i=i-- -- does gcc document, which will be done

Question

Hello

Once again, our best loved "i=i--" -like issues. In C99 we have:

6.5 Expressions #2: Between the previous and next sequence point an object shall have its stored value modified at most once

70) This paragraph renders !!undefined!! statement expressions such as

i = ++i + 1;

But for undefinded behavior there can be variants from random output to "program execution in a documented manner" (c99 3.4.3)

So, the question:

Does gcc document the behavior for i=i++, i=i--, and so on statements?

Actual code is

int main(){int i=2;i=i--;return i;}

Answer 1

It's left to the back-end implementation to decide what it does. You can use -S and inspect the generated code to determine the exact sequence of events.

Answer 2

GCC does not document this behaviour. The Warning Options page mentions sequence points issues in -Wsequence-point, but does not hint at well-defined sematics for violations.

GCC does have a nice list of C Implementation Defined Behaviour, but I could not find any reference to this issue here either.

Answer 3

It is not documented but even it it was, I wouldn't want to read it. You should never rely on what a particular implementation does when running into undefined behavior.

Answer 4

why on earth would you want to do that? Seriously. I'm curious.