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page ,132 title strncat - append n chars of string1 to string2;***;strncat.asm - append n chars of string to new string;; Copyright (c) 1985-2001, Microsoft Corporation. All rights reserved.;;Purpose:; defines strncat() - appends n characters of string onto; end of other string;;Revision History:; 10-25-83 RN initial version; 08-05-87 SKS Fixed bug: extra null was stored if n > strlen(back); 05-18-88 SJM Add model-independent (large model) ifdef; 08-04-88 SJM convert to cruntime/ add 32-bit support; 08-23-88 JCR Minor 386 cleanup; 10-26-88 JCR General cleanup for 386-only code; 03-23-90 GJF Changed to _stdcall. Also, fixed the copyright.; 05-10-91 GJF Back to _cdecl, sigh...; 12-15-96 GJF Faster version from Intel.; 12-19-96 GJF Fixed bugs in Intel code; 06-12-01 PML inc->add 1, dec->sub 1 for Pentium 4 perf (vs7#267015);;*******************************************************************************
.xlist include cruntime.inc .list
page;***;char *strncat(front, back, count) - append count chars of back onto front;;Purpose:; Appends at most count characters of the string back onto the; end of front, and ALWAYS terminates with a null character.; If count is greater than the length of back, the length of back; is used instead. (Unlike strncpy, this routine does not pad out; to count characters).;; Algorithm:; char *; strncat (front, back, count); char *front, *back;; unsigned count;; {; char *start = front;;; while (*front++); ;; front--;; while (count--); if (!(*front++ = *back++)); return(start);; *front = '\0';; return(start);; };;Entry:; char * front - string to append onto; char * back - string to append; unsigned count - count of max characters to append;;Exit:; returns a pointer to string appended onto (front).;;Uses: ECX, EDX;;Exceptions:;;*******************************************************************************
CODESEG
public strncatstrncat proc; front:ptr byte,; back:ptr byte,; count:IWORD
.FPO ( 0, 3, 0, 0, 0, 0 )
mov ecx,[esp + 0ch] ; ecx = count push edi ; preserve edi test ecx,ecx jz finish ; leave if count is zero
mov edi,[esp + 8] ; edi -> front string push esi ; preserve esi test edi,3 ; is string aligned on dword (4 bytes) push ebx ; preserve ebx je short find_end_of_front_string_loop
; simple byte loop until string is aligned
front_misaligned: mov al,byte ptr [edi] add edi,1 test al,al je short start_byte_3 test edi,3 jne short front_misaligned
find_end_of_front_string_loop: mov eax,dword ptr [edi] ; read dword (4 bytes) mov edx,7efefeffh add edx,eax xor eax,-1 xor eax,edx add edi,4 test eax,81010100h je short find_end_of_front_string_loop
; found zero byte in the loop mov eax,[edi - 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 jne short find_end_of_front_string_loop ; taken if bits 24-30 are clear and bit ; 31 is setstart_byte_3: sub edi,1 jmp short copy_startstart_byte_2: sub edi,2 jmp short copy_startstart_byte_1: sub edi,3 jmp short copy_startstart_byte_0: sub edi,4
; edi now points to the end of front string.
copy_start: mov esi,[esp + 14h] ; esi -> back string test esi,3 ; is back string is dword aligned? jnz back_misaligned
mov ebx,ecx ; store count for tail loop
shr ecx,2 jnz short main_loop_entrance jmp short tail_loop_start ; 0 < counter < 4
; simple byte loop until back string is aligned
back_misaligned: mov dl,byte ptr [esi] add esi,1 test dl,dl je short byte_0 mov [edi],dl add edi,1 sub ecx,1 jz empty_counter test esi,3 jne short back_misaligned mov ebx,ecx ; store count for tail loop shr ecx,2 ; convert ecx to dword count jnz short main_loop_entrance
tail_loop_start: mov ecx,ebx and ecx,3 ; ecx = count of leftover bytes after the ; dwords have been concatenated jz empty_counter
tail_loop: mov dl,byte ptr [esi] add esi,1 mov [edi],dl add edi,1 test dl,dl je short finish1 ; '\0' was already copied sub ecx,1 jnz tail_loop
empty_counter: mov [edi],cl ; cl=0;finish1: pop ebx pop esifinish: mov eax,[esp + 8] ; return in eax pointer to front string pop edi ret ; _cdecl return
byte_0: mov [edi],dl mov eax,[esp + 10h] ; return in eax pointer to front string pop ebx pop esi pop edi ret ; _cdecl return
main_loop: ; edx contains first dword of back string mov [edi],edx ; store one more dword add edi,4 ; kick pointer to front string
sub ecx,1 jz tail_loop_startmain_loop_entrance: mov edx,7efefeffh mov eax,dword ptr [esi] ; read 4 bytes
add edx,eax xor eax,-1
xor eax,edx mov edx,[esi] ; it's in cache now
add esi,4 ; kick pointer to back string test eax,81010100h
je short main_loop
; may be found zero byte in the loop 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 jne short main_loop ; taken if bits 24-30 are clear and bit ; 31 is setbyte_3: mov [edi],edx mov eax,[esp + 10h] ; return in eax pointer to front string pop ebx pop esi pop edi ret ; _cdecl return
byte_2: mov [edi],dx xor edx,edx mov eax,[esp + 10h] ; return in eax pointer to front string mov [edi + 2],dl pop ebx pop esi pop edi ret ; _cdecl return
byte_1: mov [edi],dx mov eax,[esp + 10h] ; return in eax pointer to front string pop ebx pop esi pop edi ret ; _cdecl return
strncat endp
end
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