狠心买下正版《UNIX网络编程》后，喵哥开始了网络编程的道路。在书中介绍的第一个例子：输出服务器端的时间信息。
官方提供的源代码都是基于自己的头文件写的，所以要使用这些源代码需要使用他们的头文件，这个不是自己靠复制一两个头文件到目的文件夹那么简单，幸好，官方提供了README，按照README基本上不会有啥问题，除了接下来要解决的几个。
终端进入unpv13e目录下，执行前面的命令比较顺利：
./configure # try to figure out all implementation differences
cd lib # build the basic library that all programs need
make # use "gmake" everywhere on BSD/OS systems
在执行第二个make的时候就报错了。
cd ../libfree # continue building the basic library
make
错误提示：
gcc -I../lib -g -O2 -D_REENTRANT -Wall -c -o in_cksum.o in_cksum.c
gcc -I../lib -g -O2 -D_REENTRANT -Wall -c -o inet_ntop.o inet_ntop.c
inet_ntop.c: In function ‘inet_ntop’:
inet_ntop.c:60:9: error: argument ‘size’ doesn’t match prototype
size_t size;
^~~~
In file included from inet_ntop.c:27:
/usr/include/arpa/inet.h:64:20: error: prototype declaration
extern const char *inet_ntop (int __af, const void *__restrict __cp,
^~~~~~~~~
make: *** [<内置>：inet_ntop.o] 错误 1
解决方法：1、在inet_ntop.c中加入
#define size_t socklen_t
2、在/usr/include/net路径下添加一个文件：if_dl.h 。if_dl.h的源代码如下：
/*
* Copyright (c) 1990, 1993
* The Regents of the University of California. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. All advertising materials mentioning features or use of this software
* must display the following acknowledgement:
* This product includes software developed by the University of
* California, Berkeley and its contributors.
* 4. Neither the name of the University nor the names of its contributors
* may be used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*
* @(#)if_dl.h 8.1 (Berkeley) 6/10/93
* $FreeBSD: src/sys/net/if_dl.h,v 1.10 2000/03/01 02:46:25 archie Exp $
*/
#ifndef _NET_IF_DL_H_
#define _NET_IF_DL_H_
/*
* A Link-Level Sockaddr may specify the interface in one of two
* ways: either by means of a system-provided index number (computed
* anew and possibly differently on every reboot), or by a human-readable
* string such as "il0" (for managerial convenience).
*
* Census taking actions, such as something akin to SIOCGCONF would return
* both the index and the human name.
*
* High volume transactions (such as giving a link-level ``from'' address
* in a recvfrom or recvmsg call) may be likely only to provide the indexed
* form, (which requires fewer copy operations and less space).
*
* The form and interpretation of the link-level address is purely a matter
* of convention between the device driver and its consumers; however, it is
* expected that all drivers for an interface of a given if_type will agree.
*/
/*
* Structure of a Link-Level sockaddr:
*/
struct sockaddr_dl {
u_char sdl_len; /* Total length of sockaddr */
u_char sdl_family; /* AF_LINK */
u_short sdl_index; /* if != 0, system given index for interface */
u_char sdl_type; /* interface type */
u_char sdl_nlen; /* interface name length, no trailing 0 reqd. */
u_char sdl_alen; /* link level address length */
u_char sdl_slen; /* link layer selector length */
char sdl_data[12]; /* minimum work area, can be larger;
contains both if name and ll address */
u_short sdl_rcf; /* source routing control */
u_short sdl_route[16]; /* source routing information */
};
#define LLADDR(s) ((caddr_t)((s)->sdl_data + (s)->sdl_nlen))
#ifndef _KERNEL
#include <sys/cdefs.h>
__BEGIN_DECLS
void link_addr __P((const char *, struct sockaddr_dl *));
char *link_ntoa __P((const struct sockaddr_dl *));
__END_DECLS
#endif /* !_KERNEL */
#endif
然后可以正常的执行上面的make了。
README接下来的两个make是针对BSD系统的，而喵哥用的是ubuntu（GNU），所以不需要执行它们，事实上直接执行是会报错的。。。
接下来，就可以进入intro，开始执行第一个程序了，至少README是这么说的:)
cd ../intro # build and test a basic client program
make daytimetcpcli
./daytimetcpcli 127.0.0.1
果真如此么？通常，你这么执行会报如下错误：
connect error: Connection refused
这个错误的意思是： daytime 服务程序没开（为了安全默认是关闭的）。
有两个解决方案：
1、安装xinetd，然后允许访问daytime服务。
sudo apt-get install xinetd
cd /etc/xinetd.d/
sudo vim daytime
文件中有两个 disable = yes ，全部修改成 disable = no，并保存退出，然后重启 xinetd
service xinetd restart
此时，直接执行，
cd ../intro # build and test a basic client program
make daytimetcpcli
./daytimetcpcli 127.0.0.1
即可得到本机的时间信息。
附：关于如何使用Xinetd：Xinetd服务的安装与配置、xinetd不太详的详解
2、开启系统原本禁止访问的daytime服务可能会导致无法预料的危险，所以不是太提倡按照方法1解决。
实际上在intro文件夹下有相应的服务端源代码，只要用它开启一个服务器即可。
make daytimetcpsrv.c
sudo ./daytimetcpsrv
然后在另外一个终端中输入：
make daytimetcpcli
./daytimetcpcli 127.0.0.1
就可以得到想要的结果了。