GNU/Linux thread implementation

Question

Recently, I've read in the "Advanced Linux Programming" book (http://www.advancedlinuxprogramming.com/alp-folder/alp-ch04-threads.pdf, Chapter 4.5) that on GNU/Linux POSIX threads are implemented as processes and that there is some kind of "manager thread" which does some control work.

When I run the following example from this book:

#include <stdio.h>
#include <pthread.h>

void* thread_func(void *arg)
{
  fprintf(stderr, "thread: %d\n", (int)getpid());
  while(1);
  return NULL;
}

int main()
{
  fprintf(stderr, "main: %d\n", (int)getpid());

  pthread_t thread;
  pthread_create(&thread, NULL, thread_func, NULL);

  while(1);
  return 0;
}

I've received the same PID for the main thread and child thread, while the book says that it can be different & that there is another PID, which corresponds to the so-called "manager thread." I've tried to find some information about this "manager thread", but it's proving difficult.

UPD. I have no doubts about behaivour of my program, but there is some confusion about behaivour, explained on the book - in particular, under which circumstances it can be true?

Answer 1

i've received the same PID both for main thread and child thread

That is normal and expected behavior. These threads you created co-exist in the same process, so getpid() returns ID for that process. If you want to distinguish threads, use pthread_self() (POSIX-compatible, but not system-wide unique) or gettid() (Linux specific).

Internally, all processes and threads in Linux are managed by universal object called task (and defined by task_struct), and each task has it's own ID. However, first task is a handle process thus called task group leader. And getpid() returns PID of that task group leader.

So in your case, thread_func() thread prints PID of its leader, main() task, and main() thread print PID of itself.

I suggest you to dive into kernel internals. It provides cleaner view of such things - try for example Robert Love book "Linux Kernel Development". Some information on threads/processes/etc. may be found here: http://www.win.tue.nl/~aeb/linux/lk/lk-10.html