title: Writing concurrently to a file
body: <p>I have this tool in which a single log-like file is written to by several processes.</p>
  
  <p>What I want to achieve is to have the file truncated when it is first opened, and then have all writes done at the end by the several processes that have it open.
  All writes are systematically flushed and mutex-protected so that I don't get jumbled output.</p>
  
  <p>First, a process creates the file, then starts a sequence of other processes, one at a time, that then open the file and write to it (the master sometimes chimes in with additional content; the slave process may or may not be open and writing something).</p>
  
  <p>I'd like, as much as possible, not to use more IPC that what already exists (all I'm doing now is writing to a popen-created pipe). I have no access to external libraries other that the CRT and Win32 API, and I would like not to start writing serialization code.</p>
  
  <p>Here is some code that shows where I've gone:</p>
  
  <pre><code>// open the file. Truncate it if we're the 'master', append to it if we're a 'slave'
  std::ofstream blah(filename, ios::out | (isClient ? ios:app : 0));
  
  // do stuff...
  
  // write stuff
  myMutex.acquire();
  blah &lt;&lt; "stuff to write" &lt;&lt; std::flush;
  myMutex.release();
  </code></pre>
  
  <p>Well, this does not work: although the output of the slave process is ordered as expected, what the master writes is either bunched together or at the wrong place, when it exists at all.</p>
  
  <p>I have two questions: is the flag combination given to the ofstream's constructor the right one ? Am I going the right way anyway ?</p>