Question

all,

The code below comes from “Advanced Programing in Unix Environment”, it creates a new thread, and prints the process id and thread id for main and new threads.

In the book, it said that in linux, the output of this code would show that two threads have different
process ids, because pthread uses lightweight process to emulate thread. But when I ran this code in Ubuntu 12.04, it has kernel 3.2, printed the same pid.

so, does the new linux kernel change the internal implementation of pthread?

#include "apue.h"
#include <pthread.h>

pthread_t ntid;

void printids(const char *s) {
  pid_t     pid;
  pthread_t tid;
  pid = getpid();
  tid = pthread_self();
  printf("%s pid %u tid %u (0x%x)\n",
         s, (unsigned int)pid, (unsigned int)tid, (unsigned int)tid);
}

void *thread_fn(void* arg) {
  printids("new thread: ");
  return (void *)0;
}

int main(void) {
  int err;
  err = pthread_create(&ntid, NULL, thread_fn, NULL);
  if (err != 0)
    err_quit("can't create thread: %s\n", strerror(err));
  printids("main thread: ");
  sleep(1);
  return 0;
}

Answer 1

On Linux pthread uses the clone syscall with a special flag CLONE_THREAD.

See the documentation of clone syscall.

CLONE_THREAD (since Linux 2.4.0-test8)

If CLONE_THREAD is set, the child is placed in the same thread group as the calling process. To make the remainder of the discussion of CLONE_THREAD more readable, the term "thread" is used to refer to the processes within a thread group.

Thread groups were a feature added in Linux 2.4 to support the POSIX threads notion of a set of threads that share a single PID. Internally, this shared PID is the so-called thread group identifier (TGID) for the thread group. Since Linux 2.4, calls to getpid(2) return the TGID of the caller.

And in fact, Linux do change its thread implementation, since POSIX.1 requires threads share a same process ID.

   In the obsolete LinuxThreads implementation, each of the threads in a process
   has a different process ID.  This is in violation of the POSIX threads
   specification, and is the source of many other nonconformances to the
   standard; see pthreads(7).