From cd7ffd8a9382c78725f3c09b317bdb4739d3654b Mon Sep 17 00:00:00 2001 From: Tom de Vries Date: Thu, 11 Apr 2019 15:36:59 +0000 Subject: [PATCH] [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); }) ... 2019-04-11 Tom de Vries * doc/extend.texi (@node Statement Exprs): Note variable shadowing at max macro using statement expression. From-SVN: r270287 --- gcc/ChangeLog | 5 +++++ gcc/doc/extend.texi | 26 ++++++++++++++++++++++++-- 2 files changed, 29 insertions(+), 2 deletions(-) diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 7131d58f0ad..cf4434a941a 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,3 +1,8 @@ +2019-04-11 Tom de Vries + + * doc/extend.texi (@node Statement Exprs): Note variable shadowing at + max macro using statement expression. + 2019-04-11 David Edelsohn * xcoffout.h (xcoff_private_rodata_section_name): Declare. diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index 8e0deac26c3..cad7ad49e56 100644 --- a/gcc/doc/extend.texi +++ b/gcc/doc/extend.texi @@ -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. -- 2.30.2