Rust / C FFI structs

help
1

I have a bit of an interesting situation that I'm looking for a better solution to. Right now I copy data around, but it's not the best solution.

I have the following C structs that I need to interact with:

struct hdr {
Int field1;
Char field2;
...
char data[1];
};

Hdr.data could be start of a new struct (there's about 30 to choose from) or the start of payload, depends on some of the fields in the header.

What I get is an array of u8 that should be converted to / from all this.

What is the most effective way of doing this in Rust, both reading data into various structs as well as dumping data from various structs.

At first I tried replicating the above, and dropping the hdr.data, but that comes with padding issues ofc.

Right now I'm dumping to / from an u8 array and into different structs, but that means a lot of memcopy.

Any guidance would be mostly appreciated.

2

for simple cases, rust has repr(C) enums, but in general, tricky to model what I would like to call "internally discriminated sum type" in a way that feels natural and idiomatic in rust. I would typically just make the ffi data structures declared as distinct types, then create a rust enum to sum them up. it works pretty well in practice.

you should properly parse/deserialize the bytes into strongly typed value, instead of clever type-punning.

did you measure the code's performance? are you sure the memcpy-es are the bottle neck? personally, I would just go the simple route and not preemptively micro-optimize in such cases.

that being said, if you do need to eliminate the copies, you can always parse the bytes "in-place" and return borrowed data, given the data layout is really compatible (e.g. alignment, padding, etc).

3

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