page    ,132
        title   strrev - reverse a string in place
;***
;strrev.asm - reverse a string in place
;
;       Copyright (c) Microsoft Corporation. All rights reserved.
;
;Purpose:
;       defines _strrev() - reverse a string in place (not including
;       '\0' character)
;
;*******************************************************************************

        .xlist
        include cruntime.inc
        .list

page
;***
;char *_strrev(string) - reverse a string in place
;
;Purpose:
;       Reverses the order of characters in the string.  The terminating
;       null character remains in place.
;
;       Algorithm:
;       char *
;       _strrev (string)
;             char *string;
;             {
;             char *start = string;
;             char *left = string;
;             char ch;
;
;             while (*string++)
;                     ;
;             string -= 2;
;             while (left < string)
;                     {
;                     ch = *left;
;                     *left++ = *string;
;                     *string-- = ch;
;                     }
;             return(start);
;             }
;
;       NOTE: There is a check for an empty string in the following code.
;       Normally, this would fall out of the "cmp si,di" instruction in the
;       loop portion of the routine.  However, if the offset of the empty
;       string is 0 (as it could be in large model), then the cmp does not
;       catch the empty string and the routine essentially hangs (i.e., loops
;       moving bytes one at a time FFFFh times).  An explicit empty string
;       check corrects this.
;
;Entry:
;       char *string - string to reverse
;
;Exit:
;       returns string - now with reversed characters
;
;Uses:
;
;Exceptions:
;
;*******************************************************************************

        CODESEG

        public  _strrev
_strrev proc \
        uses edi esi, \
        string:ptr byte

        mov     edi,[string]    ; di = string
        mov     edx,edi         ; dx=pointer to string; save return value

        mov     esi,edi         ; si=pointer to string
        xor     eax,eax         ; search value (null)
        or      ecx,-1          ; cx = -1
repne   scasb                   ; find null
        cmp     ecx,-2          ; is string empty? (if offset value is 0, the
        je      short done      ; cmp below will not catch it and we'll hang).

        sub     edi,2           ; string is not empty, move di pointer back
                                ; di points to last non-null byte

lupe:
        cmp     esi,edi         ; see if pointers have crossed yet
        jae     short done      ; exit when pointers meet (or cross)

        mov     ah,[esi]        ; get front byte...
        mov     al,[edi]        ;   and end byte
        mov     [esi],al        ; put end byte in front...
        mov     [edi],ah        ;   and front byte at end
        add     esi,1           ; front moves up...
        sub     edi,1           ;   and end moves down
        jmp     short lupe      ; keep switching bytes

done:
        mov     eax,edx         ; return value: string addr

        ret                     ; _cdecl return

_strrev endp
        end