C / C++ / MFC

1. Take out the C runtime library (idea from ATL):
   The compiler by default will link in CRT startup code before actually calling your entry point function.  So the first step (and the biggest) step is to remove those CRT stuff since you are just using Win32 API.  Borrowing idea from ATL's WinMainCRTStartup I put something like this:
   void WinMainCRTStartup(void)<br>{<br>    WinMain(GetModuleHandle(NULL), NULL, NULL, 0);<br>    return;<br>}
   This takes 20K out of the exe and it is down to 16K.
2. I tried a few things and I think the compiler just won't reduce the size further.  It always give you 3 segments and stuff.  So I thought of UPX, the exe compression tool (I know I cheated a bit here, but I don't know assembly! ).  I used it to compress my exe and boom... it is down to 2560 bytes!
3. Without resorting to assembly (I don't know the PE format too much :p ), I opened up the original exe in binary mode and I noticed a lot of blanks (to fill up the segments I suppose).  So I thought, "if I could make the segments more blank then I may be able to get more compression out of this".  I notice that I am calling 2 functions from Win32 API, killing 1 of them should make the import table more compressible.  Since I am not interested in the module handle, I make that NULL:
       WinMain(NULL, NULL, NULL, 0);
   Unfortunately that didn't work, UPX still compressed it to 2560 bytes.
4. My final step is to realize that I am doing a 2-step function call to get to the MessageBox.  So I killed my WinMain and make my entry point function call MessageBox directly:
   void WinMainCRTStartup(void)<br>{<br>    MessageBox(NULL, "Hello World","Hello", MB_OK);<br>}
   Now, that didn't do much in terms of the compiler, it still gave me 16K, but UPX made it down to 2048 bytes!  I guess my code segment is more compressible now.

                           Before UPX  After UPX
Plain old WinMain()        36384       12288
Minus CRT                  16384       2560
Minus GetModuleHandle()    16384       2560
Direct call                16384       2048

• use the TEXT() or _T() macros for all your string literals
• use TCHAR instead of char
• include tchar.h
• and if you have UNICODE and _UNICODE defined (ie you are using unicode version of wsprintf), make sure you check the "Display unicode strings" options under the Debug tab of the VC++'s Options dialog.