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Re: [PATCH] assert: Suppress pedantic warning caused by statement expression [BZ# 21242]
On 07/05/2017 10:15 PM, Zack Weinberg wrote:
> On Wed, Jul 5, 2017 at 11:51 AM, Florian Weimer <fweimer@redhat.com> wrote:
>> On 07/05/2017 05:46 PM, Zack Weinberg wrote:
>>> A problem occurs to me: expressions involving VLAs _are_ evaluated
>>> inside sizeof.
>>
>> The type of the sizeof argument would still be int (due to the
>> comparison against 0), so this doesn't actually occur.
>
> I rechecked what C99 says about sizeof and VLAs, and you're right -
> the operand of sizeof is only evaluated when sizeof is _directly_
> applied to a VLA. So this is indeed safe, but I think this wrinkle
> should be mentioned in the comment. Perhaps
>
> /* The first occurrence of EXPR is not evaluated due to the sizeof,
> but will trigger any pedantic warnings masked by the __extension__
> for the second occurrence. The explicit comparison against zero
> ensures that sizeof is not directly applied to a function pointer or
> bit-field (which would be ill-formed) or VLA (which would be evaluated). */
>
> zw
What about the attached patch?
Siddhesh, is this okay during the freeze? I'd like to backport it to
2.25 as well.
Thanks,
Florian
assert: Suppress pedantic warning caused by statement expression
2017-07-06 Florian Weimer <fweimer@redhat.com>
[BZ #21242]
* assert/assert.h [__GNUC__ && !__STRICT_ANSI__] (assert):
Suppress pedantic warning resulting from statement expression.
(__ASSERT_FUNCTION): Add missing __extendsion__.
diff --git a/assert/assert.h b/assert/assert.h
index 22f0195..6801cfe 100644
--- a/assert/assert.h
+++ b/assert/assert.h
@@ -91,13 +91,19 @@ __END_DECLS
? __ASSERT_VOID_CAST (0) \
: __assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION))
# else
+/* The first occurrence of EXPR is not evaluated due to the sizeof,
+ but will trigger any pedantic warnings masked by the __extension__
+ for the second occurrence. The explicit comparison against zero is
+ required to support function pointers and bit fields in this
+ context, and to suppress the evaluation of variable length
+ arrays. */
# define assert(expr) \
- ({ \
+ ((void) sizeof ((expr) == 0), __extension__ ({ \
if (expr) \
; /* empty */ \
else \
__assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION); \
- })
+ }))
# endif
# ifdef __USE_GNU
@@ -113,7 +119,7 @@ __END_DECLS
C9x has a similar variable called __func__, but prefer the GCC one since
it demangles C++ function names. */
# if defined __cplusplus ? __GNUC_PREREQ (2, 6) : __GNUC_PREREQ (2, 4)
-# define __ASSERT_FUNCTION __PRETTY_FUNCTION__
+# define __ASSERT_FUNCTION __extension__ __PRETTY_FUNCTION__
# else
# if defined __STDC_VERSION__ && __STDC_VERSION__ >= 199901L
# define __ASSERT_FUNCTION __func__