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一、UDP服务器与客户端的区别
对于UDP服务器与客户端，两者都可以通过sendto和recvfrom函数收发数据，它们的主要区别是：
1.服务器一般是等待并响应来自客户端的请求，客户端则是主动发送请求并且等待服务器的响应。
2.服务器端要将地址和端口号绑定，如果不绑定就无法使用recvfrom函数接受数据（也就是说服务器需要调用bind函数将一个套接字与一个地址绑定，而客户端不需要）。所以对于UDP，其服务器与其说是服务端不如说是后发端。
所以如果阅读一份源码，要快速判断其是UDP服务器还是客户端，一个简单的方法是查看代码中是否调用了bind函数就可以了。
二、UDP是否可以使用select/poll/epoll
UDP是一种无连接的传输协议，因此通常情况下没有必要使用多路复用。UDP只是逐个接收数据段（数据包），并按照其规则进行处理，其并不关心单个数据包所属的任何特定数据流或数据包。但是UDP是可以使用select/poll/epoll的，某些开源软件，比如FFmpeg内部使用了这些多路复用来处理UDP。
三、使用poll函数编写UDP客户端、服务器程序
代码摘自：《IPV4 UDP server client program with Poll system call》
UDP客户端代码udpClient.c：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <signal.h>
#include <poll.h>
#define BUFFER_SIZE 1024
int client_socket = -1;
static void sigint_handler(int signo)
{
(void)close(client_socket);
sleep(2);
(void)printf(“Caught sigINT!\n”);
exit(EXIT_SUCCESS);
}
void validate_convert_port(
char *port_str,
struct sockaddr_in *sock_addr)
{
int port;
if (port_str == NULL) {
perror(“Invalid port_str\n”);
exit(EXIT_FAILURE);
}
if (sock_addr == NULL) {
perror(“Invalid sock_addr\n”);
exit(EXIT_FAILURE);
}
port = atoi(port_str);
if (port == 0) {
perror(“Invalid port\n”);
exit(EXIT_FAILURE);
}
sock_addr->sin_port = htons(
(uint16_t)port);
printf(“Port: %d\n”,
ntohs(sock_addr->sin_port));
}
void validate_convert_addr(
char *ip_str,
struct sockaddr_in *sock_addr)
{
if (ip_str == NULL) {
perror(“Invalid ip_str\n”);
exit(EXIT_FAILURE);
}
if (sock_addr == NULL) {
perror(“Invalid sock_addr\n”);
exit(EXIT_FAILURE);
}
printf(“IP Address: %s\n”, ip_str);
if (inet_pton(AF_INET, ip_str,
&(sock_addr->sin_addr)) <= 0) {
perror(“Invalid address\n”);
exit(EXIT_FAILURE);
}
}
void recv_data(char *buffer)
{
int ret, len;
len = recvfrom(client_socket,
buffer, BUFFER_SIZE, 0, NULL, NULL);
if (len > 0) {
buffer[len] = ‘\0’;
(void)printf(“Received: %s\n”,
buffer);
} else if (len == 0) {
printf(“Connection closed\n”);
exit(EXIT_FAILURE);
}
}
void register_signal_handler(
int signum,
void (*handler)(int))
{
if (signal(signum, handler) == SIG_ERR)
{
printf(“Cannot handle signal\n”);
exit(EXIT_FAILURE);
}
}
int main(int argc, char *argv[])
{
int ret, len;
struct sockaddr_in
server_addr;
char buffer[BUFFER_SIZE];
struct pollfd fds[1];
char *str = “HI”;
register_signal_handler(SIGINT,
sigint_handler);
if (argc != 3) {
printf("%s\n",
argv[0]);
exit(EXIT_FAILURE);
}
memset(&server_addr, 0,
sizeof(server_addr));
server_addr.sin_family = AF_INET;
validate_convert_port(argv[1],
&server_addr);
validate_convert_addr(argv[2],
&server_addr);
client_socket = socket(AF_INET,
SOCK_DGRAM,
IPPROTO_UDP);
if (client_socket < 0) {
perror(“socket”);
return -1;
}
while (1) {
ret = sendto(client_socket, str,
strlen(str), 0,
(struct sockaddr*)&server_addr,
sizeof(server_addr));
printf(“sendbuffer = %s\n”, str);
if (ret < 0) {perror("send error\n");(void)close(client_socket);break;
}fds[0].fd = client_socket;
