C Programming Language :: Unusual warning


	Unusual warning Hi all, I have the following bit of code: int main() { unsigned int a = 0xAAAAAAAA; unsigned int g = 0x55555555; a = (a <<= 1) ^ g; return 0; } GCC outputs the following warning: code.c: In function `main': code.c:5: warning: operation on `a' may be undefined I was wondering if anyone could explain why "a" may be undefined? Is this referenced somewhere in the cfaq? Arash Partow __________________________________________________ Be one who knows what they don't know, Instead of being one who knows not what they don't know, Thinking they know everything about all things. http://www.partow.net Arash Partow said: - Hide quoted text - > Hi all, > I have the following bit of code: > int main() > { > unsigned int a = 0xAAAAAAAA; > unsigned int g = 0x55555555; > a = (a <<= 1) ^ g; > return 0; > } > GCC outputs the following warning: > code.c: In function `main': > code.c:5: warning: operation on `a' may be undefined > I was wondering if anyone could explain why "a" may be undefined? Because you're violating a "shall" that is not a constraint, to wit 3.3(2): "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be accessed only to determine the value to be stored." > Is this referenced somewhere in the cfaq? Not your exact expression, obviously, but the following URL: <http://c-faq.com/expr/seqpoints.html> covers the basic problem. I would guess that the following modification still expresses your intent and yet is well-defined: a = (a << 1) ^ g; -- Richard Heathfield "Usenet is a strange place" - dmr 29/7/1999 http://www.cpax.org.uk email: rjh at the above domain, - www. - Hide quoted text - Arash Partow wrote: > Hi all, > I have the following bit of code: > int main() > { > unsigned int a = 0xAAAAAAAA; > unsigned int g = 0x55555555; > a = (a <<= 1) ^ g; > return 0; > } > GCC outputs the following warning: > code.c: In function `main': > code.c:5: warning: operation on `a' may be undefined > I was wondering if anyone could explain why "a" may be undefined? The variable `a' is well defined, but the operation on it is not. The fifth line tries to modify `a' twice without an intervening sequence point. That's undefined behavior. Perhaps you actually meant a = (a << 1) ^ g; ... which suffers from no objectionable udnfenideenss. > Is this referenced somewhere in the cfaq? Yes. Questions 3.1, 3.2, 3.3, and 3.3b all address this issue. -- Eric Sosman esos@acm-dot-org.invalid