[doc] Note variable shadowing at max macro using statement expression
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); })
...
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 | 26 ++++++++++++++++++++++++--
1 file changed, 24 insertions(+), 2 deletions(-)
@@ -142,14 +142,36 @@ follows:
@cindex side effects, macro argument
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:
+type of the operands (here taken as @code{int}), you can avoid this
+problem by defining the macro 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 @code{maxint}) can
+cause variable shadowing, so while this example using the @code{max} macro
+produces correct results:
+@smallexample
+int _a = 1, _b = 2, c;
+c = max (_a, _b);
+@end smallexample
+@noindent
+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.