go - COM 中的 UTF-16 字符串

Question

请有人解释一下 Go 指针的魔法
我尝试调用 COM 函数

func (p *IServer) Authorize(user, pass string) error {
  UserName := ole.SysAllocString(login.UserName)
  defer ole.SysFreeString(UserName)
  UserPsw := ole.SysAllocString(login.UserPsw)
  defer ole.SysFreeString(UserPsw)  

  // HRESULT IServer::Authorize([in] BSTR UserName, [in] BSTR UserPsw, [out] VARIANT* SID, [out, retval] long* Result)
  hr, _, _ := Call(p.VTable().Authorize,
    uintptr(unsafe.Pointer(p)),
    uintptr(unsafe.Pointer(UserName)),
    uintptr(unsafe.Pointer(UserPsw)),
    uintptr(unsafe.Pointer(sid)),
    uintptr(unsafe.Pointer(&res)))

   ...
}

此代码运行良好，但是当我将转换替换为

UserName := syscall.StringToUTF16Ptr(login.UserName)
UserPsw := syscall.StringToUTF16Ptr(login.UserPsw)

它会导致访问违规!来自 go-ole

func SysAllocString(v string) (ss *int16) {
  pss, _, _ := procSysAllocString.Call(uintptr(unsafe.Pointer(syscall.StringToUTF16Ptr(v))))
  ss = (*int16)(unsafe.Pointer(pss))
  return
}

我究竟做错了什么？

更新:在 C/C++ 中 wchar* 指针在 386/amd64 中都可以正常工作

void Authorize(IServer* p wchar* user, wchar* pass) {
   p.Authorize(user, pass ....
}

Answer 1

SysAllocString 返回 BSTR 类型，com 类型对象。

 typedef struct {
 #ifdef _WIN64
     DWORD pad;
 #endif
     DWORD size;
     union {
         char ptr[1];
         WCHAR str[1];
         DWORD dwptr[1];
     } u; // take it as a starting point of the string 
 } bstr_t;

换句话说，它是相同的 utf16 编码字符串，但前缀是其大小(Unicode 字符的长度乘以 wchar_t 的大小(2-4 字节))。出于优化的原因，它也有填充。

由于它的 float 大小，最好使用 ole 包而不是重新发明轮子。如果你想自己实现它，并且 wchar_t 的大小为 int16(2 字节)，那么你必须执行以下操作:

(半伪代码，我没测试过)

type BSTR *uint16

func SysAllocString(str string) (result BSTR) {
    // DWORD == int32 == rune
    const padf = "\x00" // only for 64 bit system
    const sizef = "\x00"

    // int32 == 4 byte 
    // int16 == 2 byte
    const wordSize = unsafe.Sizeof(int16(0))

    utf16 := utf16.Encode([]rune(padf + sizef + str))

    /* pad is on index 0 and 1 */
    size :=  &utf16[2 /* 0 for 32 bit system */]

    // set "size" field as unicode charachers length multypled by size of wchar_t
    *(*rune)(unsafe.Pointer(size)) = rune((len(utf16)-2) * int(wordSize))


    result = BSTR(&utf16[0])

    return
}

// ...
bstr := SysAllocString(login.UserName)

uintptr(unsafe.Pointer(bstr))