How to define align for Packed based on Platform

[aeb-dev]

I have create a library for steam api. In steam, the alignment of structs changes from platform to platform. i.e. certain structs are aligned by 4 in linux and mac but 8 in windows.

I could not find a way to this in dart without writing two independent code per different configuration.

Here is an example:

import "dart:ffi";

const int align = 4;
@Packed(align)
class ActiveBeaconsUpdated extends Opaque {
}

The constant align should be able to change depending on the platform.

Now, I afaik, compiler directives are not implemented in dart so there is no way to do this in compile time.

There again afaik, the only way to do this is eliminate const from the constructor of the Packed.

If there is any workaround other that writing duplicate package per platform please let me know.

The steam api repo is here if you want to investigate further: https://github.com/aeb-dev/steamworks

[mraleph]

You should not need @Packed on an Opaque, maybe I am missing something. Did you mean to say extends Struct?

This might need either AbiSpecificInteger treatment or tie into introducing compile time constants to distinguish specific platforms.

/cc @dcharkes @mkustermann

[dcharkes]

We indeed do not have support for different alignment of the same struct for different ABIs. We could implement that, it should be a fairly small change.

Implementing ABI-specific structs would be a more general solution. That is a bit more involved change, but it would solve this issue as well (and prevent us from having two ways to solve this issue).

• [ffi] Add support for abi-specific structs #42816

[aeb-dev]

You should not need @PACKED on an Opaque, maybe I am missing something. Did you mean to say extends Struct?

I have no idea. Steam has different alignment for the same struct but the struct is empty on steam as well.

@dcharkes I checked the issue you have linked and I like the idea. Does it have any planned release date?