Broken autoconf mmap test (was Re: 1.7] BUG - GREP slows to a crawl with large number of matches on a single file)

[Eric Blake]

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[please limit replies about the patch itself to autoconf-patches]

According to Corinna Vinschen on 11/9/2009 7:05 AM:
> This part of the testcase
> 
>   data2 = (char *) malloc (2 * pagesize);
>   if (!data2)
>     return 1;
>   data2 += (pagesize - ((long int) data2 & (pagesize - 1))) & (pagesize - 1);
>   if (data2 != mmap (data2, pagesize, PROT_READ | PROT_WRITE,
>                        MAP_PRIVATE | MAP_FIXED, fd, 0L))
>     return 1;
> 
> is bad.  The chance that the address of data2 is not usable for mmap on
> Windows/Cygwin is 100%.  The problem here is that the generic HAVE_MMAP
> test tests one certain feature, which is not usable on Windows, and which
> is non-portable.

MAP_FIXED appears to be more portable when the fixed address was obtained
from a previous mmap call.  Therefore, this patch fixes the macro as well
as making diagnosing configure failures more accurately pinpoint why they
are declaring failure.  I don't have access to HP-UX 11, which is another
platform where AC_FUNC_MMAP was failing; I would appreciate if someone
else could see if this makes a difference there.  But I have verified that
this now sets HAVE_MMAP for cygwin 1.5.x and cygwin 1.7 where the old
version failed, and that it does not change behavior on Linux or OpenBSD.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             ebb9@byu.net
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