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Tell me the truth ... (piefed.jeena.net)

submitted 1 month ago by [email protected] to c/[email protected]

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[–] [email protected] 0 points 1 month ago (1 children)

but if you have a single bool in a stack frame it’s probably going to be more than a byte.

Nope. - if you can't read RISC-V assembly, look at these lines

        sb      a5,-17(s0)
...
        sb      a5,-18(s0)
...
        sb      a5,-19(s0)
...

That is it storing the bools in single bytes. Also I only used RISC-V because I'm way more familiar with it than x86, but it will do the same thing.

on the heap definitely more than a byte

Nope, you can happily malloc(1) and store a bool in it, or malloc(4) and store 4 bools in it. A bool is 1 byte. Consider this a TIL moment.

[–] [email protected] 0 points 1 month ago

c++ guarantees that calls to malloc are aligned https://en.cppreference.com/w/cpp/memory/c/malloc .

you can call malloc(1) ofc, but calling malloc_usable_size(malloc(1)) is giving me 24, so it at least allocated 24 bytes for my 1, plus any tracking overhead

yeah, as I said, in a stack frame. not surprised a compiler packed them into single bytes in the same frame (but I wouldn't be that surprised the other way either), but the system v abi guarantees at least 4 byte alignment of a stack frame on entering a fn, so if you stored a single bool it'll get 3+ extra bytes added on the next fn call.

computers align things. you normally don't have to think about it. Consider this a TIL moment.