How does gdb detect pthread? - c

Question

Recently I've been learning about pthread. Then I suddenly came out of an idea that how does gdb know I create a new thread. Then I wrote down a test code below and started up gdb. I step into pthread_create() function, but instead of letting it return normally, I use return 0 to return pthread_create() function. But gdb still shows that I have only one thread. At first, I thought that gdb got thread information from the return value from the pthread_create() function then I thought gdb might also use child process info to the get thread info so I edited my test code. But the result wasn't what I thought of.
So how does gdb get thread info? What kind of information it needs to know how many threads the main thread have and which thread I'm on.
Code
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>
#include "pthread.h"
void *foo(void *bar) {
while(1) {
printf("hello from thread: %d\n", pthread_self());
sleep(2);
}
}
int main() {
printf("Before fake pthread_create");
pid_t pid;
if ((pid = fork()) == -1) {
perror("fork error");
exit(errno);
}
if (pid == 0) {
while(1) {
sleep(3);
}
}
if (pid > 0) {
pthread_t thread;
pthread_create(&thread, NULL, foo, NULL);
while(1) {
printf("hello from thread: %d\n", pthread_self());
sleep(2);
}
return 0;
}
}

Answer 1

How does gdb detect pthread?
GDB sets internal breakpoint on _dl_debug_state, which allows it track which shared libraries are loaded (this is necessary for debugging shared libraries).
When it observes that libpthread.so is loaded, it loads libthread_db.so.1 into its own process space (into GDB itself, not into the program being debugged), and asks that library to notify GDB when new threads are created and when they are destroyed. Documentation.
The libthread_db has intimate knowledge of the internals of libpthread, and installs appropriate hooks to achieve such notification.

Answer 2

There are 2 main mechanisms that debuggers use on linux, and neither of them are very pretty. There is so much detail here that I can only point you there and hope.
One is ptrace which allows the debugger to follow along as the program does things, such as executing system commands like pthread_create, or specific events happen, such as new threads starting, and control the monitored program: http://man7.org/linux/man-pages/man2/ptrace.2.html
The other is the /proc/ file system which reveals lots of information about a process: http://man7.org/linux/man-pages/man5/proc.5.html
In particular ls -l /proc/self/tasks shows you what threads ls has (only 1).