What does RET XX Mean?

Code:

;*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_
; Author: typedef aka CHX101                  *
;**********************************************
; Description: A simple program that executes
;              a function given it's pointer.
;              It uses the mandatory value of
;              the function's parameter count to
;              loop through them and push each
;              one of them on the stack and then
;              calls the function.
;*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_
;
; I was trying to make a simple pointer "checker"
; that validates data before being passed to a function.
;
; If one of the datas or data pointers is null (which is 0)
; then return and don't call the function.
;
; But I discovered that that would be hard to make.
; For example if the caller passes 0 as a parameter,
; I wouldn't know if it's there for a purpose or it
; was just an uninitialized data value.
;
; So I'll just leave it for now. I'll be thinking though 
;
;*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_*_
format pe gui 4.0 dll

include 'win32ax.inc'
include 'api/user32.inc'
include 'api/kernel32.inc'

entry DllMain

section '.txt' data executable readable

proc DllMain hInst:DWORD, dwReason:DWORD, reserved:DWORD

mov  eax,TRUE

     ret
endp
;*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+
;UniExec : Main export function. For more info refer to uniexec.h
;
;*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+
proc UniExec,function, argc, param1, param2; etc
     ;save registers
     push ebp edi esi ebx
     ;setup stack
     mov  ebp,esp
     mov  ebx,[ebp+24] ;save function pointer ebp+24
                  ;First parameter starts at ebp+24 or 18h
     mov  ecx,dword [ebp+28] ;save argument count   +28
_push:
     cmp ecx,0x0    ;Are all the needed arguments there or does function have no parameters ?
     je  _exec             ;if so call function
     push dword [ebp+28+ecx*4]  ; else keep pushing the parameters
     dec ecx              ; decrement parameter count for each parameter pushed
     jmp _push         ; go back up for more
_exec:
     call ebx            ;function was saved in ebx now call it
     pop  ebx esi edi ebp
     mov  ebp,esp    ; restore ebp
     leave                ; restore ebp again ? It crashes if left out
     retn 4                 ;transfer handle to parent caller
endp

;******
; VERSION INFO
; Version: 1.30.20
;******
proc  About
      push MB_OK + MB_ICONINFORMATION
      push const_char_title
      push const_char_msg
      push 0
      call [MessageBox]
      ret
endp

section '.' data readable 

const_char_msg         db 'Universal Executor v1.30.20',10,\
               'Copyright (c) typedef aka CHX101 2008 - 2010',10,\
              'Email: typedef_int@live.com / elite1976@live.com',0
const_char_title db 'About Universal Executor',0

section '.idata' import data readable writeable

library kernel32,'kernel32.dll',\
   user32,'user32.dll'


section '.edata' export data readable

export 'UniExec',\
       UniExec,'UniExec',\
       About  ,'About'

section '.reloc' fixups data discardable

Include file

Code:

#ifndef U_DLL_H_
#define U_DLL_H_

#if BUILDING_DLL
# define DLLIMPORT __declspec (dllexport)
#else /* Not BUILDING_DLL */
# define DLLIMPORT __declspec (dllimport)
#endif /* Not BUILDING_DLL */

/*
Universal Function Executor

 @param function: Pointer to function cast to void*
 @param      argc: Number of arguments to be passed
 @param      ...: Any type of parameter to be passed,
                  NOTE: Must equal to argc.
*/

extern "C"
{       
int DLLIMPORT UniExec(LPVOID function,int argc=0,...);
int DLLIMPORT About(void);
}      



#endif /* U_DLL_H_ */

Now here's how I am using the function in C/C++

If I want to execute MessageBox(NULL,"Hello","World!",MB_OK);

I do

Code:

int number_of_MessageBox_args = 4;
UniExec( (LPVOID)&MessageBox,number_of_MessageBox_args,NULL,"Hello","World",MB_OK);

That runs well....
But when I run a function that returns a number I crash.
For example

Code:

int pow2( int x)
{
return x * x;
}
int argc = 1; number of arguments.

int twoSquared = UniExec( (LPVOID)&pow2,argc,2);

That crashes....

I found out that It's the ret 4

opcode. I do not know why It is like that so I came to ask.

What does RET XX Mean ? I just use it but do not know what XX does.

Any help ?

Thanks

PS: The question is not about C/C++ it's about the RET opcode. Why it makes the program crash when the called function returns a number.

And moreover MessageBox returns a number too but does not crash !

What's going on there.
If I use RET It becomes a bad DLL Image, If I use RET 4 It works but then some functions crash.
Why ?