Help about CDECL

From Programming Windows 5e

Code:

#include <windows.h>
#include <tchar.h>     
#include <stdio.h>     

int CDECL MessageBoxPrintf (TCHAR * szCaption, TCHAR * szFormat, ...)
{
     TCHAR   szBuffer [1024] ;
     va_list pArgList ;

     va_start (pArgList, szFormat) ;

      _vsntprintf (szBuffer, sizeof (szBuffer) / sizeof (TCHAR), 
                  szFormat, pArgList) ;

     va_end (pArgList) ;

     return MessageBox (NULL, szBuffer, szCaption, 0) ;
}

int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
                    PSTR szCmdLine, int iCmdShow) 
{
     int cxScreen, cyScreen ;

     cxScreen = GetSystemMetrics (SM_CXSCREEN) ;
     cyScreen = GetSystemMetrics (SM_CYSCREEN) ;
     MessageBoxPrintf (TEXT ("ScrnSize"), 
                       TEXT ("The screen is %i pixels wide by %i pixels high."),
                       cxScreen, cyScreen) ;
     return 0 ;
}

What does this CDECL mean? I found that even without CDECL, this program still compiled and run successfully. Why?

Its an instruction to the compiler on how params are removed from the stack after the function is called.

Most Windows APIs require STDCALL, but C compilers use CDECL by default. The function you are using uses CDECL as it doesnt know the numbers of params passed to it at compile time, so it uses CDECL and relies on the calling code to balance the stack.