Golang CGo: converting union field to Go type

Question

I\'m working with this C struct on a 64 bit platform, trying to access the ui32v field in the value union:

struct _GNetSnmp         


        

Answer 1

The solution was first to cast to uintptr, then cast to unsafe.Pointer ie two separate casts:

func union_to_guint32_ptr(cbytes [8]byte) (result *_Ctype_guint32) {
    buf := bytes.NewBuffer(cbytes[:])
    var ptr uint64
    if err := binary.Read(buf, binary.LittleEndian, &ptr); err == nil {
        uptr := uintptr(ptr)
        return (*_Ctype_guint32)(unsafe.Pointer(uptr))
    }   
    return nil 
}                

I checked this by comparing results with a command line tool, and it's returning correct results.

---

Context

// gsnmp._Ctype_gpointer -> *gsnmp._Ctype_GNetSnmpVarBind
data := (*C.GNetSnmpVarBind)(out.data)

switch VarBindType(data._type) {
case GNET_SNMP_VARBIND_TYPE_OBJECTID:
    result += "GNET_SNMP_VARBIND_TYPE_OBJECTID" + ":"
    guint32_star := union_to_guint32_ptr(data.value)
    result += OidArrayToString(guint32_star, data.value_len)