When is DllMain called?

I hear the DllMain function is called when the process loads the needed dll at the start time, so I write the following code to verify this:

//mydll.cpp
#include <windows.h>

BOOL WINAPI DllMain(HINSTANCE hInstance, DWORD dwReason, LPVOID lpReserved)
{
  switch (dwReason)
  {
  case DLL_PROCESS_ATTACH:
    MessageBox(NULL, "DLL_PROCESS_ATTACH", "Message", MB_OK);
    break;

  case DLL_THREAD_ATTACH:
    MessageBox(NULL, "DLL_THREAD_ATTACH", "Message", MB_OK);
    break;

  case DLL_THREAD_DETACH:
    MessageBox(NULL, "DLL_THREAD_DETACH", "Message", MB_OK);
    break;

  case DLL_PROCESS_DETACH:
    MessageBox(NULL, "DLL_PROCESS_DETACH", "Message", MB_OK);
    break;
  }

  return TRUE;
}

int __declspec(dllexport) Sum(int a, int b)
{
  return a + b;
}

//app.cpp
#include <iostream>
using namespace std;


int Sum(int a, int b);

int main()
{
  cout << Sum(5, 10);
  return 0;
}

I use Mingw g++ to compile:

g++ -g -shared -o mydll.dll mydll.cpp
g++ -g -o app.exe app.cpp mydll.dll

All are ok. Running app.exe outputs the desired 15. But when debugging app.exe, it never stops at the breakpoint set in DllMain although I can step into the function Sum in the dll. Why isn’t DllMain called?

This is because the g++ compiler mangles the function name DllMain into other name which is not considered by the linker as the entry point for a dll. (This mangled function resides in the dll as an ordinary function.) The entry point is strictly the function named after “DllMain”. The cue is using extern “C” for DllMain:

//mydll.cpp
#include <windows.h>

extern "C" BOOL WINAPI DllMain(HINSTANCE hInstance, DWORD dwReason, LPVOID lpReserved)
{
  switch (dwReason)
  {
  case DLL_PROCESS_ATTACH:
    MessageBox(NULL, "DLL_PROCESS_ATTACH", "Message", MB_OK);
    break;

  case DLL_THREAD_ATTACH:
    MessageBox(NULL, "DLL_THREAD_ATTACH", "Message", MB_OK);
    break;

  case DLL_THREAD_DETACH:
    MessageBox(NULL, "DLL_THREAD_DETACH", "Message", MB_OK);
    break;

  case DLL_PROCESS_DETACH:
    MessageBox(NULL, "DLL_PROCESS_DETACH", "Message", MB_OK);
    break;
  }

  return TRUE;
}

int __declspec(dllexport) Sum(int a, int b)
{
  return a + b;
}

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