Why doesn't this_thread::sleep_for need to be linked against pthread?

Usually when building thread related code in GCC, explicit linking against pthread is necessary:

g++ -pthread main.cxx

However, the following code compiles, links, and runs fine without being linked against pthread:

#include <iostream>
#include <thread>

using namespace std::chrono_literals;

int main() {
    std::this_thread::sleep_for(1000ms);
    return 0;
}

I guess what is happening here is that std::this_thread::sleep_for is using some POSIX function from libc (instead of something from pthread)? But if that is the case, does the execution of std::this_thread::sleep_for change depending on whether or not it was called from the main thread?

Solution

Why doesn't this_thread::sleep_for need to be linked against pthread?

Because the call to std::this_thread::sleep_for translates into underlying call to nanosleep, which is defined in libc.so.6, and not in libpthread.so.0.

Note that when linking with GLIBC-2.34 and later, using other functions (which previously required -pthread) no longer require it, because GLIBC got rid of libpthread.