Humans are curious, perhaps that makes us humans, and you might be curious about what your program is doing.
Sometimes, the program has several threads running and sometimes you can’t completely stop it neither kill it to see what it is doing.
A situation like that could be when there is a service running on a client and it has some problem, you suspect a thread (one of several) is locked or waiting for something that will never happen, but all other threads look like they are running fine; so you don’t want to interrupt neither kill the process for now.
What I do, is to use gdb and write a file with the commands I want to run and ending the file with the ‘q’ command (quit), making gdb quit so the process can continue its execution. I usually write a file called ‘commands’ with this:
thread apply all bt q
That will execute ‘bt’ (backtrace) to all threads and then ‘q’ (quit) gdb after executing backtrace. Printing the backtrace for all threads shows me (more or less) what the threads are executing.
Using the ‘commands’ file I run gdb like this:
gdb -p <pid> -x commands > /tmp/threads
Being <pid> the process pid.
Notice I redirect the output to a file, that is because unless I redirect to a file, gdb will stop the output when the console is full and wait for me to press ‘return’; which will make the process stop for a while, something I don’t want to happen.
After seeing the threads, I can write another ‘commands’ files with another instructions to gdb, like printing some variable.
[…]the ability to ask questions is probably the central cognitive element that distinguishes human and animal cognitive abilities[…] (https://en.wikipedia.org/wiki/Great_ape_language#Limitations_of_ape_language)