fds[0].events = POLLIN;ret = poll(fds, 2, 1000);if (ret < 0) {perror("poll");(void)close(client_socket);break;
}if (fds[0].revents & POLLIN) {recv_data(buffer);
}
}
(void)close(client_socket);
return 0;
}
UDP服务器代码udpServer.c：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <signal.h>
#include <poll.h>
#define BUFFER_SIZE 1024
int server_socket = -1;
static void sigint_handler(int signo)
{
(void)close(server_socket);
sleep(2);
(void)printf(“Caught sigINT!\n”);
exit(EXIT_SUCCESS);
}
void register_signal_handler(
int signum,
void (*handler)(int))
{
if (signal(signum, handler) == SIG_ERR) {
printf(“Cannot handle signal\n”);
exit(EXIT_FAILURE);
}
}
void validate_convert_port(
char *port_str,
struct sockaddr_in *sock_addr)
{
int port;
if (port_str == NULL) {
perror(“Invalid port_str\n”);
exit(EXIT_FAILURE);
}
if (sock_addr == NULL) {
perror(“Invalid sock_addr\n”);
exit(EXIT_FAILURE);
}
port = atoi(port_str);
if (port == 0) {
perror(“Invalid port\n”);
exit(EXIT_FAILURE);
}
sock_addr->sin_port = htons(
(uint16_t)port);
printf(“Port: %d\n”,
ntohs(sock_addr->sin_port));
}
void recv_send(
char *buffer,
struct sockaddr_in *client_addr)
{
int len, ret;
socklen_t client_addr_len = sizeof(
client_addr);
len = recvfrom(server_socket,
buffer, BUFFER_SIZE, 0,
(struct sockaddr*)&client_addr,
&client_addr_len);
if (len > 0) {
buffer[len] = ‘\0’;
printf(“Received: %s\n”,
buffer);
memset(buffer, 0,
sizeof(buffer));
strncpy(buffer, "HELLO",
strlen("HELLO") + 1);
buffer[strlen(buffer) + 1] = '\0';ret = sendto(server_socket,
buffer,
strlen(buffer), 0,
(struct sockaddr*)&client_addr,
client_addr_len);if (ret < 0) {perror("sendto");exit(EXIT_FAILURE);}
} else if (len < 0) {perror("recvfrom");exit(EXIT_FAILURE);
}
printf("Sentbuffer = %s\n",
buffer);
}
int main(int argc, char *argv[])
{
int ret;
struct sockaddr_in
server_addr,
client_addr;
char buffer[BUFFER_SIZE];
struct pollfd fds[1];
register_signal_handler(SIGINT,
sigint_handler);
if (argc != 2) {
printf("%s ",
argv[0]);
exit(EXIT_FAILURE);
}
memset(&server_addr, 0,
sizeof(server_addr));
server_addr.sin_family = AF_INET;
server_addr.sin_addr.s_addr =
INADDR_ANY;
validate_convert_port(argv[1],
&server_addr);
server_socket = socket(AF_INET,
SOCK_DGRAM,
IPPROTO_UDP);
if (server_socket < 0) {
perror(“socket”);
return -1;
}
ret = bind(server_socket,
(struct sockaddr*)&server_addr,
sizeof(server_addr));
if (ret < 0) {
perror(“bind”);
(void)close(server_socket);
return -2;
}
printf(“UDP listining\n”);
memset(fds, 0, sizeof(fds));
fds[0].fd = server_socket;
fds[0].events = POLLIN;
while (1) {
ret = poll(fds, 1, 1000);if (ret < 0) {
perror("poll");break;
}if (fds[0].revents & POLLIN) {recv_send(buffer, &client_addr);
}
}
(void)close(server_socket);
return 0;
}
编译：
gcc udpClient.c -o udpClient -g
gcc udpServer.c -o udpServer -g
运行：
客户端执行命令：
./udpClient 1234 127.0.0.1
服务器执行命令：
./udpServer 1234
运行后两者即可通讯。
从上面的代码可以看出来，上述UDP客户端和服务器都使用了poll函数监视文件描述符是否可读。区别在于服务器调用了bind函数将套接字与地址绑定，而客户端没有调用bind函数而已。