用lwip库与c语言，举一个udp通信的例子，用代码说明

好的，下面是一个基于lwip库和C语言的UDP通信示例代码。

假设有两个设备A和B，它们之间通过UDP协议进行通信。设备A需要向设备B发送一条消息，设备B接收到消息后回复一个确认消息。具体的实现步骤如下：

1. 在设备A上创建UDP套接字，并设置发送缓冲区大小和接收缓冲区大小：

#include "lwip/sockets.h"

#define BUF_SIZE 1024

int sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
int send_buf_size = BUF_SIZE;
int recv_buf_size = BUF_SIZE;
setsockopt(sock, SOL_SOCKET, SO_SNDBUF, (const char*)&send_buf_size, sizeof(int));
setsockopt(sock, SOL_SOCKET, SO_RCVBUF, (const char*)&recv_buf_size, sizeof(int));

2. 设置设备B的IP地址和端口号：

struct sockaddr_in addr;
addr.sin_family = AF_INET;
addr.sin_port = htons(1234); // 设备B的端口号为1234
addr.sin_addr.s_addr = inet_addr("192.168.1.2"); // 设备B的IP地址为192.168.1.2

3. 向设备B发送消息：

char buf[BUF_SIZE] = "Hello, B! This is A.";
sendto(sock, buf, strlen(buf), 0, (struct sockaddr*)&addr, sizeof(addr));

4. 在设备B上创建UDP套接字，并设置发送缓冲区大小和接收缓冲区大小：

#include "lwip/sockets.h"

#define BUF_SIZE 1024

int sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
int send_buf_size = BUF_SIZE;
int recv_buf_size = BUF_SIZE;
setsockopt(sock, SOL_SOCKET, SO_SNDBUF, (const char*)&send_buf_size, sizeof(int));
setsockopt(sock, SOL_SOCKET, SO_RCVBUF, (const char*)&recv_buf_size, sizeof(int));

5. 绑定设备B的IP地址和端口号：

struct sockaddr_in addr;
addr.sin_family = AF_INET;
addr.sin_port = htons(1234); // 设备B监听的端口号为1234
addr.sin_addr.s_addr = htonl(INADDR_ANY); // 任意IP地址
bind(sock, (struct sockaddr*)&addr, sizeof(addr));

6. 接收设备A发送的消息，并回复确认消息：

char buf[BUF_SIZE];
struct sockaddr_in from_addr;
socklen_t from_len = sizeof(from_addr);
int len = recvfrom(sock, buf, BUF_SIZE, 0, (struct sockaddr*)&from_addr, &from_len);
if (len > 0) {
    printf("Received message from %s:%d: %s\n", inet_ntoa(from_addr.sin_addr), ntohs(from_addr.sin_port), buf);
    char reply[BUF_SIZE] = "Received.";
    sendto(sock, reply, strlen(reply), 0, (struct sockaddr*)&from_addr, from_len);
}

7. 关闭UDP套接字：

close(sock);

完整的代码如下：

设备A的代码：

#include <stdio.h>
#include <string.h>
#include "lwip/sockets.h"

#define BUF_SIZE 1024

int main() {
    // 创建UDP套接字
    int sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    int send_buf_size = BUF_SIZE;
    int recv_buf_size = BUF_SIZE;
    setsockopt(sock, SOL_SOCKET, SO_SNDBUF, (const char*)&send_buf_size, sizeof(int));
    setsockopt(sock, SOL_SOCKET, SO_RCVBUF, (const char*)&recv_buf_size, sizeof(int));

    // 设置设备B的IP地址和端口号
    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons(1234); // 设备B的端口号为1234
    addr.sin_addr.s_addr = inet_addr("192.168.1.2"); // 设备B的IP地址为192.168.1.2

    // 向设备B发送消息
    char buf[BUF_SIZE] = "Hello, B! This is A.";
    sendto(sock, buf, strlen(buf), 0, (struct sockaddr*)&addr, sizeof(addr));

    // 关闭UDP套接字
    close(sock);
    return 0;
}

设备B的代码：

#include <stdio.h>
#include <string.h>
#include "lwip/sockets.h"

#define BUF_SIZE 1024

int main() {
    // 创建UDP套接字
    int sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    int send_buf_size = BUF_SIZE;
    int recv_buf_size = BUF_SIZE;
    setsockopt(sock, SOL_SOCKET, SO_SNDBUF, (const char*)&send_buf_size, sizeof(int));
    setsockopt(sock, SOL_SOCKET, SO_RCVBUF, (const char*)&recv_buf_size, sizeof(int));

    // 绑定设备B的IP地址和端口号
    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons(1234); // 设备B监听的端口号为1234
    addr.sin_addr.s_addr = htonl(INADDR_ANY); // 任意IP地址
    bind(sock, (struct sockaddr*)&addr, sizeof(addr));

    // 接收设备A发送的消息，并回复确认消息
    char buf[BUF_SIZE];
    struct sockaddr_in from_addr;
    socklen_t from_len = sizeof(from_addr);
    int len = recvfrom(sock, buf, BUF_SIZE, 0, (struct sockaddr*)&from_addr, &from_len);
    if (len > 0) {
        printf("Received message from %s:%d: %s\n", inet_ntoa(from_addr.sin_addr), ntohs(from_addr.sin_port), buf);
        char reply[BUF_SIZE] = "Received.";
        sendto(sock, reply, strlen(reply), 0, (struct sockaddr*)&from_addr, from_len);
    }

    // 关闭UDP套接字
    close(sock);
    return 0;
}

上述代码中，我们使用了lwip库提供的socket和setsockopt函数来创建和设置UDP套接字，并使用sendto和recvfrom函数来发送和接收数据报。需要注意的是，具体的IP地址和端口号需要根据实际情况进行设置。