014_UDP数据读写.md · leewooong/Linux网络编程

# UDP数据读写

* 头文件：

```C++
    #include <sys/types.h>
    #include <sys/socket.h>
    ```

## 1 recvfrom

顶替了accept

recvfrom读取sockfd上的数据，buf和len参数分别指定读缓冲区的位置和大小

UDP通信没有连接的概念，所以我们每次读取数据都需要获取发送端的socket地址，即参数src_addr所指的内容，addrlen参数则指定该地址的长度。flag传0

* 函数原型：`ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags, struct sockaddr *src_addr, socklen_t *addrlen);`
* 返回值：
  * These calls return the number of bytes received
  * or -1 if an error occurred. In the event of an error, errno is set to indicate the error.
  * When a stream socket peer has performed an orderly shutdown, the return value will be 0 (the traditional "end-of-file" return).
  * Datagram sockets in various domains (e.g., the UNIX and Internet domains) permit zero-length datagrams. When such a datagram is received, the return value is 0.
  * The value 0 may also be returned if the requested number of bytes to receive from a stream socket was 0.

## 2 sendto

顶替connect，调用sendto时自动分配IP和端口号

sendto往sockfd上写入数据，buf和len参数分别指定写缓冲区的位置和大小。dest_addr参数指定接收端的socket地址，addrlen参数则指定该地址的长度。

* 函数原型：`ssize_t sendto(int sockfd, const void *buf, size_t len, int flags, const struct sockaddr *dest_addr, socklen_t addrlen);`
* 返回值：
  * On success, these calls return the number of bytes sent.  
  * On error, -1 is returned, and errno is set appropriately.

值得一提的是，recvfrom/sendto系统调用也可以用于面向连接（STREAM）的socket的数据读写，只需要把最后两个参数都设置为NULL以忽略发送端/接收端的socket地址（因为我们已经和对方建立了连接，所以已经知道其socket地址了）

## 3 实现服务端与客户端

### 3.1 server

```C++
#include <stdio.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
#include <unistd.h>

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

if (argc != 2) {
        printf("Usage: %s <port>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

// 创建通信用的套接字，选择SOCK_DGRAM
    int server_socket = socket(PF_INET, SOCK_DGRAM, 0);

// 服务端地址结构
    struct sockaddr_in server_addr;
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family      = AF_INET;
    server_addr.sin_port        = htons(atoi(argv[1]));
    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);

// 将服务端地址结构与套接字绑定
    bind(server_socket, (struct sockaddr*)&server_addr, sizeof(server_addr));

printf("accepting connection...\n");

// 客户端地址结构，传出参数
    struct sockaddr_in client_addr;
    memset(&client_addr, 0, sizeof(client_addr));
    socklen_t client_addr_len;

char buf[BUFSIZ], ip_buf[INET_ADDRSTRLEN];
    while (1) {
        client_addr_len = sizeof(client_addr);

// 从客户端接收数据，使用server_socket通信，接收到的数据放到buf中，最大可以接受sizeof(buf)个字节
        // 返回值为实际接收到的字节数
        ssize_t bytes_size = recvfrom(server_socket, buf, sizeof(buf), 0, 
                                      (struct sockaddr*)&client_addr, &client_addr_len);
        
        // 打印实际接收到的字节数与客户端的信息
        printf("received %ld bytes from %s:%d\n", bytes_size,
                inet_ntop(AF_INET, &client_addr.sin_addr, ip_buf, INET_ADDRSTRLEN),
                ntohs(client_addr.sin_port));

for (int i = 0; i < bytes_size; ++i)
            buf[i] = toupper(buf[i]);

// 重新发送给客户端
        sendto(server_socket, buf, bytes_size, 0, 
               (struct sockaddr*)&client_addr, sizeof(client_addr));
    }

close(server_socket);

return 0;
}
```

### 3.2 client

```C++
#include <stdio.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
#include <unistd.h>

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

if (argc != 3) {
        printf("Usage: %s <ip> <port>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

// 创建通信用的套接字
    int client_socket = socket(PF_INET, SOCK_DGRAM, 0);

// 创建服务端地址结构，指定通信对象
    struct sockaddr_in server_addr;
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family      = AF_INET;
    server_addr.sin_port        = htons(atoi(argv[2]));
    inet_pton(AF_INET, argv[1], &server_addr.sin_addr);

char buf[BUFSIZ];

// 从标准输入读取数据存入buf中
    while (fgets(buf, sizeof(buf), stdin) != NULL) {

// 发送给服务端
        ssize_t bytes_size = sendto(client_socket, buf, strlen(buf), 0, 
                                    (struct sockaddr*)&server_addr, sizeof(server_addr));
        
        // 接收服务端传回的数据，由于我们不关心服务端的地址信息，所以最后两个参数传NULL, 0
        bytes_size = recvfrom(client_socket, buf, sizeof(buf), 0, NULL, 0);

// 打印到标准输出中
        write(STDOUT_FILENO, buf, bytes_size);
    }

close(client_socket);

return 0;
}
```

测试：

![udp](assets/022.png)

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