The _penter function is not part of any library and it is up to you to provide a definition for _penter.

Unless you plan to explicitly call _penter, you do not need to provide a prototype. The function must appear as if it had the following prototype, and it must push the content of all registers on entry and pop the unchanged content on exit:

void __declspec(naked) _cdecl _penter( void );

To set this compiler option in the Visual Studio development environment

To set this compiler option programmatically

The following code, when compiled with /Gh, shows how _penter is called twice; once when entering function main and once when entering function x.

// Gh_compiler_option.cpp
// compile with: /Gh
// processor: x86
#include <stdio.h>
void x() {}

int main() {
   x();
}

extern "C" void __declspec(naked) _cdecl _penter( void ) {
   _asm {
      push eax
      push ebx
      push ecx
      push edx
      push ebp
      push edi
      push esi
    }

   printf_s("\nIn a function!");

   _asm {
      pop esi
      pop edi
      pop ebp
      pop edx
      pop ecx
      pop ebx
      pop eax
      ret
    }
}

In a function!
In a function!

Reference