I don't think C standard library particularly wanted to be problematic to use, but they just weren't prepared for multithreaded general programming, and tried t

I don't think C standard library particularly wanted to be problematic to use, but they just weren't prepared for multithreaded general programming, and tried to patch it with e.g. the _r() version of methods. An example where I recently made a change is localtime() where I changed to use the _r() variant after wrongly just calling localtime(), but only later realised they were using a global per-process buffer which might produce wrong results in my logging code (e.g. if another thread calls localtime then my time buffer would be updated after I populated it 2 seconds prior). Now the clib could have made a per thread TLS slot for each threads' time values without too much overhead, which would have automatically fixed any careless uses of localtime() in multiple threads, but instead opted for the separate thread safe function calls.