UDP 协议是如此的简单，我们可以想象一下，下面是 UDP 包头，前面再加一个 IP 的包头，后面接着 UDP 的报文内容，这样一个简单 的UDP/IP 报文就形成了。
源自 https://en.wikipedia.org/wiki/User_Datagram_Protocol
UDP 是一个无连接状态的协议，每一个 UDP 报文都独立发出。理解了这点，我们再来看 UDP socket API 接口，就容易理解了。
UDP socket 编程有几乎固定的格式和步骤，我们先归纳一下。编程的细节我们可以看之后的例子。
服务器端：
1 声明一个 struct sockaddr_in 型变量，用于存储监听地址和端口。
2 声明一个 int 型变量，用于存储 UDP socket。
3 使用 bind() API，把地址和 socket 绑定在一起。
4 使用 recvfrom() 或 recvmsg() 接收 UDP 报文，得到客户端 IP 地址和端口。
5 使用 sendto() 或 sendmsg() 给客户端发送回应。
客户端：
1 声明一个 struct sockaddr_in 型变量，用于存储服务器地址和端口。
2 声明一个 int 型变量，存储 UDP socket。
3 使用 sendto() 或 sendmsg() 给服务器端发送 UDP 报文。
4 使用 recvfrom() 或 recvmsg() 接收服务器端回应。
客户端比服务器端少了一个 bind() 的步骤，这个也容易理解，客户端的发送网口和端口由操作系统自动分配，不需要程序员指定。
还有一点需要说明，虽然 sendto() 和 recvfrom() 的格式比较接近，而且 sendmsg() 和 recvmsg() 的格式比较接近，但是它们不是必须成对出现的。例如使用 sendto() 发送，并使用 recvmsg() 接收，是完全可以的。两者之间的区别是，sendmsg() 可以把要发送的内容存放在多块，相互不连续的内存中；而 sendto() 需要一块连续的内存。
例子 1
客户端接收用户输入的一串字符，并发送给服务器端。
服务器端接收后，颠倒整串字符顺序，并将其发回客户端。
这个例子中使用 sendto / recvfrom 接口。
server.c :
/*
* Simple udp server
*/
#include <stdio.h> //printf,perror
#include <string.h> //memset,strlen
#include <stdlib.h> //exit
#include <unistd.h> //close
#include <arpa/inet.h>
#include <sys/socket.h>
#define BUFLEN 512 //Max length of buffer
#define PORT 8888 //The port on which to listen for incoming data
void die(char *s)
{
perror(s);
exit(1);
}
char * reverse_string(char * s)
{
if (s != NULL)
{
char temp;
for (int i=0, len=strlen(s), k=len-1; i < (len/2); i++,k--)
{
temp = s[i];
s[i] = s[k];
s[k] = temp;
}
}
return s;
}
int main(void)
{
struct sockaddr_in si_me, si_other;
int s, i, slen=sizeof(si_me), recv_len;
char buf[BUFLEN];
//build my internet address
memset((char *)&si_me, 0, slen);
si_me.sin_family = AF_INET;
si_me.sin_port = htons(PORT);
si_me.sin_addr.s_addr = htonl(INADDR_ANY);
//create a UDP socket
if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
{
die("socket");
}
//bind socket to port
if (bind(s, (struct sockaddr*)&si_me, slen) == -1)
{
die("bind");
}
//keep listening for data
while(1)
{
memset(buf, 0, BUFLEN);
//try to receive some data, this is a blocking call
if ((recv_len=recvfrom(s, buf, BUFLEN, 0, (struct sockaddr *)&si_other, &slen)) == -1)
{
die("recvfrom()");
}
//print details of the client/peer and the data received
printf("Received packet from %s:%d\n", inet_ntoa(si_other.sin_addr), ntohs(si_other.sin_port));
printf("Data: %s\n" , buf);
//now reply the client with the reversed data
if (sendto(s, reverse_string(buf), recv_len, 0, (struct sockaddr*)&si_other, slen) == -1)
{
die("sendto()");
}
}
close(s);
return 0;
}
client.c :
/*
* Simple udp client
*/
#include <stdio.h> //printf,fgets
#include <string.h> //memset,strlen
#include <stdlib.h> //exit
#include <unistd.h> //close
#include <arpa/inet.h>
#include <sys/socket.h>
#define SERVER "127.0.0.1"
#define BUFLEN 512 //Max length of buffer
#define PORT 8888 //The port on which to send data
void die(char *s)
{
perror(s);
exit(1);
}
int main(void)
{
struct sockaddr_in si_other;
int s, i, slen=sizeof(si_other), len;
char buf[BUFLEN];
char message[BUFLEN];
//build server internet address
memset((char *) &si_other, 0, sizeof(si_other));
si_other.sin_family = AF_INET;
si_other.sin_port = htons(PORT);
if (inet_aton(SERVER, &si_other.sin_addr) == 0)
{
die("inet_aton");
}
//create a UDP socket
if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
{
die("socket");
}
while(1)
{
printf("Enter message : ");
if (fgets(message, BUFLEN-1, stdin) == NULL)
{
die("gets");
}
len = strlen(message);
if (len > 0 && message[len-1] == '\n')
{
message[len-1] = '\0';
}
//send the message
if (sendto(s, message, strlen(message), 0, (struct sockaddr *)&si_other, slen) == -1)
{
die("sendto()");
}
//receive a reply and print it
//clear the buffer by filling null, it might have previously received data
memset(buf,'\0', BUFLEN);
//try to receive some data, this is a blocking call
if (recvfrom(s, buf, BUFLEN, 0, (struct sockaddr *)&si_other, &slen) == -1)
