multiple threads writing to std::cout or std::cerr

Question

I have OpenMP threads that write to the console via cout and cerr. This of course is not safe, since output can be interleaved. I could do something like

#pr         


        

Answer 1

As others pointed out, in C++11, std::cout is thread-safe.

However if you use it like

std::cout << 1 << 2 << 3;

with different threads, the output can still be interleaved, since every << is a new function call which can be preceeded by any function call on another thread.

To avoid interleaving without a #pragma omp critical - which would lock everything - you can do the following:

std::stringstream stream; // #include  for this
stream << 1 << 2 << 3;
std::cout << stream.str();

The three calls writing 123 to the stream are happening in only one thread to a local, non-shared object, therefore aren't affected by any other threads. Then, there is only one call to the shared output stream std::cout, where the order of items 123 is already fixed, therefore won't get messed up.