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
}

function exported by golang a.k.a OnProcessAttach translates in uint32 in x64 systems to unsigned int, while uint64 translates to long long unsigned int, giving trouble because DWORD is long unsigned int, any clue?

cgo-gcc-export-header-prolog:49:35: error: conflicting types for 'OnProcessAttach'; have 'void(void *, GoUint32,  void *)' {aka 'void(void *, unsigned int,  void *)'}
In file included from main.go:2:
./dllmain.h:3:6: note: previous declaration of 'OnProcessAttach' with type 'void(struct HINSTANCE__ *, DWORD,  void *)' {aka 'void(struct HINSTANCE__ *, long unsigned int,  void *)'}
    3 | void OnProcessAttach(HINSTANCE, DWORD, LPVOID);
      |      ^~~~~~~~~~~~~~~
_cgo_export.c:24:28: error: conflicting types for 'OnProcessAttach'; have 'void(void *, GoUint32,  void *)' {aka 'void(void *, unsigned int,  void *)'}
   24 | __declspec(dllexport) void OnProcessAttach(void* hinstDLL, GoUint32 fdwReason, void* lpReserved)
      |                            ^~~~~~~~~~~~~~~
./dllmain.h:3:6: note: previous declaration of 'OnProcessAttach' with type 'void(struct HINSTANCE__ *, DWORD,  void *)' {aka 'void(struct HINSTANCE__ *, long unsigned int,  void *)'}
    3 | void OnProcessAttach(HINSTANCE, DWORD, LPVOID);

golang can even be used to make internal game cheats... wow.