1.strdup()
功能：克隆一个副本，具有独立的内存空间
声明：char *strdup(char *str)；
原型：
char * __strdup (const char *s)
{
size_t len =strlen (s) + 1;
void *new =malloc (len);
if (new == NULL)
return NULL;
return (char *)memcpy (new, s, len);
}
strndup()
功能：克隆一个n长度副本，具有独立的内存空间
声明：char *strdnup(char *str,int n)；
原型：
The strdup() function returns a pointer to a new string which is a duplicate of the string s. Memory for the new string is obtained withmalloc(3), and can be freed with free(3).
The strndup() function is similar, but only copies at most n bytes. If s is longer than n, only n bytes are copied, and a terminating null byte ('\0') is added.
如果s的长度比n大，只复制n长度，并在末尾加‘\0’
2.