Calling a Windows DLL

Go allows you to call native Windows function in several different ways.

1. Dynamically load a DLL, then call a function in it. You can call the function via SyscallX (where X is the number of parameters. If the function has fewer parameters than that, for example passing 7 arguments to a function that accepts 9, Syscall9 will still work, you just need to specify 7 as your second argument to Syscall9).

A sample Go program that calls a Windows DLL function using this method:

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")
}

2. Using syscall.NewProc 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 (
	"fmt"
	"syscall"
	"unsafe"
)

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("This test is Done."))),
		uintptr(unsafe.Pointer(syscall.StringToUTF16Ptr("Done Title"))),
		uintptr(MB_YESNOCANCEL))
	fmt.Printf("Return: %d\n", ret)

}

3. By “linking” against the library, using the “[[cgo]]” method (this way works in Linux and Windows). Example:

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

See [[cgo]] for further details.