[RFC, doc] Note variable shadowing at max macro using statement expression

Tom de Vries tdevries@suse.de
Mon Apr 8 11:38:00 GMT 2019


Hi,

When suggesting to rewrite the unsafe (with respect to multiple evaluation of
arguments) macro definition:
...
  #define max(a,b) ((a) > (b) ? (a) : (b))
...
into the safe macro definition:
...
  #define maxint(a,b) \
    ({int _a = (a), _b = (b); _a > _b ? _a : _b; })
...
mention the variable shadowing problem for:
...
  #define maxint3(a, b, c) \
    ({int _a = (a), _b = (b), _c = (c); maxint (maxint (_a, _b), _c); })
...

Any comments?

Thanks,
- Tom

[doc] Note variable shadowing at max macro using statement expression

2019-04-08  Tom de Vries  <tdevries@suse.de>

	* doc/extend.texi (@node Statement Exprs): Note variable shadowing at
	max macro using statement expression.

---
 gcc/doc/extend.texi | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 8e0deac26c3..27ed0fb014f 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -143,13 +143,34 @@ follows:
 But this definition computes either @var{a} or @var{b} twice, with bad
 results if the operand has side effects.  In GNU C, if you know the
 type of the operands (here taken as @code{int}), you can define
-the macro safely as follows:
+the macro safe (from evaluating operands more than once) as follows:
 
 @smallexample
 #define maxint(a,b) \
   (@{int _a = (a), _b = (b); _a > _b ? _a : _b; @})
 @end smallexample
 
+Note that introducing variable declarations (as we do in maxint) can
+cause variable shadowing, so while this example using the max macro will
+produce correct results:
+@smallexample
+int _a = 1, _b = 2, c;
+c = max (_a, _b);
+@end smallexample
+this example using maxint will not:
+@smallexample
+int _a = 1, _b = 2, c;
+c = maxint (_a, _b);
+@end smallexample
+
+This problem may for instance occur when we use this pattern recursively, like
+so:
+
+@smallexample
+#define maxint3(a, b, c) \
+  (@{int _a = (a), _b = (b), _c = (c); maxint (maxint (_a, _b), _c); @})
+@end smallexample
+
 Embedded statements are not allowed in constant expressions, such as
 the value of an enumeration constant, the width of a bit-field, or
 the initial value of a static variable.



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