Question

I thought pthread uses clone to spawn one new thread in linux. But if so, all of the threads should have their seperate pid. Otherwise, if they have the same pid, the global variables in the libc seem to be shared. However, as I ran the following program, I got the same pid but the different address of errno.

extern errno;
void*
f(void *arg)
{
    printf("%u,%p\n", getpid(), &errno);
    fflush(stdin);
    return NULL;
}

int
main(int argc, char **argv)
{
    pthread_t tid;
    pthread_create(&tid, NULL, f, NULL);
    printf("%u,%p\n", getpid(), &errno);
    fflush(stdin);
    pthread_join(tid, NULL);
    return 0;
}

Then, why?

Answer 1

I’m not sure exactly how clone() is used when pthread_create() is called. That said, looking at the clone() man page, it looks like there is a flag called CLONE_THREAD which:

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.

It then goes on to talk about a gettid() function for getting the unique ID of an individual thread within a process. Modifying your code:

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

int errno;
void*
f(void *arg)
{
    printf("%u,%p, %u\n", getpid(), &errno, syscall(SYS_gettid));
    fflush(stdin);
    return NULL;
}

int
main(int argc, char **argv)
{
    pthread_t tid;
    pthread_create(&tid, NULL, f, NULL);
    printf("%u,%p, %u\n", getpid(), &errno, syscall(SYS_gettid));
    fflush(stdin);
    pthread_join(tid, NULL);
    return 0;
}

(make sure to use “-lpthread”!) we can see that the individual thread id is indeed unique, while the pid remains the same.

rascher@coltrane:~$ ./a.out 
4109,0x804a034, 4109
4109,0x804a034, 4110