入门进阶：C/C++中的套接字编程

什么是套接字编程?

套接字编程是一种连接网络上的两个节点以相互通信的方法。一个套接字(节点)监听IP上的一个特定端口，而另一个套接字连接到另一个端口。当客户端到达服务器时，服务器形成侦听器套接字。

服务器和客户端模型的状态图

服务器端

套接字创建：

int sockfd = socket(domain, type, protocol)

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int sockfd = socket(domain, type, protocol)

sockfd：套接字描述符，一个整数（如文件句柄）
domain：整数，通信域，例如AF_INET（IPv4协议），AF_INET6（IPv6协议）
type：通信类型
SOCK_STREAM：TCP（可靠，面向连接）
SOCK_DGRAM：UDP（不可靠，无连接的
protocol： Internet协议（IP）的协议值，为0。这与出现在数据包IP报头的协议字段中的数字相同。（有关详细信息，请参见手动协议）
Setsockopt：

int setsockopt(int sockfd, int level, int optname, const void *optval, socklen_t optlen);

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int setsockopt(int sockfd, int level, int optname,
const void *optval, socklen_t optlen);

这有助于处理文件描述符sockfd引用的套接字的选项。这是完全可选的，但有助于地址和端口的重用。防止出现以下错误：“address already in use”。
Bind：

int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

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int bind(int sockfd, const struct sockaddr *addr,
socklen_t addrlen);

创建套接字后，bind函数将套接字绑定到addr（自定义数据结构）中指定的地址和端口号。在示例代码中，我们将服务器绑定到本地主机，因此我们使用INADDR_ANY指定IP地址。
Listen：

int listen(int sockfd, int backlog);

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int listen(int sockfd, int backlog);

它将服务器套接字置于被动模式，在该模式下它等待客户端接近服务器以建立连接。待办事项定义了sockfd的未决连接队列可以增长的最大长度。如果在队列已满时连接请求到达，则客户端可能会收到带有ECONNREFUSED指示的错误。
Accept：

int new_socket= accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

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int new_socket= accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

它为侦听套接字sockfd提取未决连接队列上的第一个连接请求，创建一个新的已连接套接字，并返回引用该套接字的新文件描述符。此时，客户端和服务器之间已建立连接，并且它们已准备好传输数据。

客户端

Socket connection：与服务器的套接字创建完全相同
Connect：

int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

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int connect(int sockfd, const struct sockaddr *addr,
socklen_t addrlen);

connect()系统调用将文件描述符sockfd引用的套接字连接到addr指定的地址。服务器的地址和端口在addr中指定。

实现
这里，我们在服务器和客户端之间交换一条问候消息，以演示客户端/服务器模型。

server.c

<span style="color: #999580;">// 服务器端C/C++程序演示套接字编程</span>
<span style="color: #b65611;">#<span style="color: #b854d4;">include</span> &lt;unistd.h&gt; </span>
<span style="color: #b65611;">#<span style="color: #b854d4;">include</span> &lt;stdio.h&gt; </span>
<span style="color: #b65611;">#<span style="color: #b854d4;">include</span> &lt;sys/socket.h&gt; </span>
<span style="color: #b65611;">#<span style="color: #b854d4;">include</span> &lt;stdlib.h&gt; </span>
<span style="color: #b65611;">#<span style="color: #b854d4;">include</span> &lt;netinet/in.h&gt; </span>
<span style="color: #b65611;">#<span style="color: #b854d4;">include</span> &lt;string.h&gt; </span>
<span style="color: #b65611;">#<span style="color: #b854d4;">define</span> PORT 8090 </span>
<span style="color: #b854d4;">int</span> main(<span style="color: #b854d4;">int</span> argc, <span style="color: #b854d4;">char</span> <span style="color: #b854d4;">const</span> *argv[]) 
{ 
    <span style="color: #b854d4;">int</span> server_fd, new_socket, valread; 
    <span style="color: #b854d4;">struct</span> sockaddr_in address; 
    <span style="color: #b854d4;">int</span> opt = <span style="color: #b65611;">1</span>; 
    <span style="color: #b854d4;">int</span> addrlen = <span style="color: #b854d4;">sizeof</span>(address); 
    <span style="color: #b854d4;">char</span> buffer[<span style="color: #b65611;">1024</span>] = {<span style="color: #b65611;">0</span>}; 
    <span style="color: #b854d4;">char</span> *hello = (<span style="color: #b854d4;">char</span>*)<span style="color: #60ac39;">"Hello from server"</span>; 

    <span style="color: #999580;">// Creating socket file descriptor </span>
    <span style="color: #b854d4;">if</span> ((server_fd = socket(AF_INET, SOCK_STREAM, <span style="color: #b65611;">0</span>)) == <span style="color: #b65611;">0</span>) 
    { 
        perror(<span style="color: #60ac39;">"socket failed"</span>); 
        <span style="color: #b65611;">exit</span>(EXIT_FAILURE); 
    } 

