Easy way to avoid clog collision from different threads?

Question

I have a multi-threaded program where two separate threads are sending debug output to std::clog and the outputs are interspersed. I would like to find an easy way to force the output to at least be kept separate except at line feeds in the output. This way, the debug output can be more readily interpreted. In some places, I've inserted a sleep(1) before the output and gather up the output into a string before sending it to clog to reduce the chances of collision, but I'd prefer a more robust and sure fired solution.

Is there an easy way to ensure that each thread writes a whole line at a time to std::clog before the other thread can get in and write its own line of output?

Answer 1

There's no particularly easy way of doing this, and there's an extended discussion about it here: http://www.cplusplus.com/forum/general/27760/

The problem is somewhat solved there with the creation of a new AtomicStream that writes an entire line atomically, before anything else is streamed (this is done with buffering tricks). You'll need to come up with a similar solution. Sorry for the non-easy answer -- thread synchronization will somehow have to make it into your solution.

This may be derivative, but if your std::clog redirects to a file, you could also have multiple files for the multiple threads.

Answer 2

You ... can't. You're writing to the same stream at the same time. The buffering in clog will help a little, but there's still no guarantees.

Unless you want to synchronize your threads' logging (kinda expensive for what you're doing) maybe you should look at using a logging facility instead (this would allow you to log to say, different files for different things).

Answer 3

Yeah, you're lookign for a cross-thread synchronization method. These are usually available in an operating system's API, you can also find one in Boost.