Re: mmap test failing on cygwin

[Bob Friesenhahn]

On Sun, 20 Mar 2005, Noah Misch wrote:

> On Sun, Mar 20, 2005 at 10:59:13PM +0100, a.rburgers wrote:
> > the autoconf test for mmap is failing for cygwin,
> > although there is a functional mmap available
> > on that platform..
> >
> > This message gives a patch:
> >
> > http://lists.gnu.org/archive/html/autoconf-patches/2000-10/msg00241.html
>
> Summary: AC_FUNC_MMAP rejects the Cygwin mmap because MAP_FIXED on a malloc'ed
> region fails; rejecting it is bad because many programs that use mmap do not
> need that to work.  Nonetheless, changing AC_FUNC_MMAP to help that majority,
> perhaps marginally, breaks programs relying on the check as it stands.

AC_FUNC_MMAP is working as designed.  Mmap can be used for a great 
many things.  Changing AC_FUNC_MMAP now would surely break programs 
which intend to test for its documented behavior.

I have attached a macro for a somewhat different mmap test which 
verifies that the system supports coherent mmap file I/O.  This means 
that it is safe to update a file via both file descriptor and memory 
mapping without the content of the file getting confused.  One of the 
systems rejected by this test is HP-UX (Cygwin passes).  The test for 
a fixed mapping is not included in this macro.

Bob
======================================
Bob Friesenhahn
address@hidden, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

mmap-fileio.m4
Description: mmap test for coherent I/O