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[PATCH 3/3] Don't test variable-length array by default


From: Antonin Décimo
Subject: [PATCH 3/3] Don't test variable-length array by default
Date: Wed, 24 Apr 2024 15:59:25 -0300

Variable-length arrays (VLA) are a feature of C99, that was made
optional in C11. A compliant C11 compiler defines __STDC_NO_VLA__ to 1
if it doesn't support VLAs.

MSVC doesn't implement VLAs in C99 (non-compliant) but defines
__STDC_NO_VLA__ in C11 mode. If we don't guard the VLA in
_AC_C_C99_TEST_MAIN, as the test is included in
_AC_C_C11_TEST_PROGRAM, MSVC won't be considered as a compiler
supporting C11 (which, to some extent, it does).

Fix this by guarding the C99 code using VLA with the C11 macro
__STDC_NO_VLA__.
---
 lib/autoconf/c.m4 | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
index 306ee63e..0ba98b28 100644
--- a/lib/autoconf/c.m4
+++ b/lib/autoconf/c.m4
@@ -1337,13 +1337,18 @@ ac_c_conftest_c99_main='
 
   ni.number = 58;
 
+#if !defined(__STDC_NO_VLA__) || __STDC_NO_VLA__ != 1
   int dynamic_array[ni.number];
   dynamic_array[0] = argv[0][0];
   dynamic_array[ni.number - 1] = 543;
+#endif
 
   // work around unused variable warnings
   ok |= (!success || bignum == 0LL || ubignum == 0uLL || newvar[0] == '\''x'\''
-        || dynamic_array[ni.number - 1] != 543);
+#if !defined(__STDC_NO_VLA__) || __STDC_NO_VLA__ != 1
+        || dynamic_array[ni.number - 1] != 543
+#endif
+       );
 '
 ]])])
 
-- 
2.44.0




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