Calling a Windows DLL

A sample program that calls Windows DLLs from Go:

package main

import (
       "syscall"
       "unsafe"
       "fmt"
)


func abort(funcname string, err int) {
       panic(funcname + " failed: " + syscall.Errstr(err))
}

var (
       kernel32, _ = syscall.LoadLibrary("kernel32.dll")
       getModuleHandle, _ = syscall.GetProcAddress(kernel32, "GetModuleHandleW")
       
       user32, _ = syscall.LoadLibrary("user32.dll")
       messageBox, _ = syscall.GetProcAddress(user32, "MessageBoxW")
)


const (
       MB_OK                      = 0x00000000
       MB_OKCANCEL                = 0x00000001
       MB_ABORTRETRYIGNORE        = 0x00000002
       MB_YESNOCANCEL             = 0x00000003
       MB_YESNO                   = 0x00000004
       MB_RETRYCANCEL             = 0x00000005
       MB_CANCELTRYCONTINUE       = 0x00000006
       MB_ICONHAND                = 0x00000010
       MB_ICONQUESTION            = 0x00000020
       MB_ICONEXCLAMATION         = 0x00000030
       MB_ICONASTERISK            = 0x00000040
       MB_USERICON                = 0x00000080
       MB_ICONWARNING             = MB_ICONEXCLAMATION
       MB_ICONERROR               = MB_ICONHAND
       MB_ICONINFORMATION         = MB_ICONASTERISK
       MB_ICONSTOP                = MB_ICONHAND

       MB_DEFBUTTON1              = 0x00000000
       MB_DEFBUTTON2              = 0x00000100
       MB_DEFBUTTON3              = 0x00000200
       MB_DEFBUTTON4              = 0x00000300
)

func MessageBox(caption, text string, style uintptr) (result int) {
       ret, _, callErr := syscall.Syscall9(uintptr(messageBox),
               0,
               uintptr(unsafe.Pointer(syscall.StringToUTF16Ptr(text))),
               uintptr(unsafe.Pointer(syscall.StringToUTF16Ptr(caption))),
               style,
               0,
               0,
               0,
               0,
               0)
       if callErr != 0 {
               abort("Call MessageBox", int(callErr))
       }
       result = int(ret)
       return
}

func GetModuleHandle() (handle uintptr) {
       if ret, _, callErr := syscall.Syscall(uintptr(getModuleHandle), 0, 0, 0); callErr != 0 {
               abort("Call GetModuleHandle", int(callErr))
       } else {
               handle = ret
       }
       return
}

func main() {
       defer syscall.FreeLibrary(kernel32)
       defer syscall.FreeLibrary(user32)
       
       fmt.Printf("Retern: %d\n", MessageBox("Done Title", "This test is Done.", MB_YESNOCANCEL))
}

func init() {
       fmt.Print("Starting Up\n")
}