string/strncat.c

   1 /* Copyright (C) 1991, 1997 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, write to the Free
  16    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  17    02111-1307 USA.  */
  18
  19 #include <string.h>
  20
  21 #ifdef _LIBC
  22 # include <memcopy.h>
  23 #else
  24 typedef char reg_char;
  25 #endif
  26
  27 #undef strncat
  28
  29 char *
  30 strncat (s1, s2, n)
  31      char *s1;
  32      const char *s2;
  33      size_t n;
  34 {
  35   reg_char c;
  36   char *s = s1;
  37
  38   /* Find the end of S1.  */
  39   do
  40     c = *s1++;
  41   while (c != '\0');
  42
  43   /* Make S1 point before next character, so we can increment
  44      it while memory is read (wins on pipelined cpus).  */
  45   s1 -= 2;
  46
  47   if (n >= 4)
  48     {
  49       size_t n4 = n >> 2;
  50       do
  51         {
  52           c = *s2++;
  53           *++s1 = c;
  54           if (c == '\0')
  55             return s;
  56           c = *s2++;
  57           *++s1 = c;
  58           if (c == '\0')
  59             return s;
  60           c = *s2++;
  61           *++s1 = c;
  62           if (c == '\0')
  63             return s;
  64           c = *s2++;
  65           *++s1 = c;
  66           if (c == '\0')
  67             return s;
  68         } while (--n4 > 0);
  69       n &= 3;
  70     }
  71
  72   while (n > 0)
  73     {
  74       c = *s2++;
  75       *++s1 = c;
  76       if (c == '\0')
  77         return s;
  78       n--;
  79     }
  80
  81   if (c != '\0')
  82     *++s1 = '\0';
  83
  84   return s;
  85 }