Re: detect empty functions

[vijay nag <vijunag at gmail dot com>]

Hello,

You can use the below program to find the length of the function
compiled for X86 target.

Note that assembly instruction for "function()" is immediately
followed by assembly for "functionEnd"

Key things to note here

1) Compiler still emits epilogue/prologue for a function with empty
body. However you could disable this using -fomit-frame-ptr in which
case function
    body will only have "ret" instruction and in which case difference
would be 1.
    0x80483a4 <function>            ret

                                   â
   â0x80483a5 <functionEnd>         ret

   So size of the function would be 0x80483a5 - 0x80483a4 = 1

 2)In case if you need frame-ptr, there will be a minimum of 4
instructions and in this case difference would be 5.
    0x80483a4 <function>            push   %ebp

                                   â
   â0x80483a5 <function+1>          mov    %esp,%ebp

                                   â
   â0x80483a7 <function+3>          pop    %ebp

                                   â
   â0x80483a8 <function+4>          ret

                                   â
   â0x80483a9 <functionEnd>         push   %ebp

                                   â
   â0x80483aa <functionEnd+1>       mov    %esp,%ebp

                                   â
   â0x80483ac <functionEnd+3>       pop    %ebp

                                   â
   â0x80483ad <functionEnd+4>       ret

     So size of the function would be 0x80483a9 - 0x80483a4 = 5


cat fsize.c
#include <stdio.h>

int function(){
}

int functionEnd(){
}

int main()
{
  printf("%x\n", &functionEnd - &function);
}

vijayNag

On Wed, Feb 5, 2014 at 7:00 PM, Prathamesh Kulkarni
<bilbotheelffriend@gmail.com> wrote:
> Hi, is there a way to find if a function has empty body ?
>
> Thanks and Regards,
> Prathamesh