string/strncat.c

   1 /* Copyright (C) 1991-2014 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 #endif
  23
  24 #ifndef STRNCAT
  25 # undef strncat
  26 # define STRNCAT  strncat
  27 #endif
  28
  29 char *
  30 STRNCAT (char *s1, const char *s2, size_t n)
  31 {
  32   char c;
  33   char *s = s1;
  34
  35   /* Find the end of S1.  */
  36   do
  37     c = *s1++;
  38   while (c != '\0');
  39
  40   /* Make S1 point before next character, so we can increment
  41      it while memory is read (wins on pipelined cpus).  */
  42   s1 -= 2;
  43
  44   if (n >= 4)
  45     {
  46       size_t n4 = n >> 2;
  47       do
  48         {
  49           c = *s2++;
  50           *++s1 = c;
  51           if (c == '\0')
  52             return s;
  53           c = *s2++;
  54           *++s1 = c;
  55           if (c == '\0')
  56             return s;
  57           c = *s2++;
  58           *++s1 = c;
  59           if (c == '\0')
  60             return s;
  61           c = *s2++;
  62           *++s1 = c;
  63           if (c == '\0')
  64             return s;
  65         } while (--n4 > 0);
  66       n &= 3;
  67     }
  68
  69   while (n > 0)
  70     {
  71       c = *s2++;
  72       *++s1 = c;
  73       if (c == '\0')
  74         return s;
  75       n--;
  76     }
  77
  78   if (c != '\0')
  79     *++s1 = '\0';
  80
  81   return s;
  82 }