How to get all threads of a definite process?

Hi, everyone:

I want to get all threads of one definite process, and I've found some
data structure such as 'all_threads', which is not what I want, i
guess. I don't understand the relationship between process and thread,
from the gdb-source-code-level (pointers for example),  and I don't
understand the meaning of 'ptid_t', does this mean 'process_thread_id'
? and I've found this data's value is not the thread's PID that Linux
shell tell me, it just some number like 0 or 1.

If you know something about this, please help me. Thanks.

BR.