Specified by whom? Not the C standard for sure. It is indeed soecified by individual ABIs, and ABIs don't tend to do anything too weird, but that's another question.

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jstimpfle 7 hours ago | parent [-]

looks like I was wrong, but here is the de-facto standard I was relying on over the years ;-). Not that I've memcpied many structs to file directly btw. http://www.catb.org/esr/structure-packing/

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jcranmer 6 hours ago | parent [-]

The general struct layout algorithm is that you lay out the first member at the address of the struct (this is guaranteed by C), and then subsequent fields in order (also guaranteed by C). What isn't guaranteed is how fields get their alignment, in particular shenanigans you can do with allocating fields in the padding of their prior field, and bitfields in general are horribly underspecified.

In practice, C doesn't do any padding shenanigans, but C++ does (but only for non-POD structs, and then you discover there's several slightly different definitions that mean basically "POD", so have fun predicting which one is the one that actually matters for your use case).

	▲	RossBencina 10 minutes ago \| parent [-]
		If you sort your fields by size or manually pad them with natural alignment, and use #pragma pack or equivalent non-standard directives that gets you most of the way there. But yes, avoid bitfields. C++ "standard layout type" is the modern equivalent of "POD" I think.