    <span style="color: #999580;">// Forcefully attaching socket to the port 8080 </span>
    <span style="color: #b854d4;">if</span> (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, 
                                                  &amp;opt, <span style="color: #b854d4;">sizeof</span>(opt))) 
    { 
        perror(<span style="color: #60ac39;">"setsockopt"</span>); 
        <span style="color: #b65611;">exit</span>(EXIT_FAILURE); 
    } 
    address.sin_family = AF_INET; 
    address.sin_addr.s_addr = INADDR_ANY; 
    address.sin_port = htons( PORT ); 

    <span style="color: #999580;">// Forcefully attaching socket to the port 8080 </span>
    <span style="color: #b854d4;">if</span> (bind(server_fd, (<span style="color: #b854d4;">struct</span> sockaddr *)&amp;address,  
                                 <span style="color: #b854d4;">sizeof</span>(address))&lt;<span style="color: #b65611;">0</span>) 
    { 
        perror(<span style="color: #60ac39;">"bind failed"</span>); 
        <span style="color: #b65611;">exit</span>(EXIT_FAILURE); 
    } 
    <span style="color: #b854d4;">if</span> (listen(server_fd, <span style="color: #b65611;">3</span>) &lt; <span style="color: #b65611;">0</span>) 
    { 
        perror(<span style="color: #60ac39;">"listen"</span>); 
        <span style="color: #b65611;">exit</span>(EXIT_FAILURE); 
    } 
    <span style="color: #b854d4;">if</span> ((new_socket = accept(server_fd, (<span style="color: #b854d4;">struct</span> sockaddr *)&amp;address,  
                       (socklen_t*)&amp;addrlen))&lt;<span style="color: #b65611;">0</span>) 
    { 
        perror(<span style="color: #60ac39;">"accept"</span>); 
        <span style="color: #b65611;">exit</span>(EXIT_FAILURE); 
    } 
    valread = read( new_socket , buffer, <span style="color: #b65611;">1024</span>); 
    <span style="color: #b65611;">printf</span>(<span style="color: #60ac39;">"%s\n"</span>,buffer ); 
    send(new_socket , hello , <span style="color: #b65611;">strlen</span>(hello) , <span style="color: #b65611;">0</span> ); 
    <span style="color: #b65611;">printf</span>(<span style="color: #60ac39;">"Hello message sent,www.linuxmi.com\n"</span>); 
    <span style="color: #b854d4;">return</span> <span style="color: #b65611;">0</span>; 
}

// 服务器端C/C++程序演示套接字编程

#include <unistd.h>

#include <stdio.h>

#include <sys/socket.h>

#include <stdlib.h>

#include <netinet/in.h>

#include <string.h>

#define PORT 8090

int main(int argc, char const *argv[])

{

int server_fd, new_socket, valread;

struct sockaddr_in address;

int opt = 1;

int addrlen = sizeof(address);

char buffer[1024] = {0};

char *hello = (char*)"Hello from server";

// Creating socket file descriptor

if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0)

{

perror("socket failed");

exit(EXIT_FAILURE);

}

// Forcefully attaching socket to the port 8080

if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT,

&opt, sizeof(opt)))

{

perror("setsockopt");

exit(EXIT_FAILURE);

}

address.sin_family = AF_INET;

address.sin_addr.s_addr = INADDR_ANY;

address.sin_port = htons( PORT );

// Forcefully attaching socket to the port 8080

if (bind(server_fd, (struct sockaddr *)&address,

sizeof(address))<0)

{

perror("bind failed");

exit(EXIT_FAILURE);

}

if (listen(server_fd, 3) < 0)

{

perror("listen");

exit(EXIT_FAILURE);

}

if ((new_socket = accept(server_fd, (struct sockaddr *)&address,

(socklen_t*)&addrlen))<0)

{

perror("accept");

exit(EXIT_FAILURE);

}

valread = read( new_socket , buffer, 1024);

printf("%s\n",buffer );

send(new_socket , hello , strlen(hello) , 0 );

printf("Hello message sent,www.linuxmi.com\n");

return 0;

}

client.c

<span style="color: #999580;">// 客户端C/C++程序演示套接字编程</span>
<span style="color: #b65611;">#<span style="color: #b854d4;">include</span> &lt;stdio.h&gt; </span>
<span style="color: #b65611;">#<span style="color: #b854d4;">include</span> &lt;sys/socket.h&gt; </span>
<span style="color: #b65611;">#<span style="color: #b854d4;">include</span> &lt;arpa/inet.h&gt; </span>
<span style="color: #b65611;">#<span style="color: #b854d4;">include</span> &lt;unistd.h&gt; </span>
<span style="color: #b65611;">#<span style="color: #b854d4;">include</span> &lt;string.h&gt; </span>
<span style="color: #b65611;">#<span style="color: #b854d4;">define</span> PORT 8090 </span>

