glibc/sysdeps/generic/strncat.c

/* Copyright (C) 1991 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 Library General Public License as
published by the Free Software Foundation; either version 2 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
Library General Public License for more details.

You should have received a copy of the GNU Library General Public
License along with the GNU C Library; see the file COPYING.LIB.  If
not, write to the Free Software Foundation, Inc., 675 Mass Ave,
Cambridge, MA 02139, USA.  */

#include <ansidecl.h>
#include <string.h>
#include <memcopy.h>

char *
DEFUN(strncat, (s1, s2, n), char *s1 AND CONST char *s2 AND size_t n)
{
  reg_char c;
  char *s = s1;

  /* Find the end of S1.  */
  do
    c = *s1++;
  while (c != '\0');

  /* Make S1 point before next character, so we can increment
     it while memory is read (wins on pipelined cpus).  */
  s1 -= 2;

  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;
}