Feedback on design implementation of a alloc/strncpy

From: Perry S. Morris (relvinian_at_mindspring.com)
Date: 04/24/04 

Date: Sat, 24 Apr 2004 17:32:29 +0000 (UTC)

Hello,

I had a routine in c/c++ that allocated a buffer and copied a non-null
terminated string into it and terminated it. I have re-written it in
assembly using MASM 6.15 (mainly to help me learn assembly better then I do)
and looking for feedback on ways I can write better ASM code. Here is my
full function. Also, this function will allocate the buffer with the size
in four-byte alignments.

;--------------------------------------------------------------
;prototype:
;void __stdcall StrNCopy(char** pDest, char* pSrc, int len);
;--------------------------------------------------------------
;example usage:
;char* pszString = NULL;
;StrNCopy(&pszString, pBuffer, nLen);
; do something with pszString;
;if (pszString)
; free(pszString);

--------------------------------------------------------------
title "StrNCopy.asm"
.586
.model flat

extrn __imp__malloc:near ; malloc()

.data
align 8
_TEXT SEGMENT
_pDest$ = 8
_pSrc$ = 12
_nLen$ = 16
_TEXT ENDS

.code
_StrNCopy@12 proc near
 push ebp
 mov ebp, esp

 push ebx
 push esi
 push edi

 ; get the source buffer into ESI
 mov esi, dword ptr _pSrc$[ebp]

 ; align the outgoing buffer length on a four-byte boundary.
 ; we also need to make room for the NULL terminator
 mov ebx, dword ptr _nLen$[ebp] ; need to remember len!
 xor edx, edx
 lea ecx, dword ptr [ebx+1]

 ; aee if we are already aligned
 and ecx, 3
 je short @Aligned

 ; we aren't so align it now
 mov edx, 4
 sub edx, ecx

@Aligned:
 ; remember new length
 lea eax, dword ptr [ebx+1+edx]

 ; allocate the new buffer
 push eax
 call dword ptr __imp__malloc
 pop ecx

 ; see if we have a successfull allocation
 test eax, eax
 je short @Done

 ; copy the string
 mov edi, eax
 shr ecx, 2
 rep movsd

 ; terminate our string
 mov byte ptr [eax+ebx], cl

 ; set the outgoing buffer ptr to the new string
 mov edi, dword ptr _pDest$[ebp]
 mov dword ptr [edi], eax

@Done:
 pop ebx
 pop edi
 pop esi
 pop ebp
 ret 12

_StrNCopy@12 ENDP

end

--------------------------------------------------------------

Thank you

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