multithreading - Why is -pthread necessary for usage of std::thread in GCC and Clang?

Question

Why does specifying -std=c++11 when compiling a program that directly or indirectly uses std::thread not imply -pthread? It seems strange that the implementation detail of std::thread using pthreads under the hood is exposed to the programmer; if it's a matter of giving the user a choice of posix-compatible threading libraries, why not just default to pthreads and have some --threading-model=<your_favorite_posix_threads_library> argument to override it?

Answer 1

The -pthread option is not universally required to use std::thread - it's an implementation quirk of whatever platform you're building on.

Compiling:

#include <thread>
#include <iostream>

int main()
{
    std::thread t{[]()
        {
            std::cout << "Hello World
";
        }};
    t.join();
    return 0;
}

with

clang -std=c++11 ThreadTest.cpp -lc++

On MacOSX, builds and runs, and if we do:

otool -L a.out 
a.out:
    /usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 120.1.0)
    /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1225.0.0)

We can see that we've needed to link nothing extra to make this work - nor has it happened behind the scenes. It seems to be very much a platform implementation detail that pthreads is a separate library.

Having a choice of threading libraries with the pthread interface is legacy baggage on *NIX systems, many of which started off without thread support, then went through a phase of user-space threads before having full kernel support. I guess it's still there because nobody likes making breaking changes.