Memory layout in Zig with formulas

71 points by raymondtana 8 hours ago on hackernews | 17 comments

[OP] raymondtana | 8 hours ago

I've been learning Zig, and needed a refresher on memory layout (@sizeOf and @alignOf).

Wrote this blog post to summarize what I think are the right ways to understand alignment and size for various data types in Zig, just through experimentation.

Let me know any and all feedback!

dnautics | 4 hours ago

i could be wrong but i believe the zig compiler reserves the right to lay things out differently depending on compilation mode? especially debug. unless it's extern or packed, in which case the layout will be defined.

rvrb | 4 hours ago

`extern` and `packed` container types have well defined layouts. a regular `struct` is an "auto" layout - and the compiler can and will rearrange whenever it wants.

if you need a well defined layout, use `extern`. if your struct makes sense to represent as an integer, use `packed`. I think it is often ill advisable to use `packed` otherwise.

you can explore this yourself on the Type info returned from @TypeInfo(T):

https://ziglang.org/documentation/master/std/#std.builtin.Ty...

LexiMax | 4 hours ago

To wit: https://ziglang.org/documentation/master/#extern-struct

> An extern struct has in-memory layout matching the C ABI for the target.

Zig is really good at speaking the C ABI of the target, but the upshot seems to be that it appears there is no stable Zig-native ABI.

If I'm correct, I wonder if there are plans to settle on a stable ABI at some point in the future. I do know that in other languages the lack of a stable ABI is brought up as a downside, and although I've been burned by C++ ABI stability too many times to agree, I can understand why people would want one.

Cloudef | 3 hours ago

I doubt zig will have stable abi any time soon. It may have some sort of "zig extern" when it gets mature. But stable abi isnt very usful if no-one else can talk it. I have project that uses codegen to effectively implement zig like ABI on top of the C abi.

Heres the kind of code it generates https://zigbin.io/6dba68

It can also generate javascript, heres doom running on browser: https://cloudef.pw/sorvi/#doom.wasm

peesem | 3 hours ago

Andrew Kelley has said relatively recently that there are no plans to introduce a Zig ABI: https://github.com/ziglang/zig/issues/3786#issuecomment-2646...

LexiMax | 2 hours ago

What's interesting is that the scope of the proposal isn't a Zig-specific ABI, but a codified way of expressing certain Zig concepts using the existing C ABI.

That could be an interesting middle ground.

: Cloudef | 2 hours ago
Yeah the new translate-c package already kind of does that.

dnautics | 2 hours ago

in practice, as long as you match the version and release mode, it's fine (though you are playing with fire). I pass raw pointers to zig structs/unions/etc from the zig compiler into a dynamically loaded .so file (via dlload) and as long as my .so file is compiled with the same compiler as the parent (both LLVM, in my case) it's peachy keen.

: Cloudef | an hour ago
You are still playing with fire as the data inside those pointers may be different even if they are the same type. Zig is free to optimize them in anyway it likes depending on the code that touches them (aka its free to assume they never leave the program).

bk496 | 3 hours ago

useful!

thechao | 3 hours ago

I know this is a bit cursed; but, I always wanted a bitfield-on-steroids construct:

    struct Dang : bits 64    // 64 bits wide, int total
    {
        foo : bits 5 @ 0;    // 5 bits wide at bit offset 0
        bar : bits 5 @ 0;
        baz : bits 16 @ 4;   // 16 bits wide at bit offset 4
        tom : bits 11 @ 32;
    };

: _bohm | 2 hours ago
You can kinda do this with Zig’s packed structs and arbitrary-width integers
: sestep | 2 hours ago
I think you can do this with Virgil, but I'm having trouble finding the exact doc page at the moment: https://github.com/titzer/virgil
: Lvl999Noob | 2 hours ago
Are you saying you want foo and bar to completely overlap? And baz and foo / bar to partially overlap? And have lots of unused bits in there too?
: bsder | 26 minutes ago
Look at Erlang bit syntax: https://www.erlang.org/doc/system/bit_syntax.html
It can even be used for pattern matching.
I don't know whether Gleam or Elixir inherited it.

ivanjermakov | an hour ago

I also had to learn struct alignment the hard way working on WebGPU path tracer and struggling to understand why struct fields not aligning (ironically).