CGO: Convert between LPCWSTR and string

Question

I'm writing CGO bindings for a library which uses LPCWSTR for all of its string types. How do I convert from C.LPCWSTR to string and vice-versa?

Answer 1

You should be able to "cast" the LPCWSTR as a []uint16, and use the utf16 package to decode the characters

// take a C.wstr pointer, and convert it to a go slice
// `sz` is the length of the LPCWSTR
wstr := (*[1 << 30-1]uint16)(unsafe.Pointer(C.wstr))[:sz:sz]
runes := utf16.Decode(wstr)
goString := string(runes)

You generally don't want to be passing Go pointers into your C code, so when converting from a string to an LPCWSTR, you will want to alloc the memory in C. A solution for converting from a Go string s might look like:

func Encode(s string) C.LPCWSTR {
    wstr := utf16.Encode([]rune(s))

    p := C.calloc(C.size_t(len(wstr)+1), C.sizeof_uint16_t)
    pp := (*[1 << 30]uint16)(p)
    copy(pp[:], wstr)

    return (C.LPCWSTR)(p)
}

There may also be some MFC macros that could help convert to and from cstrings which you could take advantage of with simple wrapper functions in C. This way you could easily copy the data in and out just using the builtin C.CString and C.GoString functions.

Answer 2

As an alternative to allocating memory in C, one could just call syscall.UTF16PtrFromString:

func Encode(s string) C.LPCWSTR {
    ptr, _ := syscall.UTF16PtrFromString(s)
    return C.LPCWSTR(unsafe.Pointer(ptr))
}

This assumes that the API you're trying to call (supposedly a win32 API since you're using an LPCWSTR) does not making any copies of the string pointers. If this is indeed the case, not allocating C memory should be safe if the GoLang's code scope is appropriate.

For example, something along the lines of the code below should be okay:

func DoSomeWindowsStuff(arg string) {
  CallWin32API(Encode(arg))
}

Here the memory allocated for the string should survive until CallWin32API() returns.

Answer 3

If you are sure input does not contain null bytes, you can do the encoding yourself:

import (
   // #include <windows.h>
   "C"
   "unicode/utf16"
   "unsafe"
)

func PtrFromString(s string) C.LPCWSTR {
   r := []rune(s + "\x00")
   e := utf16.Encode(r)
   p := unsafe.Pointer(&e[0])
   return (C.LPCWSTR)(p)
}

https://golang.org/pkg/unicode/utf16