How to tell what line of code created new thread (gdb)?

Working on Linux, catch syscall clone should break on all threads (and possibly some processes) creation. Notice that it will break in the creator thread (=the new thread is yet to be started).

Since you get the full backtrace that leads to the clone, if you need to extract the new thread entry point you should do up until you reach the pthread_create (or similar library function) stack frame and take it from its parameters (you can also directly check the parameters to clone, but I fear that the address there will be of some pthread library stub).

Threads have their own call stack. The only thing you can see is the value on the bottom of the stack. Point the thread id in t <thread id> or thread <thread id> and get call stack using bt or backtrace. You may obtain thread ids during pausing execution of your application in gdb and running info threads.

For example, my gdb session look like (specially tried to make be more clear for you) this:

(gdb) t 23[Switching to thread 23 (Thread 0x7fff8ffff700 (LWP 32334))]#0  0x00007fffc0cb829e in ?? () from /usr/lib/x86_64-linux-gnu/libgomp.so.1(gdb) bt#0  0x00007fffc0cb829e in ?? () from /usr/lib/x86_64-linux-gnu/libgomp.so.1#1  0x00007fffc0cb5bb0 in ?? () from /usr/lib/x86_64-linux-gnu/libgomp.so.1#2  0x00007ffff52b10a5 in start_thread (arg=0x7fff8ffff700) at pthread_create.c:309#3  0x00007ffff591a88d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

Here gdb says that first value of the call stack is somewhere in libgomp.so (OpenMP library). Next you can see pthread_create.c which is system-dependent method of starting thread.