https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97976
--- Comment #7 from Peter Bisroev <peter at int19h dot net> --- Thank you for your response Jonathan. If you have a minute, could you please clarify a few things. I have been talking about this behavior with a few colleagues and we are all slightly confused by the same issue. So I think the answers here can definitely help a few people besides myself. >It is undefined to decrement a pointer "before" the start of an object I am sorry, but I am not sure where I am doing this? For example, lets say I am accessing raw memory on an embedded system and I have bytes 0, 1, 1, 1 at addresses 0, 1, 2 and 3 respectively. I know this is a trivial example and makes no sense on x86 arch (almost, in real mode maybe). So if I call containsBackwardsSafe(p, 2) with p == 3, shouldn't I get back -1? I guess it all depends on what is the "object" in this context. Conceivably some other function could have mapped that memory at that address on that system and passed the pointer to containsBackwardsSafe() function. In that case wouldn't the responsibility of "object" be up to the system and not the compiler? I have also just tried going through C11 draft (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf) to try to find relevant standard sections that would describe this behavior. I have read through sections: * 6.2.5 Types (specifically paragraphs 14 and 20) * 6.3.2.3 Pointers * 6.5.3.1 Prefix increment and decrement operators * 6.5.6 Additive operators (specifically paragraphs 7, 8 and 9) * 6.5.16.2 Compound assignment And I cannot seem to find relevant information that forbids pointer decrement as shown in containsBackwardsSafe() or containsBackwards() from the last comment. If you could point me to the right section of the standard it would be incredibly helpful. I am sure I must be missing something obvious. Once again, thank you for your time and help on this. Regards, --peter
