On 2024-04-26 08:10, Zack Weinberg wrote:
I think what we should do here is fold AC_C_VARARRAYS into AC_PROG_CC.
Take the test for VLAs completely out of _AC_C_C99_TEST_MAIN, but
unconditionally *run* a test for VLAs as part of AC_PROG_CC. If that
test fails, and __STDC_NO_VLA__ was not defined by the compiler,
then AC_DEFINE([__STDC_NO_VLA__], 1)
Although the basic strategy sounds reasonable, AC_PROG_CC is documented
to operate by setting compiler options not via AC_DEFINE, so presumably
this should put -D__STDC_NO_VLA__ into $CC rather than use AC_DEFINE.
I installed the attached patch, which is less ambitious but should
address the original problem report. The attached patch doesn't preclude
your suggestion, which can be done later as needed.From e9fee73dba5d2156abc48734b5a9faff89dcdc11 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@xxxxxxxxxxx>
Date: Thu, 25 Apr 2024 14:47:20 -0700
Subject: [PATCH] Port better to MSVC
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Problems reported by Antonin Décimo in:
https://lists.gnu.org/r/autoconf/2024-04/msg00001.html
* lib/autoconf/c.m4 (_AC_C_C89_TEST_GLOBALS):
Do not test the value of __STDC__.
(_AC_C_C99_TEST_MAIN): Do not test for VLAs.
(_AC_C_C11_OPTIONS): Also test -std:c11.
---
NEWS | 7 +++++++
doc/autoconf.texi | 9 +++++++--
lib/autoconf/c.m4 | 17 +++++++----------
3 files changed, 21 insertions(+), 12 deletions(-)
diff --git a/NEWS b/NEWS
index 01a058aa..3d968c4c 100644
--- a/NEWS
+++ b/NEWS
@@ -2,6 +2,13 @@ GNU Autoconf NEWS - User visible changes.
* Noteworthy changes in release ?.? (????-??-??) [?]
+** Backward incompatibilities
+
+*** AC_PROG_CC no longer tests for VLAs, or whether __STDC__ is defined.
+ This ports better to MSVC, which does not support variable length
+ arrays and does not define __STDC__. Although C99 requires VLAs,
+ they are optional in C11 and later. Programs can use AC_C_VARARRAYS
+ and __STDC_NO_VLA__ to use VLAs if available.
* Noteworthy changes in release 2.72 (2023-12-22) [release]
diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 186c033b..0fd82056 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -7444,9 +7444,14 @@ the C standard, and @samp{no} if the compiler does not support compiling
standard C at all.
The tests for standard conformance are not comprehensive. They test the
-values of @code{__STDC__} and @code{__STDC_VERSION__}, and a
+value of @code{__STDC_VERSION__}, and a
representative sample of the language features added in each version of
-the C standard. They do not test the C standard library, because the C
+the C standard. They do not examine @code{__STDC__}
+because some compilers by default leave it undefined.
+They do not test for variable-length arrays,
+a C99 feature that was made optional in C11;
+if you need to use this feature when available, use @code{AC_C_VARARRAYS}.
+They do not test the C standard library, because the C
compiler might be generating code for a ``freestanding environment''
(in which most of the standard library is optional). If you need to know
whether a particular C standard header exists, use @code{AC_CHECK_HEADER}.
diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
index d8eab7bb..d410f8bc 100644
--- a/lib/autoconf/c.m4
+++ b/lib/autoconf/c.m4
@@ -1120,12 +1120,8 @@ AC_DEFUN([_AC_C_C89_TEST_GLOBALS],
[m4_divert_text([INIT_PREPARE],
[[# Test code for whether the C compiler supports C89 (global declarations)
ac_c_conftest_c89_globals='
-/* Does the compiler advertise C89 conformance?
- Do not test the value of __STDC__, because some compilers set it to 0
- while being otherwise adequately conformant. */
-#if !defined __STDC__
-# error "Compiler does not advertise C89 conformance"
-#endif
+/* Do not test the value of __STDC__, because some compilers define it to 0
+ or do not define it, while otherwise adequately conforming. */
#include <stddef.h>
#include <stdarg.h>
@@ -1337,13 +1333,12 @@ ac_c_conftest_c99_main='
ni.number = 58;
- int dynamic_array[ni.number];
- dynamic_array[0] = argv[0][0];
- dynamic_array[ni.number - 1] = 543;
+ // Do not test for VLAs, as some otherwise-conforming compilers lack them.
+ // C code should instead use __STDC_NO_VLA__; see Autoconf manual.
// work around unused variable warnings
ok |= (!success || bignum == 0LL || ubignum == 0uLL || newvar[0] == '\''x'\''
- || dynamic_array[ni.number - 1] != 543);
+ || ni.number != 58);
'
]])])
@@ -1553,6 +1548,7 @@ m4_define([_AC_C_C99_OPTIONS], [
# shell quotes around the group.
#
# GCC, Clang -std=gnu11
+# MSVC -std:c11
#
# For IBM XL C for AIX V16.1 or later, '-std=gnu11' should work if
# the user configured with CC='xlclang'. Otherwise, do not try
@@ -1562,6 +1558,7 @@ m4_define([_AC_C_C99_OPTIONS], [
# _Noreturn, which is a win.
m4_define([_AC_C_C11_OPTIONS], [
-std=gnu11
+ -std:c11
])
--
2.40.1
