Strange error during debugging program with threads: Unable open /proc file

Question

During debugging a program with threads, I have a strange error:

(gdb) info threads 
warning: unable to open /proc file '/proc/3663/status'
warning: Can't attach LWP 3663: No child processes
Cannot find new threads: debugger service failed

I get no other warnings during the debug session.

The process/thread 3663 doesn't exist.

How can it be that the gdb misunderstands the pid of the process/thread? What additional checks could I do to localize the problem?

Answer 1

I have found the reason of the failure.

The process I wanted to debug was in a process namespace; and the debugger was out of it. When I started the debugger in the same namespace the problem gone away.