难以访问 go 中的嵌套 C 联合成员

Question

Disclaimer: I am new to Go/CGo.

I'm working with this C struct on a 64 bit platform, trying to access the uint32 member of a union

typedef enum {
    n = 0,
    ix = 1,
    iy = 3 
} enum_x;

struct smallStruct_s {
     union {
        uint32 a[4];
        uint32 b[8];
        uint32 c[16];
    } u;
} smallStruct_t;

struct bigStruct_s {
   enum_x fa;
   union {
        uint32 member_to_access; <<<<<< This is member that needs to be accessed
        smallStruct_t an;
   } un_t;
} bigStruct_t;

I am facing difficulty accessing/mutating member_to_access in Go given I have access to bigStruct_t which can be accessed using C.bigStruct_t .

How can I pass the address of member_to_access using unsafe.Pointer to a function which accepts void* in C without violating any memory constraints.

The machine is little endian

I tried using byteArray and C buffers as mentioned in Golang CGo: converting union field to Go type but couldn't understand why the function takes size of 8 bytes array as parameter.

Answer 1

Union can be represented as array of bytes, so this field weighs as size of largest element ([sizeof_union's_largest_element]byte). Enum can be represented as int.

I propose to use offsets for that:

type bigStruct struct {
    instance unsafe.Pointer
}

func (bs *bigStruct) fa() int {
    return int(*(*C.int)(bs.instance))
}

func (bs *bigStruct) memberToAccess() uint32 {
    // C.sizeof_int refers to sizeof(enum_x)
    return uint32(*(*C.uint32_t)(unsafe.Pointer(uintptr(bs.instance) + C.sizeof_int)))
}

func (bs *bigStruct) an() *C.smallStruct_t {
    return (*C.smallStruct_t)(unsafe.Pointer(uintptr(bs.instance) + C.sizeof_int))
}

func (bs *bigStruct) an_u() []byte {
    // cgo having same thought about and takes smallStruct_t.u as array
    return (*C.smallStruct_t)(unsafe.Pointer(uintptr(bs.instance) + C.sizeof_int)).u[:]
}

func (bs *bigStruct) next_field_after_un_t() *Type {
    return (*Type)(unsafe.Pointer(uintptr(bs.instance) + C.sizeof_int + C.sizeof_smallStruct_t))
}