關於strncpy的小坑

  strncpy是比strcpy更安全的字符串拷貝函數，但在使用中須要注意一點：

No null-character is implicitly appended at the end of destination if source is longer than num. Thus, in this case, destination shall not be considered a null terminated C string (reading it as such would overflow).

  若是在strncpy後不顯式追加'\0'，那麼你不可直接把它當作字符串使用處理，否則可能會出現亂碼，甚至程序奔潰。

  追加下strncpy glibc的實現版本

char *
STRNCPY (char *s1, const char *s2, size_t n)
{
  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;
}