Subj : Re: Is std::cerr thread safe
To   : comp.programming.threads
From : Joe Seigh
Date : Wed Jul 20 2005 04:33 pm

Sean Kelly wrote:
> The STL is guaranteed to be thread-safe, but thread safety in this case
> just means that it won't crash horribly if you use it in a
> multi-threaded program.  In my experience, writing to std::cerr from
> multiple threads works just fine, but the elements may be intermixed if
> writes are simultaneous.  A fairly cheap way around this is to do every
> write into a stringstream and then dump it en masse into the error
> stream:
> 
> std::ostringstream ostr;
> ostr << "error: " << errno << '\n';
> std::cerr << ostr.rdbuf();
> 
> The preferable alternative would be to wrap writes in a critical
> section or create your own error stream that does something like using
> thread local storage for its buffer and only flushes when explicitly
> instructed to do so

I see what was happening to the OP.  Using cerr that way is equivalent
to mulitple invocations of unbuffered fprintf, e.g.
  fprintf(stderr, "error: ");
  fprintf(stderr, "%d", errno);
  fprintf(stderr, "\n");

Even so at the filesystem level, the i/o from write() is allowed to be interleaved
though in practice you will almost never see that.

-- 
Joe Seigh

When you get lemons, you make lemonade.
When you get hardware, you make software.

.