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分类: C/C++

2008-05-31 13:35:14

以下程序段取自glibc,做了简单修改.
/* Copy SRC to DEST.  */
char *
strcpy (dest, src)
     char *dest;
     const char *src;
{
  char c;
  char *s = (char *)src;
  const ptrdiff_t off = dest - s - 1;

  do
    {
      c = *s++;
      s[off] = c;
    }
  while (c != '\0');

  return dest;
}
以下程序段取自VC6.0

/*

  下面是strcpy库函数的实现,因为库函数讲究的就是精练、简洁。所以没有其他的异常处理代码。主要的异常处理还是交给了函数的使用者,在调用前请确认目的和源指针是否都存在(不能为Null),请确认目标指针空间是否大于源字符串的空间。

Copies the string src into the spot specified by dest;

  assumes enough room.

  目标指针空间必须大于源字符串空间。

*/


char * __cdecl strcpy(char * dst, const char * src)
{
        char * cp = dst;

        while( *cp++ = *src++ )
                ;               /* Copy src over dst */

        return( dst );
}

glibc中取出的那段,其实我还删了一些,主要是范围检查,而取自VC的这段却是十分简洁.
这并不能说明哪个更好,看看这些源代码,使自己能提高编码水平吧.
===========================================================
Assembly code
        page    ,132
        title   strcat - concatenate (append) one string to another
;***
;
strcat.asm - contains strcat() and strcpy() routines
;
;
       Copyright (c) Microsoft Corporation. All rights reserved.
;
;
Purpose:
;
       STRCAT concatenates (appends) a copy of the source string to the
;
       end of the destination string, returning the destination string.
;
;
*******************************************************************************

        .xlist
        include cruntime.
inc
        .list


page
;***
;
char *strcat(dst, src) - concatenate (append) one string to another
;
;
Purpose:
;
       Concatenates src onto the end of dest.  Assumes enough
;
       space in dest.
;
;
       Algorithm:
;
       char * strcat (char * dst, char * src)
;
       {
;
           char * cp = dst;
;
;
           while( *cp )
;
                   ++cp;           /* Find end of dst */
;
           while( *cp++ = *src++ )
;
                   ;               /* Copy src to end of dst */
;
           return( dst );
;
       }
;
;
Entry:
;
       char *dst - string to which "src" is to be appended
;
       const char *src - string to be appended to the end of "dst"
;
;
Exit:
;
       The address of "dst" in EAX
;
;
Uses:
;
       EAX, ECX
;
;
Exceptions:
;
;
*******************************************************************************

page
;***
;
char *strcpy(dst, src) - copy one string over another
;
;
Purpose:
;
       Copies the string src into the spot specified by
;
       dest; assumes enough room.
;
;
       Algorithm:
;
       char * strcpy (char * dst, char * src)
;
       {
;
           char * cp = dst;
;
;
           while( *cp++ = *src++ )
;
                   ;               /* Copy src over dst */
;
           return( dst );
;
       }
;
;
Entry:
;
char * dst - string over which "src" is to be copied
;
       const char * src - string to be copied over "dst"
;
;
Exit:
;
       The address of "dst" in EAX
;
;
Uses:
;
       EAX, ECX
;
;
Exceptions:
;
*******************************************************************************


        CODESEG

%       public  strcat, strcpy     
; make both functions available
strcpy  proc \
       
dst:ptr byte, \
       
src:ptr byte

        OPTION
PROLOGUE:NONE, EPILOGUE:NONE

       
push    edi                 ; preserve edi
        mov     edi,[esp+8]         ; edi points to dest string
        jmp     short copy_start

strcpy  endp

        align  
16

strcat  proc \
       
dst:ptr byte, \
       
src:ptr byte

        OPTION
PROLOGUE:NONE, EPILOGUE:NONE

        .FPO    (
0, 2, 0, 0, 0, 0 )

