Red Hat Bugzilla – Bug 82681
gcc warning on possible code problem is issuedinconstently
Last modified: 2008-05-01 11:38:04 EDT
From Bugzilla Helper: User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.3b) Gecko/20030119 Description of problem: Two problems with gcc-3.2-7 1. gcc miscompiles my code (which might be a bordercase but is accepted with all other compilers i've tested against) 2. gcc issues a warning in non-optimizing mode but not when -O is present. Stupid example is below ... --- a.c --- #include <stdio.h> #include <ctype.h> #include <stdlib.h> int main(int argc, char *argv[]) { char *p; p = argv[1]; while(*p) *p++ = toupper(*p); printf("%s\n", argv[1]); return 0; } --- end a.c --- $ cc -Wall a.c a.c: In function `main': a.c:11: warning: operation on `p' may be undefined $ ./a.out hallo HALLO $ cc -Wall -O a.c ./a.out hallo ALLO Version-Release number of selected component (if applicable): How reproducible: Always Steps to Reproduce: see above Additional info:
Your code triggers undefined behaviour, and as such gcc can do anything it wants with it.
As i wrote above, it might be undefined behavior and i'm not arguing that (it's just likely to break lots of existing code). However (IMHO) gcc should issue the warning consistently if -Wall is present. I've seen a number of cases where the warning was not issued, with -O used. I'm not pushing this but please consider to reopen the bug for the issue #2 (warning) - i've updated the subject accordingly.