Forcing fsync of ofstream with libstdc++-3?

Forcing fsync of ofstream with libstdc++-3?

[Kenneth C. Schalk]

I'm trying to get some older C++ code to work correctly on Fedora Core
1, which uses gcc 3.3.2.  The application is a server designed to be
fault-tolerant.  It uses fstreams for writing files.  Under certain
circumstances, it needs to guarantee that some information has been
committed to disk.

With older C++ run-times, it would do something like:

  fsync(ofs.rdbuf()->fd())

But since the fd method was removed from the filebuf, that's no longer
an option.  It's no longer possible to construct an fstream from a
file descriptor or to attach an fstream to a file descriptor, so it's
not possible to get a fstream and a corresponding file descriptor that
way.  I've run a simple test program under strace, and I'm sure that
flushing an ofstream does *not* call fsync.

I'm aware of the __gnu_cxx::stdio_filebuf class, but I'd rather not
use that as I'd like to maintain portability to different compilers
and C++ run-times.

Essentially it looks to me as though my choices are:

- Write my own fstream-like class that allows me to force an fsync.

- Re-write significant pieces of code to use C stdio instead of C++
streams.

- Use temporary ostringstream objects to build up the bytes to be
written and then write them to the file using C stdio.

None of these particularly appeals to me, so I'm hoping someone can
suggest another reasonably portable option.

--Ken Schalk                            Vesta: Build Smarter
                                        www.vestasys.org