An implementation example of DllMain() entrypoint with Golang. $ go build --buildmode=c-shared -o my.dll && rundll32.exe my.dll Test

dllmain.go

package main

//#include "dllmain.h"
import "C"

dllmain.h

#include <windows.h>

void OnProcessAttach(HINSTANCE, DWORD, LPVOID);

typedef struct {
    HINSTANCE hinstDLL;  // handle to DLL module
    DWORD fdwReason;     // reason for calling function // reserved
    LPVOID lpReserved;   // reserved
} MyThreadParams;

DWORD WINAPI MyThreadFunction(LPVOID lpParam) {
    MyThreadParams params = *((MyThreadParams*)lpParam);
    OnProcessAttach(params.hinstDLL, params.fdwReason, params.lpReserved);
    free(lpParam);
    return 0;
}

BOOL WINAPI DllMain(
    HINSTANCE _hinstDLL,  // handle to DLL module
    DWORD _fdwReason,     // reason for calling function
    LPVOID _lpReserved)   // reserved
{
    switch (_fdwReason) {
    case DLL_PROCESS_ATTACH:
		    // Initialize once for each new process.
        // Return FALSE to fail DLL load.
        {
            MyThreadParams* lpThrdParam = (MyThreadParams*)malloc(sizeof(MyThreadParams));
            lpThrdParam->hinstDLL = _hinstDLL;
            lpThrdParam->fdwReason = _fdwReason;
            lpThrdParam->lpReserved = _lpReserved;
            HANDLE hThread = CreateThread(NULL, 0, MyThreadFunction, lpThrdParam, 0, NULL);
            // CreateThread() because otherwise DllMain() is highly likely to deadlock.
        }
        break;
    case DLL_PROCESS_DETACH:
        // Perform any necessary cleanup.
        break;
    case DLL_THREAD_DETACH:
        // Do thread-specific cleanup.
        break;
    case DLL_THREAD_ATTACH:
		// Do thread-specific initialization.
        break;
    }
    return TRUE; // Successful.
}

main.go

package main

import "C"

import (
	"unsafe"

	"github.com/nanitefactory/winmb"
)

//export Test
func Test() {
	winmb.MessageBoxPlain("export Test", "export Test")
}

// OnProcessAttach is an async callback (hook).
//export OnProcessAttach
func OnProcessAttach(
	hinstDLL unsafe.Pointer, // handle to DLL module
	fdwReason uint32, // reason for calling function
	lpReserved unsafe.Pointer, // reserved
) {
	winmb.MessageBoxPlain("DLL_PROCESS_ATTACH", "DLL_PROCESS_ATTACH")
}

func main() {
	// nothing really. xD
}