c++ - Is it a bug for g++ and clang++ optimization? -

#include <unistd.h> #include <pthread.h> #include <stdio.h>  bool m_ok = false;  void* run(void*) {     usleep(1000000);     m_ok = true;     printf ("good bye!\n");     return nullptr; }     int main() {     pthread_t my_thread;     pthread_create(&my_thread, nullptr, &run, nullptr);     while (!m_ok)         continue;     printf("yes!!!\n");     return 0; } 

when compiled above code following commands, good:

$ g++ test.cpp -lpthread -std=c++11 $ clang++ test.cpp -lpthread -std=c++11 

but when tried use optimization flags, program didn't finish. tested of following commands:

$ g++ test.cpp -lpthread -std=c++11 -o1 $ clang++ test.cpp -lpthread -std=c++11 -o1 $ g++ test.cpp -lpthread -std=c++11 -o2 $ clang++ test.cpp -lpthread -std=c++11 -o2 

also versions of g++ , clangs were:

$ g++ --version g++ (ubuntu/linaro 4.8.1-10ubuntu9) 4.8.1 copyright (c) 2013 free software foundation, inc. free software; see source copying conditions.  there no warranty; not merchantability or fitness particular purpose.  $ clang++ --version debian clang version 3.2-7ubuntu1 (tags/release_32/final) (based on llvm 3.2) target: x86_64-pc-linux-gnu thread model: posix 

no. c , c++ memory models not permit cross-thread interactions non-atomic variables, or when not protected mutex/lock. data race , in violation of abstract machine, optimizer within rights launch nethack.

the optimizer written assume, explicitly permitted standard, external sources (including other threads) cannot magically mutate program's state without explicit control user.