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.

Answer 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()];
}