Sometimes it's harded than that. For example in Windows XP, if you have __declspec(thread) variable then it won't work if it's in DLL (this has been fixed since

Sometimes it's harded than that. For example in Windows XP, if you have __declspec(thread) variable then it won't work if it's in DLL (this has been fixed since Vista). Not really a compiler, but more like linker/loader problem. If the __declspec(thread) is in the main .exe it works. Things like this affect for example ANGLE (WebGL) when compiling for XP, as such one would have to resort to TlsAlloc/TlsFree. It's just an example, where an OSS might've used the gcc/clang primitive for thread-safe var, and later someone does it for Visual Studio, only to understand later that it won't work when in DLL for XP. Now I know XP is no longer supported, but a lot of OSS projects still target it. (and probably lots of others). Then again, it's not such biggie, for example Elmindreda, the GLFW developer was really kind to re-implement the feature once I reported the problem.