Why two threads in NPTL have different pid in Ubuntu12.04

I tested some code in Ubuntu 12.04 LTS server x64(3.2 kernel), which I think is using NPTL.

when I run

$ getconf GNU_LIBPTHREAD_VERSION

I get

NPTL 2.15

The following is the test code. I compiled it with gcc -g -Wall -pthread

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <pthread.h>

void *func1(void *arg)
{
    printf("func1:[%d]%lu\n", getpid(), pthread_self());
    for(;;);
    return (void *)0;
}

int main(void)
{
    printf("main:[%d]%lu\n", getpid(), pthread_self());

    pthread_t tid1;
    pthread_create(&tid1, NULL, func1, NULL);
    void *tret = NULL;
    pthread_join(tid1, &tret);

    return 0;
}

when I run the program, it seems all to be expected: two threads have the same pid

$ ./a.out
main:[2107]139745753233152
func1:[2107]139745744897792

but in htop (a top like tool, you can get it by apt-get) I see this: two threads have different pid

PID  Command
2108 ./a.out
2107 ./a.out

If I kill the pid 2108, the process will be killed

$ kill -9 2108

$./a.out
main:[2107]139745753233152
func1:[2107]139745744897792
Killed

And if I run the program through gdb, I can see LWP

[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
main:[2183]140737354069760
[New Thread 0x7ffff77fd700 (LWP 2186)]
func1:[2183]140737345738496

I think NPTL's threads share one PID and LWP is for LinuxThreads before kernel 2.6. The Above seems that NPTL is still using LWP under. Am I right? I want to know the truth about NTPL and LWP.

Thanks.

The two threads do share one PID, as shown by your first example.

htop is showing you TIDs (Thread IDs) in the field marked as PID.