将具有联合字段的C结构映射到Go结构

Question

I'm getting results from syscall to some WinApi in Go. I map simple structs from C code easily, but how to deal with C structs like the following?

typedef struct SPC_LINK_
{
    DWORD dwLinkChoice;
#               define          SPC_URL_LINK_CHOICE         1
#               define          SPC_MONIKER_LINK_CHOICE     2
#               define          SPC_FILE_LINK_CHOICE        3

    union
    {
        LPWSTR                  pwszUrl;
        SPC_SERIALIZED_OBJECT   Moniker;
        LPWSTR                  pwszFile;
    };

} SPC_LINK, *PSPC_LINK;

If all possible types are defined in Go

type SPC_LINK struct {
    dwLinkChoice  DWORD
    Moniker       SPC_SERIALIZED_OBJECT
    pwszFile      LPWSTR
    pwszUrl       LPWSTR
}

after the syscall with unsafe.Pointer to this Go struct as a parameter, I've got it in memory and can access it in Go as usual, but only the first field after dwLinkChoice (Moniker in the code above) is always filled, the other two are always empty. I know this is expected behavior in C, as you can have only one union field at a time. Taking this into account, should I ignore the whole union structure and have some sort of a placeholder in my Go struct?

type SPC_LINK struct {
    dwLinkChoice  DWORD
    dwLink        uintptr // a placeholder, will hold any possible value
}

I set type for the placeholder to uintptr, but what if the original C structure has some other non-pointer types in the union block? I'm really not sure how to handle C unions and looking for any suggestions.

Answer 1

As stated in the cgo documentation,

As Go doesn't have support for C's union type in the general case, C's union types are represented as a Go byte array with the same length.

Maybe you should try this

type SPC_LINK struct {
    dwLinkChoice  DWORD
    dwLink        [{size of the union}]byte
}