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would using casts fix the problem?would using casts fix the problem?Posted Apr 16, 2008 19:46 UTC (Wed) by MathFox (subscriber, #6104)In reply to: would using casts fix the problem? by roskegg Parent article: GCC and pointer overflows I recall that one programmer asked me why
int main() {
char a[10];
char *aend = a+10;
printf("%ld\n", (int)aend - (int)a);
}
did print something different from 10 on a Cray vector computer...
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would using casts fix the problem? Posted Apr 16, 2008 19:50 UTC (Wed) by MathFox (subscriber, #6104) [Link] Make the printf line printf("%lu\n", (unsigned long)aend - (unsigned long)a);
would using casts fix the problem? Posted Apr 16, 2008 20:27 UTC (Wed) by jengelh (subscriber, #33263) [Link] Or better yet don't cast at all.%td for printf can take a ptrdiff_t, which is what such a subtraction should usually yield.
would using casts fix the problem? Posted Apr 16, 2008 21:29 UTC (Wed) by MathFox (subscriber, #6104) [Link] I guess that you missed the point completely. aend-a (no casts needed) should give 10 on any platform. The bit pattern in a pointer is implementation defined and that particular platform had (64-bits IIRC) word-based addresses in a 48 bit address space. The low 48 bits of a character pointer were the address of the word containing the character; the high bits addressed the byte in the word.Standard conforming programs don't rely on pointer layout.
would using casts fix the problem? Posted Apr 17, 2008 18:55 UTC (Thu) by hummassa (subscriber, #307) [Link] yes, aend - a (without cast) _is_ 10 in any platform, but (int)aend - (int)a is _undefined_ behaviour, because the bit pattern when converting from pointer to int is implementation-defined.
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