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title ms.custom ms.date ms.reviewer ms.suite ms.technology ms.tgt_pltfrm ms.topic dev_langs helpviewer_keywords ms.assetid caps.latest.revision author ms.author manager
Considerations When Writing Prolog-Epilog Code | Microsoft Docs
11/04/2016
cpp-language
article
C++
layouts, stack frame
stack frame layout
__LOCAL_SIZE constant
stack, stack frame layout
3b8addec-e809-48e4-b1d0-5bad133bd4b8
8
mikeblome
mblome
ghogen

Considerations When Writing Prolog/Epilog Code

Microsoft Specific

Before writing your own prolog and epilog code sequences, it is important to understand how the stack frame is laid out. It is also useful to know how to use the __LOCAL_SIZE predefined constant.

C Stack Frame Layout

This example shows the standard prolog code that might appear in a 32-bit function:

push     ebp                 ; Save ebp  
mov      ebp, esp            ; Set stack frame pointer  
sub      esp, localbytes     ; Allocate space for locals  
push     <registers>         ; Save registers

The localbytes variable represents the number of bytes needed on the stack for local variables, and the registers variable is a placeholder that represents the list of registers to be saved on the stack. After pushing the registers, you can place any other appropriate data on the stack. The following is the corresponding epilog code:

pop      <registers>         ; Restore registers  
mov      esp, ebp            ; Restore stack pointer  
pop      ebp                 ; Restore ebp  
ret                          ; Return from function

The stack always grows down (from high to low memory addresses). The base pointer (ebp) points to the pushed value of ebp. The local variables area begins at ebp-2. To access local variables, calculate an offset from ebp by subtracting the appropriate value from ebp.

The __LOCAL_SIZE Constant

The compiler provides a constant, __LOCAL_SIZE, for use in the inline assembler block of function prolog code. This constant is used to allocate space for local variables on the stack frame in custom prolog code.

The compiler determines the value of __LOCAL_SIZE. The value is the total number of bytes of all user-defined local variables and compiler-generated temporary variables. __LOCAL_SIZE can be used only as an immediate operand; it cannot be used in an expression. You must not change or redefine the value of this constant. For example:

mov      eax, __LOCAL_SIZE           ;Immediate operand--Okay  
mov      eax, [ebp - __LOCAL_SIZE]   ;Error

The following example of a naked function containing custom prolog and epilog sequences uses __LOCAL_SIZE in the prolog sequence:

__declspec ( naked ) func()  
{  
   int i;  
   int j;  
  
   __asm      /* prolog */  
      {  
      push   ebp  
      mov      ebp, esp  
      sub      esp, __LOCAL_SIZE  
      }  
  
   /* Function body */  
  
   __asm      /* epilog */  
      {  
      mov      esp, ebp  
      pop      ebp  
      ret  
      }  
}

END Microsoft Specific

See Also

Naked Functions