Why do I see one more thread than the number I cre

Question

When I create a thread (pthread_create()) from my main process, I see three (3) threads in the ps listing, why is this? That is, I see the process for the main thread, one for the created thread, and third for something else. What is the something else? Everything works just fine, I'm just wondering what the extra listed process is.

~/ cat test.c
#include <errno.h>
#include <pthread.h>
static          pthread_t       thread;
void * test_thread(void * ptr)
{
    sleep(30);
    return(ptr);
}
void thread_init(void)
{
     if (pthread_create( &thread , NULL, test_thread, NULL))
          perror("Thread not created!");
}
int main(int argc, char ** argv)
{
    thread_init();
    sleep(30);
}

When I execute this code on a system running Linux 2.6.14 and BusyBox (but using bash 2.04g), the ps listing I get after rebooting and launching my test program above:

...
   52 root            SW  [kswapd0]
  667 root            SW  [mtdblockd]
  710 root            SWN [jffs2_gcd_mtd4]
  759 root        980 S   /bin/sh
  760 root        500 S   /bin/inetd
  761 root        516 S   /bin/boa
  762 root        644 S   /sbin/syslogd -n
  763 root        640 S   /sbin/klogd -n
  766 root       1516 S   /bin/sshd -i
  767 root       1036 S   -sh
  768 root        420 S   ./test
  769 root        420 S   ./test
  770 root        420 S   ./test
  771 root        652 R   ps

The kernel is a 2.6.14 kernel with a few added drivers modules.

Answer 1

Could it be that ps is displaying 1 line for the process and 2 lines for both threads. You don't show how ps is being issued, what version, nor do you contain the entire about of the ps command.

ps typically shows only processes, not threads.

According to busybox.net/downloads/BusyBox.html the ps command will not show threads. ps -T will show threads. So if you are sure only ps is being issued (I don't know about alias in BusyBox or anything at all, I've never used it) then you are seeing 3 processes, not threads.

Also you may be using an old version of BusyBox? Refer to this bug report: bugs.busybox.net/show_bug.cgi?id=3835

Answer 2

You are seeing one thread more than you are creating because you are not counting the programs main thread.

Every time you start a program, you fire up a process that has 1 thread running. If you pthread_create one thread, then you have two threads running. You pthread_create a second one and you get three threads running.

That is why your ps (which according to you on one of the comments) shows threads, is showing you one more than the number of your pthread_creates.

Answer 3

It is likely the "thread manager" thread. See answer D.5 at this link.

You won't see the extra process listed on most modern Linux systems if they are using NPTL. But I searched and it sounds like BusyBox uses ulibc which I think only added NPTL support recently. So I don't know for sure, but my guess is you are using LinuxThreads and seeing the manager thread as the extra thread.