work in progress: [PATCH 0/2+] open_memstream

[Eric Blake]

The libvirt project would be very interested in using open_memstream,
since it is now part of POSIX 2008.  We even tossed around an idea
on IRC on how to implement it for mingw, details below.

For now, I've got a test working on Linux and cygwin 1.7 as the golden
reference (two independent implementations both satisfying the example
from POSIX 2008), then for BSD systems with funopen(), a light-weight
implementation that does all the work using stdio hooking (borrowing
heavily from my cygwin implementation of open_memstream).  I'm posting
the first two patches now, for any feedback, although the third patch
for a heavy-weight implementation for Solaris, cygwin 1.5.x, and mingw
will not be any sooner than Monday.

For platforms without stdio hooking, the simplest thing we could think
of is to create a temporary file under the hood, then provide gnulib
overrides of fflush and fclose (the only two points at which POSIX
requires that the original arguments to open_memstream are in sync
with what has happened to the stream).  Those hooks would then dump
the current contents of the temp file into malloc'd memory.  So it is
I/O intensive, but seems feasible to implement.  Other alternatives we
considered were using mmap of a temporary file, and doing a gnulib
override of malloc/realloc to recognize the mmap pointer, which is
less I/O intensive but still requires the disk; but that runs into
mmap difficulties (and makes mingw porting harder).  We also
considered hooking every single function that takes a FILE* to
understand string streams, but that was ruled out as too invasive to
be practical.

 ChangeLog                               |   17 +++
 doc/posix-functions/open_memstream.texi |    9 +-
 lib/open_memstream.c                    |  191 +++++++++++++++++++++++++++++++
 lib/stdio.in.h                          |   19 +++
 m4/open_memstream.m4                    |   26 ++++
 m4/stdio_h.m4                           |    8 +-
 modules/open_memstream                  |   29 +++++
 modules/open_memstream-tests            |   12 ++
 modules/stdio                           |    2 +
 tests/test-open_memstream.c             |   58 ++++++++++
 10 files changed, 365 insertions(+), 6 deletions(-)