Calling a Windows DLL

There are a few ways to call “C” code from inside Go

First way: Dynamically load a dll, then call a method on it. You can call the method via “syscallXX” (the XX is number of parameters, but if it has few than that, like if you need seven parameter, then syscall9 will still work, you just tell it the number of arguments is 7). This way also works with Linux shared libraries, as well, if you're targeting linux:

A sample program that calls Windows DLLs from Go:

package main

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

func abort(funcname string, err error) {
	panic(fmt.Sprintf("%s failed: %v", funcname, 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) {
	var nargs uintptr = 4
	ret, _, callErr := syscall.Syscall9(uintptr(messageBox),
		nargs,
		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", callErr)
	}
	result = int(ret)
	return
}

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

func main() {
	defer syscall.FreeLibrary(kernel32)
	defer syscall.FreeLibrary(user32)

	fmt.Printf("Return: %d\n", MessageBox("Done Title", "This test is Done.", MB_YESNOCANCEL))
}

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

Second way is via syscall.NewProc (etc.) instead of syscall.GetProcAddress. These are basically some helper methods over the syscall ones, you saw above, and are available in Windows only: http://golang.org/src/pkg/syscall/dll_windows.go

package main

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

func main() {
 var mod = syscall.NewLazyDLL("user32.dll")
 var proc = mod.NewProc("MessageBoxW");
 var MB_YESNOCANCEL       = 0x00000003

 ret, _, _ := proc.Call(0,
  uintptr(unsafe.Pointer(syscall.StringToUTF16Ptr("Done Title"))),
  uintptr(unsafe.Pointer(syscall.StringToUTF16Ptr("This test is Done."))),
  uintptr(MB_YESNOCANCEL));
  fmt.Printf("Return: %d\n", ret)
  
}

A third way would be to call into libraries basically by “linking” against the library, using the “cgo” method (this way works in Linux and Windows):

This way would look something like this

import ("C")
...
C.MessageBoxW(...)

See cgo for further details.