memccpy.asm

C标准库源代码

        page    ,132
        title   memccpy - copy bytes until character found
;***
;memccpy.asm - copy bytes until a character is found
;
;       Copyright (c) 1985-1997, Microsoft Corporation. All rights reserved.
;
;Purpose:
;       defines _memccpy() - copies bytes until a specifed character
;       is found, or a maximum number of characters have been copied.
;
;*******************************************************************************

        .xlist
        include cruntime.inc
        .list

page
;***
;char *_memccpy(dest, src, _c, count) - copy bytes until character found
;
;Purpose:
;       Copies bytes from src to dest until count bytes have been
;       copied, or up to and including the character _c, whichever
;       comes first.
;
;       Algorithm:
;       char *
;       _memccpy (dest, src, _c, count)
;             char *dest, *src, _c;
;             unsigned int count;
;             {
;             while (count && (*dest++ = *src++) != _c)
;                     count--;
;
;             return(count ? dest : NULL);
;             }
;
;Entry:
;       char *dest - pointer to memory to receive copy
;       char *src - source of bytes
;       char _c - character to stop copy at
;       int count - max number of bytes to copy
;
;Exit:
;       returns pointer to byte immediately after _c in dest;
;       returns NULL if _c was never found
;
;Uses:
;
;Exceptions:
;
;*******************************************************************************

        CODESEG

        public  _memccpy
_memccpy proc

        .FPO    ( 0, 4, 0, 0, 0, 0 )

        mov     ecx,[esp + 10h] ; ecx = max byte count
        push    ebx             ; save ebx

        test    ecx,ecx         ; if it's nothing to move
        jz      ret_zero_len    ; restore ebx, and return NULL

        mov     bh,[esp + 10h]  ; bh = byte to look for
        push    esi             ; save esi

        test    ecx,1           ; test if counter is odd or even

        mov     eax,[esp + 0ch] ; eax = dest   , don't affect flags
        mov     esi,[esp + 10h] ; esi = source , don't affect flags

;       nop
        jz      lupe2           ; if counter is even, do double loop
                                ; else do one iteration, and drop into double loop
        mov     bl,[esi]        ; get first byte into bl
        inc     esi             ; kick src (esi points to src)

        mov     [eax],bl        ; store it in dest
        inc     eax             ; kick dest

        cmp     bl,bh           ; see if we just moved the byte
        je      short toend

        dec     ecx             ; decriment counter
        jz      retnull         ; drop into double loop if nonzero

lupe2:
        mov     bl,[esi]        ; get first byte into bl
        add     esi,2           ; kick esi (src)

        cmp     bl,bh           ; check if we just moved the byte (from bl)
        je      toend_mov_inc   ; store bl & exit

        mov     [eax],bl        ; store first byte from bl
        mov     bl,[esi - 1]    ; get second byte  into bl

        mov     [eax + 1],bl    ; store second byte from bl
        add     eax,2           ; kick eax (dest)

        cmp     bl,bh           ; see if we just moved the byte
        je      short toend     ; end of string

        sub     ecx,2           ; modify counter, and if nonzero continue
        jnz     lupe2           ; else drop out & return NULL

retnull:
        pop     esi
ret_zero_len:
        xor     eax,eax         ; null pointer
        pop     ebx

        ret                     ; _cdecl return

toend_mov_inc:
        mov     [eax],bl        ; store first byte from bl
        inc     eax             ; eax points rihgt after the value

toend:  pop     esi
        pop     ebx

        ret                     ; _cdecl return

_memccpy endp

        end