colossus:~# strace -p 18313
Process 18313 attached - interrupt to quit
restart_syscall(<... resuming interrupted call ...>^C <unfinished ...>
Process 18313 detached

root@colossus:~# ps -ef | grep 18313
root     19089 19034  0 09:46 pts/0    00:00:00 grep --color=auto 18313

root@colossus:~# ps -p 18313
PID TTY          TIME CMD

My interpretation of these commands is that 18310 is waiting for its child 18313 to complete.

Process 18313 is trying to restart an interrupted system call.

Here's where it gets weird. Although I can attach to 18313, I cannot see it in the list of current processes, when I run ps.

Can someone help me understand what is going on here, please?

That's a thread. Use `ps -L`. See also `strace -f` to strace all the threads in that process (and future children) — Stéphane Chazelas, Apr 23 '14 at 15:20

score 3 · Answer 1 · answered Mar 25 '15 at 16:44

3

Per Stéphane's comment, what I'm seeing is a thread, not a process.

Threads are listed in the output from ps -L

One can strace all current and future threads of a process with strace -f

answered Mar 25 '15 at 16:44

Dancrumb

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How is it that I can attach strace to a process that isn't in the output of ps?

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