https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103949
--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to Jörn Heusipp from comment #8) > > Note in newer versions of glibc, libpthread is all intergrated into libc and > > there is no issues again. > > > > For Mac OS X/darwin you don't need -lm -pthread because libc has it. > > Two examples where it works by chance is not sufficient to invalidate my > point about the general case. But it does mean the problem you're raising doesn't exist on two of GCC's primary platforms. So the problem is "fixing itself" for many users. > > >gcc libstdc++ implements std::thread. So what's your point again? > > > > Because it implements it on top of pthreads. > > Yeah, so that is an implementation detail because of which I am required to > pass -pthread. Why should the user of gcc even care how std::thread is > implemented? It literally makes no sense. You are supporting my point. It's like that because linking to libpthread has historically caused performance degradation for C++ programs, due to reference counting in shared_ptr and the COW std::string using slower atomic ops if libpthread is linked into the executable. So if the compiler implicitly added -lpthread to every C++ program using C++11 or later, it would hurt the performance of single-threaded programs. It matters less for the std::string reference counting now, because most people use the SSO string not the COW string, but it still affects std::shared_ptr. With recent glibc versions we have a better way to detect whether the process is single-threaded, and no longer make it depend on whether libpthread is linked in or not. And with very recent versions, you don't need -pthread at all. > Debian MinGW-w64 gcc is built in 2 flavours. One with Win32 and one with > posix threading model support. The version with posix threading model > support uses pthreads, and knows how to implement std::thread. Why is GCC > assuming single-threaded in that context? The fact the runtime is capable of supporting threads doesn't mean a given program compiled by the user actually uses threads. I understand your complaint, but I think it's unlikely we're going to change anything in GCC. The docs do not say that -std=c++17 is sufficient to get a fully C++17 conforming environment, so strictly speaking this report is not a bug. You're complaining about something we don't aim for anyway. The implementation documents that you need to add certain other flags for some features: https://gcc.gnu.org/onlinedocs/libstdc++/manual/using.html#manual.intro.using.flags This isn't ideal, but it's not easy to fix. And as already stated, some platforms are changing so it Just Works anyway.
