How can you get the Linux thread Id of a std::thread()

Question

I was playing with std::thread and I was wondering how is it possible to get the thread id of a new std::thread(), I am not talking about std::thread::id but rather the OS Id given to the thread ( you can view it using pstree). This is only for my knowledge, and it's targeted only to Linux platforms (no need to be portable).

I can get the Linux Thread Id within the thread like this :

#include <iostream>
#include <thread>
#include <unistd.h>
#include <sys/syscall.h>
#include <sys/types.h>

void SayHello()
{
    std::cout << "Hello ! my id is " << (long int)syscall(SYS_gettid) << std::endl;
}

int main (int argc, char *argv[])
{
    std::thread t1(&SayHello);
    t1.join();
    return 0;
}

But how can I retrieve the same id within the main loop ? I did not find a way using std::thread::native_handle. I believed it was possible to get it trough pid_t gettid(void); since the c++11 implementation relies on pthreads, but i must be wrong.

Any advices ? Thank you.

Solution 1

Assuming you're using GCC standard library, std::thread::native_handle() returns the pthread_t thread ID returned by pthread_self(), not the OS thread ID returned by gettid(). std::thread::id() is a wrapper around that same pthread_t, and GCC's std::thread doesn't provide any way to get the OS thread ID, but you could create your own mapping:

std::mutex m;
std::map<std::thread::id, pid_t> threads;
void add_tid_mapping()
{
  std::lock_guard<std::mutex> l(m);
  threads[std::this_thread::get_id()] = syscall(SYS_gettid);
}
void wrap(void (*f)())
{
  add_tid_mapping();
  f();
}

Then create your thread with:

std::thread t1(&wrap, &SayHello);

then get the ID with something like:

pid_t tid = 0;
while (tid == 0)
{
  std::lock_guard<std::mutex> l(m);
  if (threads.count(t1.get_id()))
    tid = threads[t1.get_id()];
}

Solution 2

Some pthread implementations, e.g. Android 21+, provide

pid_t pthread_gettid_np(pthread_t);

The implementation may use the internal structure of struct pthread_t to retrieve the native thread id, same as the one returned by gettid() or syscall(SYS_gettid) when called in the context of that thread.

Comments

[Char Aznable]

I think this is useful if you have a wrapper around std:thread or pthreads, but I can't call it from outside the thread. (correct me if I'm wrong). Thanks.

[Char Aznable]

Oh getting the id this way is smart, that's great , thanks you !

[the swine]

Why would you build a std::map? Is the syscall() expected to be so expensive? Also, you never seem to remove threads from the map, which could be viewed as a memory leak (e.g. in a webserver setting where many threads are spawned over time and the process typically runs for months without restarting).

[Jonathan Wakely]

@theswine, the syscall gives you the ID of the current thread, the map lets you find the ID of any thread, given its thread::id. That's more flexible. It's not strictly a leak, since the memory is still reachable, but to keep the memory usage bounded (and to cope with thread IDs being reused) you could use a thread_local object that adds to the map in its constructor and removes from the map in its destructor. The answer was a sketch demonstrating a solution, if you want something more robust then make it more robust.

[the swine]

@JonathanWakely You are right, I did not realize that you might want to get thread id outside of the thread. Thanks for the explanation.

[rph]

I just wanted to emphasize that the while(tid ==0) is very important because the parent thread might reach the get thread id statement earlier than the mapping happens.