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This is absolutely undefined behavior.

The C standard doesn't say anything about stacks or how they should behave or how local variables should be allocated on them. The word "stack" doesn't even occur in the C standard^[Feel free to do a string search on [this working draft version of the 2018 C standard](https://web.archive.org/web/20181230041359if_/http://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).].

C does say that (for arrays) that it is only valid to index one past the end of them, e.g. given `int x[2]`, `x+2` is a valid pointer but it is undefined behavior to dereference that pointer. If we view `&b` as a array of length 1, then you would be invoking undefined behavior when you dereference `&b+1`.

Ultimately, there is absolutely nothing that states how the locations of two separate variables are related to each other in memory. This isn't surprising as a variable doesn't need to even be allocated in memory. It would be a completely valid and not uncommon optimization for the compiler to register allocate the variable `a` or even just completely eliminate it by constant propagation/folding. In that case, your code would most likely be mutating something like a return address or a stack frame pointer which will almost certainly lead to a crash and/or erratic behavior.