网络编程–创建tcp连接

一直都认为网络编程是一块很重要的部分，之前没有时间和门道来学习。这次心血来潮，开始学习socket编程。

这次只是简单的创建了server和client之间的连接，期间也遇到一点问题，这次算是做个记录。
目标：了解测试方法，掌握网络编程的具体api

验证方法

运行server代码后，要测试server是否正常启动且端口正常暴露出来。最简单的方式，可以用telnet来测试
（0.0.0.0表示本地网络，如果要本地写server/client，用该地址）

ll@ll-VirtualBox:~/share/mycode/c++/client$ telnet 0.0.0.0 12351
Trying 0.0.0.0...
Connected to 0.0.0.0.
Escape character is '^]'

server端打印log如下，

ll@ll-VirtualBox:~/share/mycode/c++/tcp-server$ ./bin/Debug/tcp-server 
socket fd is 3
local ip =0.0.0.0
accept connect success, client port=52426ip =127.0.0.1
newsockfd=4

同时也可以用netstat命令来检查网络端口的状态。其中12351是我的server端用到的端口

ll@ll-VirtualBox:~/share/mycode/c++/tcp-server$ netstat -ta
激活Internet连接 (服务器和已建立连接的)
Proto Recv-Q Send-Q Local Address           Foreign Address         State      
tcp        0      0 *:microsoft-ds          *:*                     LISTEN     
tcp        0      0 *:12351                 *:*                     LISTEN     
tcp        0      0 localhost:27017         *:*                     LISTEN     
tcp        0      0 localhost:mysql         *:*                     LISTEN 

问题

server在写的过程中遇到的都是些基本的网络参数问题，比如struct sockaddr_in该 如何填写。
在client连接server端时却出现一个问题 connect failed: Connection refused？？？权限？还是哪里不对？？

ll@ll-VirtualBox:~/share/mycode/c++/client$ bin/Debug/client 
local ip =0.0.0.0
connect failed: Connection refused

到网络上搜，一直没有搜到答案。通过telnet测试，说明server端是没有问题的，问题肯定在client端。后来检查代码，打印端口号，发现了问题。打印出的端口竟然是0。

ll@ll-VirtualBox:~/share/mycode/c++/client$ bin/Debug/client 
sin port = 0
port = 0
 local ip =0.0.0.0
connect failed: Connection refused

检查sockaddr_in 的字段类型，发现端口号是2字节，而不是4字节。所以我在进行大小端转换时应该用htons，而不是用htonl。用htonl转换后的端口号在赋值时，被截断了，因此打印出的端口号是0。

server端

如下是server代码

#include <iostream>
#include <arpa/inet.h>
#include <err.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>

#define PORT 12351
#define  MAXSIZE 10240

using namespace std;

int main(int args, char *argv[])
{
//    cout << "Hello world!" << endl;
    int sockfd, newsockfd;
    struct sockaddr_in server_addr;
    struct sockaddr_in client_addr;

    int sin_size, portnum;
    char buf[MAXSIZE];

    if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1){
        fprintf(stderr, "create socket failed!");
        strerror(errno);
        exit(1);
    }

    std::cout<<"socket fd is " << sockfd <<std::endl;

    bzero(&server_addr, sizeof(struct sockaddr_in));
    server_addr.sin_port = htons(PORT);
    server_addr.sin_family = AF_INET;
//    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    server_addr.sin_addr.s_addr = inet_addr("0.0.0.0");

    if( bind(sockfd, (struct sockaddr*)(&server_addr), sizeof(struct sockaddr))){
        fprintf(stderr, "bind failed");
        strerror(errno);
        exit(2);
    }

    struct sockaddr_in local_addr;
    int len;
    if (getsockname(sockfd, (struct sockaddr *)&local_addr, (socklen_t* )&len) == 0) {
        struct sockaddr_in* sin = (struct sockaddr_in*)(&local_addr);
//        *port = sin->sin_port;
        char addr_buffer[INET_ADDRSTRLEN];
        void * tmp = &(sin->sin_addr);
        if (inet_ntop(AF_INET, tmp, addr_buffer, INET_ADDRSTRLEN) == NULL){
         cerr << "inet_ntop err";
         return false;
        }
        printf("local ip =%s\n", addr_buffer);
    }

    if(listen(sockfd, 10) < 0){
        fprintf(stderr,"listen failed");
        strerror(errno);
        exit(3);
    }
    sin_size = sizeof(struct sockaddr_in);
    while(1){
        if( (newsockfd = accept(sockfd, (struct sockaddr *) (&client_addr),(socklen_t*) &sin_size))< 0){
            fprintf(stderr, "listen failed");
            strerror(errno);
            exit(3);
        }
        std::cout<<"accept connect success, client port=" << client_addr.sin_port<<
            "ip =" <<inet_ntoa(client_addr.sin_addr)<<std::endl;
        std::cout<< "newsockfd=" << newsockfd<<std::endl;
    }


    return 0;
}

client端代码

#include <iostream>
#include <string.h>
#include <stdlib.h>
#include <error.h>
#include <sys/socket.h>
#include <unistd.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define MAXLINE 4096


using namespace std;

int main()
{
//    cout << "Hello world!" << endl;
    int sockfd, n;

    sockfd = socket( AF_INET, SOCK_STREAM, 0);
    if( -1 ==sockfd){
        perror("create socket failed");
        strerror(errno);
        exit(errno);
    }

    struct sockaddr_in server;
    memset(&server, 0, sizeof(struct sockaddr_in));
    server.sin_family = AF_INET;

    server.sin_port = htons(12351);
    server.sin_addr.s_addr = inet_addr("0.0.0.0");//192.168.1.106
    printf("sin port = %x\n" ,server.sin_port);
    printf("port = %x\n " ,ntohs(server.sin_port));
    struct sockaddr_in local_addr;
    int len;
    if (getsockname(sockfd, (struct sockaddr *)&local_addr, (socklen_t* )&len) == 0) {
        struct sockaddr_in* sin = (struct sockaddr_in*)(&local_addr);
//        *port = sin->sin_port;
        char addr_buffer[INET_ADDRSTRLEN];
        void * tmp = &(sin->sin_addr);
        if (inet_ntop(AF_INET, tmp, addr_buffer, INET_ADDRSTRLEN) == NULL){
         cerr << "inet_ntop err";
         return false;
        }
        printf("local ip =%s\n", addr_buffer);
    }

    int res = 0 ;
    res = connect(sockfd, (struct sockaddr* )&server, sizeof(server));
    if(res == -1){
        perror("connect failed");
        strerror(errno);
        exit(errno);
    }

    return 0;
}