Check for `mmap' fails on Cygwin/Windows

[Corinna Vinschen]

When running configure on a Cygwin system, the check for `mmap()'
fails for the following reason:

Windows doesn't support the mmap flags MAP_FIXED the same way
Unix/Linux does. The returned address may be different from the
given address as well as it may fail if the address is in a mem
range already allocated by the calling process.

The reason I think it's a bug in autoconf, though, is that the
usage of MAP_FIXED is discouraged which is documented in the Linux
man pages as well as in SUSv2 documentation:

"Use of MAP_FIXED may result in unspecified behaviour in further
use of brk(), sbrk(), malloc() and shmat(). The use of MAP_FIXED is
discouraged, as it may prevent an implementation from making the
most effective use of resources."

I have attached a patch to acfunctions.m4 which will result in a
working check for `mmap()' on all systems.

Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Developer                        mailto:address@hidden
Red Hat, Inc.
mailto:address@hidden

Index: acfunctions.m4
===================================================================
RCS file: /cvs/autoconf/acfunctions.m4,v
retrieving revision 1.9
diff -u -p -r1.9 acfunctions.m4
--- acfunctions.m4      2000/09/28 14:26:50     1.9
+++ acfunctions.m4      2000/10/05 09:17:26
@@ -1033,12 +1033,8 @@ main ()
   fd = open ("conftestmmap", O_RDWR);
   if (fd < 0)
     exit (1);
-  data2 = (char *) malloc (2 * pagesize);
-  if (!data2)
-    exit (1);
-  data2 += (pagesize - ((int) data2 & (pagesize - 1))) & (pagesize - 1);
-  if (data2 != mmap (data2, pagesize, PROT_READ | PROT_WRITE,
-                     MAP_PRIVATE | MAP_FIXED, fd, 0L))
+  if ((data2 = mmap (NULL, pagesize, PROT_READ | PROT_WRITE,
+                     MAP_PRIVATE, fd, 0L)) == MAP_FAILED)
     exit (1);
   for (i = 0; i < pagesize; ++i)
     if (*(data + i) != *(data2 + i))