Golang下通过syscall调用win32的dll（calling Windows DLLs from Go ）

很多同学比如我虽然很喜欢golang，但是还是需要调用很多遗留项目或者其他优秀的开源项目，这时怎么办呢？我们想到的方法是用package里的syscall结合cgo

注意此处有坑：

在我调试时显示not enough arguments in call to syscall.Syscall

[ `go run dms.go` | done: 260.3744ms ]
# command-line-arguments
.\dms.go:72: not enough arguments in call to syscall.Syscall

exit status 2

因为我参照的是http://golang.org/pkg/syscall/#Syscall ，而其默认的是Linux/Unix的syscall API doc说明，

如何看windows的golang doc呢？

通过godoc command, 调用 godoc -http=:6060
然后在浏览器打开http://localhost:6060/pkg/syscall/#Syscall，这才是windows的golang pakage api

func Syscall

func Syscall(trap, nargs, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)

Implemented in ../runtime/syscall_windows.goc.

func Syscall12

func Syscall12(trap, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12 uintptr) (r1, r2 uintptr, err Errno)

func Syscall15

func Syscall15(trap, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15 uintptr) (r1, r2 uintptr, err Errno)

func Syscall6

func Syscall6(trap, nargs, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)

func Syscall9

func Syscall9(trap, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, err Errno)

例子1：

func test(){
//首先,准备输入参数, GetDiskFreeSpaceEx需要4个参数, 可查MSDN
lpFreeBytesAvailable := int64(0) //注意类型需要跟API的类型相符
lpTotalNumberOfBytes := int64(0)
lpTotalNumberOfFreeBytes := int64(0)

//获取方法的引用
kernel32, _ := syscall.LoadLibrary("Kernel32.dll") // 严格来说需要加上 defer syscall.FreeLibrary(kernel32)
GetDiskFreeSpaceEx, _ := syscall.GetProcAddress(syscall.Handle(kernel32), "GetDiskFreeSpaceExW")

//执行之. 因为有4个参数,故取Syscall6才能放得下. 最后2个参数,自然就是0了
r, _, _ := syscall.Syscall6(uintptr(GetDiskFreeSpaceEx), 4,
            uintptr(unsafe.Pointer(syscall.StringToUTF16Ptr("C:"))),
            uintptr(unsafe.Pointer(&lpFreeBytesAvailable)),
            uintptr(unsafe.Pointer(&lpTotalNumberOfBytes)),
            uintptr(unsafe.Pointer(&lpTotalNumberOfFreeBytes)), 0, 0)
            
// 注意, errno并非error接口的, 不可能是nil
// 而且,根据MSDN的说明,返回值为0就fail, 不为0就是成功
if r != 0 {
    log.Printf("Free %dmb", lpTotalNumberOfFreeBytes/1024/1024)
}
}

例子2，弹出一个简单的对话框messagebox， http://code.google.com/p/go-wiki/wiki/WindowsDLLs：

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")}

http://golang.org/pkg/syscall/#Syscall 里默认的是Linux/Unix的syscall API doc说明，