This code is based on a recent patch to glheretic needed to make heretic work correctly when compiled using gcc-3.3.*: #include <ctype.h> #include <stdio.h> void strupr_OLD (char *s) { while (*s) { *s++ = toupper(*s); } } void strupr_NEW (char *s) { while (*s) { *s = toupper(*s); s++; } } int main(void) { char test_old[] = "this is using strupr_OLD"; char test_new[] = "this is using strupr_NEW"; strupr_OLD(test_old); printf("%s\n", test_old); strupr_NEW(test_new); printf("%s\n", test_new); return 0; } When compiled WITH optimization: markgray@soyo:/usr/src/packages/BUILD/bug> gcc -O2 -Wall -W bug.c markgray@soyo:/usr/src/packages/BUILD/bug> ./a.out HIS IS USING STRUPR_OLD THIS IS USING STRUPR_NEW markgray@soyo:/usr/src/packages/BUILD/bug> Notice that there is NO warning given and that the old code has an off-by-one error in it. When compiled WITHOUT optimization: markgray@soyo:/usr/src/packages/BUILD/bug> gcc -Wall -W bug.c bug.c: In function `strupr_OLD': bug.c:7: warning: operation on `s' may be undefined markgray@soyo:/usr/src/packages/BUILD/bug> ./a.out THIS IS USING STRUPR_OLD THIS IS USING STRUPR_NEW markgray@soyo:/usr/src/packages/BUILD/bug> It does give a nice warning message -- but the code it produces works as the original programmer intended. Why is there no warning given when optimization is used -- and the warning is needed? The programmer who posted the patch said that the change is due to an ISO C change made to allow more aggressive optimization. Does anyone have a list of what kinds of problematic code one should watch out for in old C code? I would argue that the old code has long been considered to be a perfectly well defined C idiom (unlike the newby mistake i=i++), and that code very much like it is very common in older C programs -- glheretic, for one, is still very buggy even after this one fix, so I suspect that there are more "ISO C gotchas" lurking in it. Looking at the kernel source code: markgray@soyo:/usr/src/linux> find -name '*.[ch]' -print0|xargs --null \ grep -P '\*\w+\+\+\s*='|wc -l 3137 markgray@soyo:/usr/src/linux> That's 3137 occurrences of '*symbol++ =' -- and looking at them in context shows that a lot of them are part of a 'while(*something)' loop. What exactly makes the old heretic code so wrong in the eyes of ISO C? A list of example problematic code would be most welcome. (A proper warning from gcc when using optimization would be nice as well.)