嵌入式Linux网络编程，UDP循环服务器，sendto()，recvfrom()

1，UDP循环服务器模型：

socket(...);
bind(...);
while(1){
   recvfrom(...);
   process(...);
   sendto(...);
}

2，UDP的使用场景

1. 实时的音视频传输
2. DNS的域名解析

3，UDP数据发送和接受sendto()、recvfrom()

ssize_t sendto(int socket, void *message, size_t length, int flags, struct sockaddr *dest_addr, socklen_t dest_len);

ssize_t recvfrom(int socket, void *buffer, size_t length, int flags, struct sockaddr *address, socklen_t *address_len);

这两个函数一般在使用UDP协议时使用

4，UDP循环服务器示例（可同时连接多个客户端）

4.1，头文件 net.h

#ifndef __NET_H__
#define __NET_H__

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <strings.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>

//#define SERV_IP_ADDR "192.168.31.100"
#define SERV_PORT 5005
//#define BACKLOG 5
#define QUIT_STR "quite"

#endif

4.2，客户端代码client.c

/* UDP demo */
/*./client serv_ip serv_port */
#include "net.h"

void usage(char *s)
{
	printf("this is a UDP demo!\n");
	printf("Usage:\n\t%s <serv_ip> <serv_port>\n",s);
	printf("\tserv_ip: UDP server ip address\n");
	printf("\tserv_port: UDP server port(>5000)\n ");
}
int main(int argc, const char *argv[])
{
	int fd;
	short port;
	struct sockaddr_in sin;
	if(argc != 3)
	{
		usage((char *)argv[0]);
		exit(1);
	}
	if((port = atoi(argv[2])) < 5000)
	{
		usage((char *)argv[0]);
		exit(1);
	}
	/* 1 创建socket fd */
	if((fd = socket(AF_INET,SOCK_DGRAM,0)) < 0)//UDP编程
	{
		perror("socket");
		exit(-1);
	}
	
	/* 2 连接服务器 */
	/* 2.1 填充struct sockaddr_in结构体变量*/
	bzero(&sin,sizeof(sin));
	sin.sin_family = AF_INET;
	sin.sin_port = htons(port);//转为网络字节序端口号
#if 0
#else
	if(inet_pton(AF_INET,argv[1],(void *)&sin.sin_addr.s_addr) < 0)
	{
		perror("inet_pton");
		goto _error1;
	}
#endif
	printf("UDP client starting ...... OK!\n");
	/* 3 读写*/
	char buf[BUFSIZ];
	while(1)
	{
		fprintf(stderr,"Please input the string to server:");
		bzero(buf,BUFSIZ);
		if(fgets(buf,BUFSIZ-1,stdin) == NULL)
		{
			perror("fgets");
			continue;
		}
		sendto(fd,buf,strlen(buf),0,(struct sockaddr *)&sin,sizeof(sin));
		if(strncasecmp(buf,QUIT_STR,strlen(QUIT_STR)) == 0)
		{
			printf("client is existed!\n");
			break;
		}
	}

_error1:
	close(fd);
	return 0;
}

4.3，服务器端代码server.c

#include "net.h"

int main(int argc, const char *argv[])
{
	int fd;		
	struct sockaddr_in sin;

	/* 1 创建socket fd */
	if((fd = socket(AF_INET,SOCK_DGRAM,0)) < 0)//UDP编程，套接字类型为SOCK_DGRAM
	{
		perror("socket");
		exit(-1);
	}

	/* 2 允许绑定地址快速重用 */ 
	int b_reuse = 1;
	setsockopt(fd,SOL_SOCKET,SO_REUSEADDR,&b_reuse,sizeof(int)); 
	
	/* 3 绑定 */
	/* 3.1 填充struct sockaddr_in 结构体变量*/
	bzero(&sin,sizeof(sin));
	sin.sin_family = AF_INET;
	sin.sin_port = htons(SERV_PORT);
#if 1
	/* 让服务器可以绑定在任意的IP上*/
	sin.sin_addr.s_addr = htonl(INADDR_ANY);
#else
	if(inet_pton(AF_INET,SERV_IP_ADDR,(void *)&sin.sin_addr.s_addr) < 0)
	{
		perror("inet_pton");
		goto _error1;
	}
#endif
	/* 3.2 绑定*/
	if(bind(fd,(struct sockaddr *)&sin,sizeof(sin)))
	{
		perror("bind");
		goto _error1;
	}
	printf("UDP server starting .... OK！\n");
	
	char buf[BUFSIZ];
	struct sockaddr_in cin;
	socklen_t addrlen = sizeof(cin);
	while(1)
	{
		/* 让系统自动获取客户端的信息*/
		bzero(buf,BUFSIZ);
		if(recvfrom(fd,buf,BUFSIZ-1,0,(struct sockaddr *)&cin,&addrlen) < 0)
		{
			perror("recvfrom");
			continue ;
		}
		char cin_ipv4_addr[16];
		if(inet_ntop(AF_INET,&cin.sin_addr.s_addr,cin_ipv4_addr,sizeof(cin_ipv4_addr)) < 0)
		{
			perror("inet_ntop");
			goto _error1;
		}

		printf("Receive from (%s:%d),Data: %s",cin_ipv4_addr,ntohs(cin.sin_port),buf);
		
		if(strncasecmp(buf,QUIT_STR,strlen(QUIT_STR)) == 0)
		{
			printf("client (%s:%d) is existing!\n",cin_ipv4_addr,ntohs(cin.sin_port));
		}
	}
_error1:
	close(fd);
	return 0;
}