<span style="color: #b854d4;">int</span> main(<span style="color: #b854d4;">int</span> argc, <span style="color: #b854d4;">char</span> <span style="color: #b854d4;">const</span> *argv[]) 
{ 
    <span style="color: #b854d4;">int</span> sock = <span style="color: #b65611;">0</span>, valread; 
    <span style="color: #b854d4;">struct</span> sockaddr_in serv_addr; 
    <span style="color: #b854d4;">char</span> *hello = <span style="color: #60ac39;">"Hello from client"</span>; 
    <span style="color: #b854d4;">char</span> buffer[<span style="color: #b65611;">1024</span>] = {<span style="color: #b65611;">0</span>}; 
    <span style="color: #b854d4;">if</span> ((sock = socket(AF_INET, SOCK_STREAM, <span style="color: #b65611;">0</span>)) &lt; <span style="color: #b65611;">0</span>) 
    { 
        <span style="color: #b65611;">printf</span>(<span style="color: #60ac39;">"\n Socket creation error \n"</span>); 
        <span style="color: #b854d4;">return</span> -<span style="color: #b65611;">1</span>; 
    } 

    serv_addr.sin_family = AF_INET; 
    serv_addr.sin_port = htons(PORT); 

    <span style="color: #999580;">// Convert IPv4 and IPv6 addresses from text to binary form </span>
    <span style="color: #b854d4;">if</span>(inet_pton(AF_INET, <span style="color: #60ac39;">"127.0.0.1"</span>, &amp;serv_addr.sin_addr)&lt;=<span style="color: #b65611;">0</span>)  
    { 
        <span style="color: #b65611;">printf</span>(<span style="color: #60ac39;">"\nInvalid address/ Address not supported \n"</span>); 
        <span style="color: #b854d4;">return</span> -<span style="color: #b65611;">1</span>; 
    } 

    <span style="color: #b854d4;">if</span> (connect(sock, (<span style="color: #b854d4;">struct</span> sockaddr *)&amp;serv_addr, <span style="color: #b854d4;">sizeof</span>(serv_addr)) &lt; <span style="color: #b65611;">0</span>) 
    { 
        <span style="color: #b65611;">printf</span>(<span style="color: #60ac39;">"\nConnection Failed \n"</span>); 
        <span style="color: #b854d4;">return</span> -<span style="color: #b65611;">1</span>; 
    } 
    send(sock , hello , <span style="color: #b65611;">strlen</span>(hello) , <span style="color: #b65611;">0</span> ); 
    <span style="color: #b65611;">printf</span>(<span style="color: #60ac39;">"Hello message sent,www.linuxmi.com\n"</span>); 
    valread = read( sock , buffer, <span style="color: #b65611;">1024</span>); 
    <span style="color: #b65611;">printf</span>(<span style="color: #60ac39;">"%s\n"</span>,buffer ); 
    <span style="color: #b854d4;">return</span> <span style="color: #b65611;">0</span>; 
}

// 客户端C/C++程序演示套接字编程

#include <stdio.h>

#include <sys/socket.h>

#include <arpa/inet.h>

#include <unistd.h>

#include <string.h>

#define PORT 8090

int main(int argc, char const *argv[])

{

int sock = 0, valread;

struct sockaddr_in serv_addr;

char *hello = "Hello from client";

char buffer[1024] = {0};

if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0)

{

printf("\n Socket creation error \n");

return -1;

}

serv_addr.sin_family = AF_INET;

serv_addr.sin_port = htons(PORT);

// Convert IPv4 and IPv6 addresses from text to binary form

if(inet_pton(AF_INET, "127.0.0.1", &serv_addr.sin_addr)<=0)

{

printf("\nInvalid address/ Address not supported \n");

return -1;

}

if (connect(sock, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)

{

printf("\nConnection Failed \n");

return -1;

}

send(sock , hello , strlen(hello) , 0 );

printf("Hello message sent,www.linuxmi.com\n");

valread = read( sock , buffer, 1024);

printf("%s\n",buffer );

return 0;

}

编译:

linuxmi@linuxmi:~/www.linuxmi.com$ gcc client.c -o client
linuxmi@linuxmi:~/www.linuxmi.com$ gcc server.c -o server

输出：

linuxmi@linuxmi:~/www.linuxmi.com$ ./server
Hello from client
Hello message sent,www.linuxmi.com

linuxmi@linuxmi:~/www.linuxmi.com$ ./client
Hello message sent,www.linuxmi.com
Hello from server

server.c

client.c

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