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•1h 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•50m 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):
> 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.
raymondtana•5h ago
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•1h ago
rvrb•50m ago
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...
https://ziglang.org/documentation/master/std/#std.builtin.Ty...
https://ziglang.org/documentation/master/std/#std.builtin.Ty...
LexiMax•35m ago
> 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.