Socket 基本概念

Socket 是对 TCP/IP 协议族的一种封装，是应用层与TCP/IP协议族通信的中间软件抽象层。从设计模式的角度看来，Socket其实就是一个门面模式，它把复杂的TCP/IP协议族隐藏在Socket接口后面，对用户来说，一组简单的接口就是全部，让Socket去组织数据，以符合指定的协议。

Socket 还可以认为是一种网络间不同计算机上的进程通信的一种方法，利用三元组（ip地址，协议，端口）就可以唯一标识网络中的进程，网络中的进程通信可以利用这个标志与其它进程进行交互。

Socket 起源于 Unix ，Unix/Linux 基本哲学之一就是“一切皆文件”，都可以用“打开(open) –> 读写(write/read) –> 关闭(close)”模式来进行操作。因此 Socket 也被处理为一种特殊的文件。

写一个简易的 WebServer

一个简易的 Server 的流程如下：

• 1.建立连接，接受一个客户端连接。
• 2.接受请求，从网络中读取一条 HTTP 请求报文。
• 3.处理请求，访问资源。
• 4.构建响应，创建带有 header 的 HTTP 响应报文。
• 5.发送响应，传给客户端。

省略流程 3，大体的程序与调用的函数逻辑如下：

• socket() 创建套接字
• bind() 分配套接字地址
• listen() 等待连接请求
• accept() 允许连接请求
• read()/write() 数据交换
• close() 关闭连接

代码如下：

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <unistd.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <string>
#include <cstring>
#include <iostream>
using namespace std;
const int port = 9090;
const int buffer_size = 1<<20;
const int method_size = 1<<10;
const int filename_size = 1<<10;
const int common_buffer_size = 1<<10;
void handleError(const string &message);
void requestHandling(int *sock);
void sendError(int *sock);
void sendData(int *sock, char *filename);
void sendHTML(int *sock, char *filename);
void sendJPG(int *sock, char *filename);
int main()
{
    int server_sock;
    int client_sock;
    struct sockaddr_in server_address;
    struct sockaddr_in client_address;
    socklen_t client_address_size;
    server_sock = socket(PF_INET, SOCK_STREAM, 0);
    if (server_sock == -1)
    {
        handleError("socket error");
    }
    memset(&server_address,0,sizeof(server_address));
    server_address.sin_family = AF_INET;
    server_address.sin_addr.s_addr = htonl(INADDR_ANY);
    server_address.sin_port = htons(port);
    if(bind(server_sock,(struct sockaddr*)&server_address, sizeof(server_address)) == -1){
        handleError("bind error");
    }
    if(listen(server_sock, 5) == -1) {
        handleError("listen error");
    }
    while(true) {
        client_address_size = sizeof(client_address);
        client_sock = accept(server_sock, (struct sockaddr*) &client_address, &client_address_size);
        if (client_sock == -1) {
            handleError("accept error");
        }
        requestHandling(&client_sock);
    }
    //system("open http://127.0.0.1:9090/index.html");
    close(server_sock);
    return 0;
}
void requestHandling(int *sock){
    int client_sock = *sock;
    char buffer[buffer_size];
    char method[method_size];
    char filename[filename_size];
    read(client_sock, buffer, sizeof(buffer)-1);
    if(!strstr(buffer, "HTTP/")) {
        sendError(sock);
        close(client_sock);
        return;
    }
    strcpy(method, strtok(buffer," /"));
    strcpy(filename, strtok(NULL, " /"));
    if(0 != strcmp(method, "GET")) {
        sendError(sock);
        close(client_sock);
        return;
    }
    sendData(sock, filename);
}
void sendData(int *sock, char *filename) {
    int client_sock = *sock;
    char buffer[common_buffer_size];
    char type[common_buffer_size];
    strcpy(buffer, filename);
    strtok(buffer, ".");
    strcpy(type, strtok(NULL, "."));
    if(0 == strcmp(type, "html")){
        sendHTML(sock, filename);
    }else if(0 == strcmp(type, "jpg")){
        sendJPG(sock, filename);
    }else{
        sendError(sock);
        close(client_sock);
        return ;
    }
}
void sendHTML(int *sock, char *filename) {
    int client_sock = *sock;
    char buffer[buffer_size];
    FILE *fp;
    char status[] = "HTTP/1.0 200 OK\r\n";
    char header[] = "Server: A Simple Web Server\r\nContent-Type: text/html\r\n\r\n";
    write(client_sock, status, strlen(status));
    write(client_sock, header, strlen(header));
    fp = fopen(filename, "r");
    if(!fp){
        sendError(sock);
        close(client_sock);
        handleError("failed to open file");
        return ;
    }
    fgets(buffer,sizeof(buffer), fp);
    while(!feof(fp)) {
        write(client_sock, buffer, strlen(buffer));
        fgets(buffer, sizeof(buffer), fp);
    }
    fclose(fp);
    close(client_sock);
}
void sendJPG(int *sock, char *filename) {
    int client_sock = *sock;
    char buffer[buffer_size];
    FILE *fp;
    FILE *fw;
    char status[] = "HTTP/1.0 200 OK\r\n";
    char header[] = "Server: A Simple Web Server\r\nContent-Type: image/jpeg\r\n\r\n";
    write(client_sock, status, strlen(status));
    write(client_sock, header, strlen(header));
    fp = fopen(filename, "rb");
    if(NULL == fp){
        sendError(sock);
        close(client_sock);
        handleError("failed to open file");
        return ;
    }
    fw = fdopen(client_sock, "w");
    fread(buffer, 1, sizeof(buffer), fp);
    while (!feof(fp)){
        fwrite(buffer, 1, sizeof(buffer), fw);
        fread(buffer, 1, sizeof(buffer), fp);
    }
    fclose(fw);
    fclose(fp);
    close(client_sock);
}
void handleError(const string &message) {
    cout<<message;
    exit(1);
}
void sendError(int *sock){
    int client_sock = *sock;
    char status[] = "HTTP/1.0 400 Bad Request\r\n";
    char header[] = "Server: A Simple Web Server\r\nContent-Type: text/html\r\n\r\n";
    char body[] = "<html><head><title>Bad Request</title></head><body><p>400 Bad Request</p></body></html>";
    write(client_sock, status, sizeof(status));
    write(client_sock, header, sizeof(header));
    write(client_sock, body, sizeof(body));
}

参考资料

1. Linux Socket编程
2. 揭开 Socket 编程的面纱