Calling Windows SendMessageW from GOlang never returns

Question

Trying to write a very simple GO program for Windows that turns off the monitor. The code looks like this:

//go:build windows
// +build windows

package main

import (
    log "github.com/sirupsen/logrus"

    "golang.org/x/sys/windows"
)

func main() {
    user32DLL := windows.NewLazyDLL("user32.dll")
    procSendMsg := user32DLL.NewProc("SendMessageW")

    a, b, err := procSendMsg.Call(0xFFFF, 0x0112, 0xF170, 2)
    if err != nil {
        log.Errorf("Error returned from SendMsg: %v", err)
    }
    log.Infof("a = %v, b = %v:", a, b)
}

The program does work (IE> the monitor turns off), but never returns from the procSendMsg.Call() -- no errors, and I never see the output of 'a' and 'b'. I have to Ctrl-C to get out of the program.

Obviously I'm doing something wrong w.r.t. calling the user32.dll function...this IS my first attempt at writing a Windows program with GO.

What am I doing wrong?

[FYI: I got the idea from this PowerShell script:

(Add-Type -MemberDefinition "[DllImport(""user32.dll"")]`npublic static extern int SendMessage(int hWnd, int hMsg, int wParam, int lParam);" -Name "Win32SendMessage" -Namespace Win32Functions -PassThru)::SendMessage(0xffff, 0x0112, 0xF170, 2)

]

Thanks for your feedback!!

JD Allen · Answer

FYI: Here is the working version.

//go:build windows
// +build windows

package main

//
//  turnOffMonitor  --  Shuts down the signal to the monitor on Windows.  A better power saver
//    over using 'scrnsave.scr /s' to just blank the screen.
//
//  John D. Allen
//  September, 2022
//

import (
    "strings"

    log "github.com/sirupsen/logrus"

    "golang.org/x/sys/windows"
)

func main() {
    user32DLL := windows.NewLazyDLL("user32.dll")
    procPostMsg := user32DLL.NewProc("PostMessageW")

    _, _, err := procPostMsg.Call(0xFFFF, 0x0112, 0xF170, 2)

    //
    // I get a "Access is denied." error when I run this from a user that does
    // not have SYSTEM rights...but it still works anyway for some reason.
    if err != nil && !strings.Contains(err.Error(), "Access is denied.") {
        log.Errorf("Error returned from PostMsg(): %v", err)
    }
}

Calling Windows SendMessageW from GOlang never returns

1 Answers