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redeexpressos

•Created by redeexpressos on 1/21/2025 in #help

compile-time `IntrPtr` size for union-like structs with `FieldOffset`

hi. im developing a FFI in Rust in order to allow C# code to call methods from it. im running through an low-level issue, but maybe there is a solution. What I want to represent is something like:

enum Encryption {
    NoEncryption,
    EncryptionKey(*mut c_char),
    EncryptionKeys(EncryptionKeys),
}

enum Encryption {
    NoEncryption,
    EncryptionKey(*mut c_char),
    EncryptionKeys(EncryptionKeys),
}

so my idea was to represent this as the following in C#:

    [StructLayout(LayoutKind.Explicit)]
    public unsafe struct Encryption
    {
        [FieldOffset(0)]
        public EncryptionTag Tag; // discriminator

        [FieldOffset(4)]
        public IntPtr EncryptionKey; // pointer to c string, will cast later

        [FieldOffset(4)]
        public IntPtr EncryptionKeys; // pointer to another struct, will cast later
    }

    [StructLayout(LayoutKind.Explicit)]
    public unsafe struct Encryption
    {
        [FieldOffset(0)]
        public EncryptionTag Tag; // discriminator

        [FieldOffset(4)]
        public IntPtr EncryptionKey; // pointer to c string, will cast later

        [FieldOffset(4)]
        public IntPtr EncryptionKeys; // pointer to another struct, will cast later
    }

however, there is a issue. IntPtr size can differ on different architectures, which means my memory alignment offsets are wrong. I've tried [FieldOffset(4)] but compiler does not like it. Is there a good solution for this?

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