string/strncat.c

   1 /* Copyright (C) 1991,1997,2011 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <http://www.gnu.org/licenses/>.  */
  17
  18 #include <string.h>
  19
  20 #ifdef _LIBC
  21 # include <memcopy.h>
  22 #else
  23 typedef char reg_char;
  24 #endif
  25
  26 #ifndef STRNCAT
  27 # undef strncat
  28 # define STRNCAT  strncat
  29 #endif
  30
  31 char *
  32 STRNCAT (char *s1, const char *s2, size_t n)
  33 {
  34   reg_char c;
  35   char *s = s1;
  36
  37   /* Find the end of S1.  */
  38   do
  39     c = *s1++;
  40   while (c != '\0');
  41
  42   /* Make S1 point before next character, so we can increment
  43      it while memory is read (wins on pipelined cpus).  */
  44   s1 -= 2;
  45
  46   if (n >= 4)
  47     {
  48       size_t n4 = n >> 2;
  49       do
  50         {
  51           c = *s2++;
  52           *++s1 = c;
  53           if (c == '\0')
  54             return s;
  55           c = *s2++;
  56           *++s1 = c;
  57           if (c == '\0')
  58             return s;
  59           c = *s2++;
  60           *++s1 = c;
  61           if (c == '\0')
  62             return s;
  63           c = *s2++;
  64           *++s1 = c;
  65           if (c == '\0')
  66             return s;
  67         } while (--n4 > 0);
  68       n &= 3;
  69     }
  70
  71   while (n > 0)
  72     {
  73       c = *s2++;
  74       *++s1 = c;
  75       if (c == '\0')
  76         return s;
  77       n--;
  78     }
  79
  80   if (c != '\0')
  81     *++s1 = '\0';
  82
  83   return s;
  84 }