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

#pragma omp critical(cerr)
{
   cerr << "my variable: " << variable << endl;
}

It would be nicer if could replace cerr with a thread-safe version, similar to the approach explained in the valgrind DRD manual (http://valgrind.org/docs/manual/drd-manual.html#drd-manual.effective-use) which involves deriving a class from std::ostreambuf. Ideally in the end I would just replace cerr with my own threaded cerr, e.g. simply:

tcerr << "my variable: " << variable << endl;

Such a class could print to the console as soon as it encounters an "endl". I do not mind if lines from different threads are interleaved, but each line should come only from one thread.

I do not really understand how all this streaming in C++ works, it is too complicated. Has anybody such a class or can show me how to create such a class for that purpose?

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 <sstream> 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.