c++ multiple processes writing to the same file - Interprocess mutex?

My question is this: what is the best way (or at least an effective way) to write to a file from multiple processes?

Note: I am using c++11 and I want this to run on any platform (i.e. pure c++ code only).

I have done some research and here is what I have concluded:

1. In my processes I have multiple threads. This is easily handled within each process using a mutex to serialise access to the file.
2. A c++/c++11 mutex or conditional variable cannot be used to serialise between processes.
3. I need some sort of external semaphore / lock file to act as a "mutex"... but I am not sure how to go about doing this.

I have seen applications use things like creating a ".lock" file when in use. But for multiple rapid access it seems like this may not work (i.e. after one process has decided the file does not exist another could create it and then the first process will also try to create it) because the operation to test and create the file is not atomic.

Note: Each process always writes one entire line at a time. I had thought that this might be enough to make the operation "atomic" (in that a whole line would get buffered before the next one), but this does not appear to be the case (unless I have my code wrong) since I (rarely) get a mangled line.

This must be a common-ish issue, but I have not yet found a workable solution to it. Any code snippets would be welcome.