Struct layouts in C are defined by the platform's ABI, and in every sane platform, that just looks like "lay out each element in order, adding the smallest-possible amount of padding to satisfy alignment requirements" [0]. There are presumably oddball platforms which do something else, but good luck actually finding one that has lots of RAM, an ordinary filesystem, and so on. (Within the realm of sane platforms, there are a few alignment oddities, but it's always safe to build packed structs as if each type is aligned to its size.)

Struct layouts for FFI in other languages tend to follow the C convention and/or allow explicit field offsets to be specified. Regardless, if you use the proper language constructs, it's nowhere near as undefined as "whatever the compiler happens to generate".

[0] https://www.gnu.org/software/c-intro-and-ref/manual/html_nod...