网络编程学习，给程序做注释

从github上找了3段程序，借此来学习socket编程，从一开始看不懂，到现在能看出来个轮廓，还是有进步的。

#注意，本文所示程序除注释外均非本人所写。

#ifndef SERVER_H
#define SERVER_H            // line 3 to 11 : include lib
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

struct Server;                //声明结构体

struct Server * new_server(char *host, char *port); //声明函数

//主机连接函数
int host_connect(struct Server *this);
//信息接收
char *get_message(struct Server *this);
//信息发送函数
void send_message(struct Server *this, const char *msg);
// 错误处理函数
void error(struct Server *this, const char *msg);

struct Server {
  int port;                        //端口
  char *host;                    //主机地址

  int sockfd;                    //描述符
  char buffer[256];                //应该是传送的信息

  int (*host_connect)(struct Server *this);//函数指针
  char *(*get_message)(struct Server *this);
  void (*send_message)(struct Server *this, const char *msg);
  void (*error)(struct Server *this, const char *msg);

  struct sockaddr_in server_address;;
  struct hostent *server;
};

#endif

#include "server.h"

//函数名字为new_server的返回值是指向结构体Server的指针
struct Server * new_server(char *host, char *port) {
  struct Server *server = malloc(sizeof(struct Server));

  // line 8~13 赋值
  server->host_connect = &host_connect;
  server->get_message = &get_message;
  server->send_message = &send_message;
  server->error = &error;
  server->host = host;
  server->port = atoi(port);
  
  return server;
}

int host_connect(struct Server *this) {
  // int socket(int domain,int type,int protocol);
  //                域            类型      协议
  //创建套接字
  // AF_INET  IPv4因特网域
  // SOCK_STREAM  有序，可靠，双向到面向连接字节流
  // AF_INET + SOCK_STREAM ----->  tcp协议
  this->sockfd = socket(AF_INET, SOCK_STREAM, 0);
  if (this->sockfd < 0) {
    return 0;
  }

  this->server = gethostbyname(this->host);
  if (this->server == NULL) {
    return 0;
  }

  //bzero函数功能，置字节字符串s到前n个字节为零且包括''
  bzero((char *) &this->server_address, sizeof(this->server_address));
  this->server_address.sin_family = AF_INET;

  //bcopy函数功能应该和bzero差不多
  //extern void bcopy(const void *src,void *dest,int n);
  //将字符串src到前n个字节复制到dest中
  bcopy((char *)this->server->h_addr,
        (char *)&this->server_address.sin_addr.s_addr,
        this->server->h_length);

  this->server_address.sin_port = htons(this->port);

  if (connect(this->sockfd,
      (struct sockaddr *) &this->server_address,
      sizeof(this->server_address)) < 0) {
    this->error(this, "Could not connect to host!");
  }

  return 1;
}

char * get_message(struct Server *this) {
  bzero(this->buffer, 256);

  if(read(this->sockfd, this->buffer, 255) < 0) {
    this->error(this, "Error reading from socket");
  }

  return this->buffer;
}

void send_message(struct Server *this, const char *msg) {
  bzero(this->buffer, 256);

  if(write(this->sockfd, msg, strlen(msg)) < 0) {
    this->error(this, "Error writing to socket");
  }
}

void error(struct Server *this, const char *msg) {
  perror(msg);
  close(this->sockfd);
  exit(-1);
}

#include "server.h"

int main(int argc, char *argv[]) {
  if (argc < 2) {
    puts("Usage: ./chat [ip address] [port]");
    return -1;
  }

  char *line;
  size_t length = 80;

  struct Server *server = new_server(argv[1], argv[2]);
  server->host_connect(server);

  printf("## %s
", server->get_message(server));

  while (1) {
    printf("> ");
    fflush(stdout);

    if (getline(&line, &length, stdin) < 0) {
      server->error(server, "Error while getting input!");
    }

    if (strcmp(line, "/quit
") == 0) {
      puts("Goodbye!");
      break;
    }

    server->send_message(server, line);
    printf("## %s
", server->get_message(server));
  }

  close(server->sockfd);
  return 0;
}