undefined | Better HN

0 pointsbodhiandpysics14y ago0 comments

When dealing with unix syscalls, you actually sometimes need to pass structs that aren't fully initialized, or are initialized to zero with the exception of some fields. The quintessential example is the sigaction struct.

0 comments

pjmlp4y ago

Another good example is Win32, in many cases only the length is initialized and the API does the rest, this allows them to change the ABI across versions without impacting the caller.

ATsch4y ago

That should work perfectly with multiple structs. Define

    #[repr("C")]
    struct MyThing {
        length: usize,
        data: MaybeUninit<MyInner>
    }

Then initialize with sizeof(data) and MaybeUninit::uninitialized(). When the call is complete, assume_init() and access the fields of the result struct as normal.

pjmlp4y ago

I see, thanks.

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0 pointsbodhiandpysics14y ago0 comments

0 comments

pjmlp4y ago

Another good example is Win32, in many cases only the length is initialized and the API does the rest, this allows them to change the ABI across versions without impacting the caller.

ATsch4y ago

That should work perfectly with multiple structs. Define

    #[repr("C")]
    struct MyThing {
        length: usize,
        data: MaybeUninit<MyInner>
    }

Then initialize with sizeof(data) and MaybeUninit::uninitialized(). When the call is complete, assume_init() and access the fields of the result struct as normal.

pjmlp4y ago

I see, thanks.

j / k navigate · click thread line to collapse