Re: [Bug-gnulib] addition: size_max.m4, ptrdiff_max.m4

[Paul Eggert]

Bruno Haible <address@hidden> writes:

>     dnl The _AC_COMPUTE_INT macro works up to LONG_MAX, since it uses 'expr',
>     dnl which is guaranteed to work from LONG_MIN to LONG_MAX.
>     ...
>    _AC_COMPUTE_INT([~(size_t)0 / 10], res_hi,
>      [#include <stddef.h>], result=?)
>    _AC_COMPUTE_INT([~(size_t)0 % 10], res_lo,
>      [#include <stddef.h>], result=?)

Ouch.  This is a hassle.  Perhaps Autoconf should be fixed so that
_AC_COMPUTE_INT can handle values outside the range
LONG_MIN..LONG_MAX?  That would be better than having to do this sort
of thing by hand all the time.  (This wouldn't fix the immediate
problem of course; I'm thinking more long-term.)


I thought about the problem with xalloc.h a bit more, and came up with
a different heuristic that should be just as good in practice, and
does not rely on SIZE_MAX or PTRDIFF_MAX, so we can remove that
dependency.  This also avoids the need for xalloc.h to include
<limits.h> and <stdint.h>, which is a minor win.  Here's what I
installed:

2003-11-12  Paul Eggert  <address@hidden>

        * lib/xalloc.h: Do not include <limits.h> or <stdint.h>.
        (xalloc_oversized): Use sizeof (ptrdiff_t) and sizeof (size_t) for
        the heuristic, rather than PTRDIFF_MAX and SIZE_MAX.  This
        heuristic is just as accurate as far as we know, and it removes a
        dependency on size_max.m4 and ptrdiff_max.m4.
        * m4/xalloc.m4 (gl_PREREQ_XALLOC): Do not require gl_SIZE_MAX or
        gl_PTRDIFF_MAX.
        * modules/xalloc (Files): Undo latest change, since xalloc.h
        no longer needs SIZE_MAX or PTRDIFF_MAX.

Index: lib/xalloc.h
===================================================================
RCS file: /cvsroot/gnulib/gnulib/lib/xalloc.h,v
retrieving revision 1.23
diff -p -u -r1.23 xalloc.h
--- lib/xalloc.h        11 Nov 2003 11:58:43 -0000      1.23
+++ lib/xalloc.h        13 Nov 2003 07:17:18 -0000
@@ -20,15 +20,8 @@
 #ifndef XALLOC_H_
 # define XALLOC_H_
 
-/* Get size_t.  */
 # include <stddef.h>
 
-/* Get SIZE_MAX, PTRDIFF_MAX.  */
-# include <limits.h>
-# if HAVE_STDINT_H
-#  include <stdint.h>
-# endif
-
 # ifndef __attribute__
 #  if __GNUC__ < 2 || (__GNUC__ == 2 && __GNUC_MINOR__ < 8) || __STRICT_ANSI__
 #   define __attribute__(x)
@@ -75,14 +68,11 @@ char *xstrdup (const char *str);
    calculations, so the conservative dividend to use here is
    SIZE_MAX - 1, since SIZE_MAX might represent an overflowed value.
    However, malloc (SIZE_MAX) fails on all known hosts where
-   PTRDIFF_MAX < SIZE_MAX, so do not bother to test for
+   sizeof (ptrdiff_t) <= sizeof (size_t), so do not bother to test for
    exactly-SIZE_MAX allocations on such hosts; this avoids a test and
    branch when S is known to be 1.  */
-# if PTRDIFF_MAX < SIZE_MAX
-#  define xalloc_oversized(n, s) (SIZE_MAX / (s) < (n))
-# else
-#  define xalloc_oversized(n, s) ((SIZE_MAX - 1) / (s) < (n))
-# endif
+# define xalloc_oversized(n, s) \
+    ((size_t) (sizeof (ptrdiff_t) <= sizeof (size_t) ? -1 : -2) / (s) < (n))
 
 /* These macros are deprecated; they will go away soon, and are retained
    temporarily only to ease conversion to the functions described above.  */
Index: m4/xalloc.m4
===================================================================
RCS file: /cvsroot/gnulib/gnulib/m4/xalloc.m4,v
retrieving revision 1.7
diff -p -u -r1.7 xalloc.m4
--- m4/xalloc.m4        11 Nov 2003 11:58:43 -0000      1.7
+++ m4/xalloc.m4        13 Nov 2003 07:17:19 -0000
@@ -1,4 +1,4 @@
-# xalloc.m4 serial 7
+# xalloc.m4 serial 8
 dnl Copyright (C) 2002-2003 Free Software Foundation, Inc.
 dnl This file is free software, distributed under the terms of the GNU
 dnl General Public License.  As a special exception to the GNU General
@@ -15,8 +15,6 @@ AC_DEFUN([gl_XALLOC],
 
 # Prerequisites of lib/xalloc.h.
 AC_DEFUN([gl_PREREQ_XALLOC], [
-  AC_REQUIRE([gl_SIZE_MAX])
-  AC_REQUIRE([gl_PTRDIFF_MAX])
   :
 ])
 
Index: modules/xalloc
===================================================================
RCS file: /cvsroot/gnulib/gnulib/modules/xalloc,v
retrieving revision 1.8
diff -p -u -r1.8 xalloc
--- modules/xalloc      11 Nov 2003 11:58:43 -0000      1.8
+++ modules/xalloc      13 Nov 2003 07:17:19 -0000
@@ -6,8 +6,6 @@ lib/xalloc.h
 lib/xmalloc.c
 lib/xstrdup.c
 m4/xalloc.m4
-m4/size_max.m4
-m4/ptrdiff_max.m4
 
 Depends-on:
 malloc