       
mov     ecx,[esp+4]         ; ecx -> dest string
        push    edi                 ; preserve edi
        test    ecx,3               ; test if string is aligned on 32 bits
        je      short find_end_of_dest_string_loop

dest_misaligned:                    ; simple byte loop until string is aligned
        mov     al,byte ptr [ecx]
       
add     ecx,1
       
test    al,al
       
je      short start_byte_3
       
test    ecx,3
       
jne     short dest_misaligned

        align  
4

find_end_of_dest_string_loop:
       
mov     eax,dword ptr [ecx] ; read 4 bytes
        mov     edx,7efefeffh
       
add     edx,eax
       
xor     eax,-1
       
xor     eax,edx
       
add     ecx,4
       
test    eax,81010100h
       
je      short find_end_of_dest_string_loop
       
; found zero byte in the loop
        mov     eax,[ecx - 4]
       
test    al,al               ; is it byte 0
        je      short start_byte_0
       
test    ah,ah               ; is it byte 1
        je      short start_byte_1
       
test    eax,00ff0000h       ; is it byte 2
        je      short start_byte_2
       
test    eax,0ff000000h      ; is it byte 3
        je      short start_byte_3
       
jmp     short find_end_of_dest_string_loop
                                   
; taken if bits 24-30 are clear and bit
                                    ; 31 is set
start_byte_3:
       
lea     edi,[ecx - 1]
       
jmp     short copy_start
start_byte_2:
       
lea     edi,[ecx - 2]
       
jmp     short copy_start
start_byte_1:
       
lea     edi,[ecx - 3]
       
jmp     short copy_start
start_byte_0:
       
lea     edi,[ecx - 4]
;       jmp     short copy_start

;       edi points to the end of dest string.
copy_start::
       
mov     ecx,[esp+0ch]       ; ecx -> sorc string
        test    ecx,3               ; test if string is aligned on 32 bits
        je      short main_loop_entrance

src_misaligned:                     ; simple byte loop until string is aligned
        mov     dl,byte ptr [ecx]
       
add     ecx,1
       
test    dl,dl
       
je      short byte_0
       
mov     [edi],dl
       
add     edi,1
       
test    ecx,3
       
jne     short src_misaligned
       
jmp     short main_loop_entrance

main_loop:                          ; edx contains first dword of sorc string
        mov     [edi],edx           ; store one more dword
        add     edi,4               ; kick dest pointer
main_loop_entrance:
       
mov     edx,7efefeffh
       
mov     eax,dword ptr [ecx] ; read 4 bytes

       
add     edx,eax
       
xor     eax,-1

       
xor     eax,edx
       
mov     edx,[ecx]           ; it's in cache now

       
add     ecx,4               ; kick dest pointer
        test    eax,81010100h

       
je      short main_loop
       
; found zero byte in the loop
;
main_loop_end:
        test    dl,dl               ; is it byte 0
        je      short byte_0
       
test    dh,dh               ; is it byte 1
        je      short byte_1
       
test    edx,00ff0000h       ; is it byte 2
        je      short byte_2
       
test    edx,0ff000000h      ; is it byte 3
        je      short byte_3
       
jmp     short main_loop     ; taken if bits 24-30 are clear and bit
                                    ; 31 is set
byte_3:
       
mov     [edi],edx
       
mov     eax,[esp+8]         ; return in eax pointer to dest string
        pop     edi
       
ret
byte_2:
       
mov     [edi],dx
       
mov     eax,[esp+8]         ; return in eax pointer to dest string
        mov     byte ptr [edi+2],0
       
pop     edi
       
ret
byte_1:
       
mov     [edi],dx
       
mov     eax,[esp+8]         ; return in eax pointer to dest string
        pop     edi
       
ret
byte_0:
       
mov     [edi],dl
       
mov     eax,[esp+8]         ; return in eax pointer to dest string
        pop     edi
       
ret

strcat  endp

        end


 
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