How does the pstack command work?

Question

I am curious to find how does the pstack command prints the stack trace of all the threads running under the PID?

It has to be someway different than the way gdb does since the process runs inside the gdb environment, but pstack is executed after the execution of the process.

Answer 1

It's the same general idea as gdb. pstack uses ptrace, which allows an external process to attach to a known pid and print out the information (stack is known via the current registers).

If you want to know exactly how it's done, look for information about ptrace.

Also, processes don't really run "inside the gdb". You can attach gdb to a running process without much trouble by running gdb executable pid.

Answer 2

pstack print similar output as cat /proc/"pid"/tasks/*/stack so it most likely that it read the procfs rather than using the ptrace.