{
die("recvfrom()");
}
printf("Receive: %s\n", buf);
}
close(s);
return 0;
}
# gcc server.c -o server && ./server
Received packet from 127.0.0.1:56190
Data: abcdef
# gcc client.c -o client && ./client
Enter message : abcdef
Receive: fedcba
例子 2
要实现的功能，和编译运行方法，与例子１完全一样；但是使用了 sendmsg() 和 recvmsg() 接口。
server.c
/*
* Simple udp server
*/
#include <stdio.h> //printf, perror
#include <string.h> //memset, strlen
#include <stdlib.h> //exit
#include <unistd.h> //close
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/uio.h> //recvmsg
#define BUFLEN 512 //Max length of buffer
#define PORT 8888 //THe port on which to listen for incoming data
void die(char * s)
{
perror(s);
exit(1);
}
char * reverse_string(char * s)
{
if (s != NULL)
{
char temp;
for (int i=0, len=strlen(s), k=len-1; i < (len/2); i++,k--)
{
temp = s[i];
s[i] = s[k];
s[k] = temp;
}
}
return s;
}
int main(void)
{
struct sockaddr_in si_me, si_other;
int s, i, slen=sizeof(si_me), recv_len;
char buf[BUFLEN];
struct msghdr msgrecv;
struct iovec iovrecv[1];
//build my internet address
memset((char *)&si_me, 0, slen);
si_me.sin_family = AF_INET;
si_me.sin_port = htons(PORT);
si_me.sin_addr.s_addr = htonl(INADDR_ANY);
//create a UDP socket
if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
{
die("socket");
}
//bind socket to port
if (bind(s, (struct sockaddr*)&si_me, slen) == -1)
{
die("bind");
}
memset(&msgrecv, 0, sizeof(msgrecv));
memset(iovrecv, 0, sizeof(iovrecv));
msgrecv.msg_name = &si_other;
msgrecv.msg_namelen = sizeof(si_other);;
msgrecv.msg_iov = iovrecv;
msgrecv.msg_iovlen = 1;
iovrecv[0].iov_base = buf;
iovrecv[0].iov_len = sizeof(buf);
while(1)
{
memset(buf, 0, BUFLEN);
//try to receive some data, this is a blocking call
if ((recv_len=recvmsg(s, &msgrecv, 0)) < 0)
{
die("recvmsg");
}
//print details of the client/peer and the data received
printf("Received packet from %s:%d\n", inet_ntoa(si_other.sin_addr), ntohs(si_other.sin_port));
printf("Data: %s\n" , buf);
//now reply the client with the reversed data
reverse_string(buf);
if (sendmsg(s, &msgrecv, 0) == -1)
{
die("sendmsg()");
}
}
close(s);
return 0;
}
client.c
/*
* Simple udp client
*/
#include <stdio.h> //printf, perror
#include <string.h> //memset, strlen
#include <stdlib.h> //exit
#include <unistd.h> //close
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/uio.h> //recvmsg
#define SERVER "127.0.0.1"
#define BUFLEN 512 //Max length of buffer
#define PORT 8888 //THe port on which to listen for incoming data
void die(char *s)
{
perror(s);
exit(1);
}
int main(void)
{
struct sockaddr_in si_other;
int s, i, slen=sizeof(si_other), len;
char buf[BUFLEN];
struct msghdr msgsend;
struct iovec iovsend[1];
//build server internet address
memset((char *) &si_other, 0, sizeof(si_other));
si_other.sin_family = AF_INET;
si_other.sin_port = htons(PORT);
if (inet_aton(SERVER, &si_other.sin_addr) == 0)
{
die("inet_aton");
}
//create a UDP socket
if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
{
die("socket");
}
memset(&msgsend, 0, sizeof(msgsend));
memset(iovsend, 0, sizeof(iovsend));
msgsend.msg_name = &si_other;
msgsend.msg_namelen = sizeof(si_other);;
msgsend.msg_iov = iovsend;
msgsend.msg_iovlen = 1;
iovsend[0].iov_base = buf;
iovsend[0].iov_len = 0;
while(1)
{
printf("Enter message : ");
memset (buf, 0, BUFLEN);
if (fgets(buf, BUFLEN-1, stdin) == NULL)
{
die("gets");
}
len = strlen(buf);
if (len > 0 && buf[len-1] == '\n')
{
buf[len-1] = '\0';
iovsend[0].iov_len = len-1;
}
else {
iovsend[0].iov_len = len;
}
//send the message
if (sendmsg(s, &msgsend, 0) == -1)
{
die("sendmsg()");
}
//receive a reply and print it
//clear the buffer by filling null, it might have previously received data
memset(buf,'\0', BUFLEN);
//try to receive some data, this is a blocking call
if (recvmsg(s, &msgsend, 0) == -1)
{
die("recvmsg()");
}
printf("Receive: %s\n", buf);
}
close(s);
return 0;
}
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