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::t

Answer 1

How about this:

pid_t gettid (void)
{
    return syscall(__NR_gettid);
}

http://yusufonlinux.blogspot.com/2010/11/get-thread-id-from-linux.html

Looks like __NR_gettid is defined in unistd.h

Answer 2

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()];
}

Answer 3

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.