strncpy.c

来自「一个C源代码分析器」· C语言 代码 · 共 83 行

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

  --s1;

  if (n >= 4)
    {
      size_t n4 = n >> 2;

      for (;;)
	{
	  c = *s2++;
	  *++s1 = c;
	  if (c == '\0')
	    break;
	  c = *s2++;
	  *++s1 = c;
	  if (c == '\0')
	    break;
	  c = *s2++;
	  *++s1 = c;
	  if (c == '\0')
	    break;
	  c = *s2++;
	  *++s1 = c;
	  if (c == '\0')
	    break;
	  if (--n4 == 0)
	    goto last_chars;
	}
      n = n - (s1 - s) - 1;
      if (n == 0)
	return s;
      goto zero_fill;
    }

 last_chars:
  n &= 3;
  if (n == 0)
    return s;

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

 zero_fill:
  do
    *++s1 = '\0';
  while (--n > 0);

  return s;
}