glibc/string/strncat.c

/* Copyright (C) 1991-2014 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <http://www.gnu.org/licenses/>.  */

#include <string.h>

#ifdef _LIBC
# include <memcopy.h>
#endif

#ifndef STRNCAT
# undef strncat
# define STRNCAT  strncat
#endif

char *
STRNCAT (char *s1, const char *s2, size_t n)
{
  char c;
  char *s = s1;

  /* Find the end of S1.  */
  s1 += strlen (s1);

  /* Make S1 point before next character, so we can increment
     it while memory is read (wins on pipelined cpus).  */
  s1 -= 1;

  if (n >= 4)
    {
      size_t n4 = n >> 2;
      do
	{
	  c = *s2++;
	  *++s1 = c;
	  if (c == '\0')
	    return s;
	  c = *s2++;
	  *++s1 = c;
	  if (c == '\0')
	    return s;
	  c = *s2++;
	  *++s1 = c;
	  if (c == '\0')
	    return s;
	  c = *s2++;
	  *++s1 = c;
	  if (c == '\0')
	    return s;
	} while (--n4 > 0);
      n &= 3;
    }

  while (n > 0)
    {
      c = *s2++;
      *++s1 = c;
      if (c == '\0')
	return s;
      n--;
    }

  if (c != '\0')
    *++s1 = '\0';

  return s;
}