地址的转换：
主机字节序和网络字节序的转换函数，通常X86用的是大端格式，而网络是小端格式，所以在设定IP地址时要来一个转换
#include <netinet/in.h>
uint32_t htonl(uint32_t hostlong);
uint16_t htons(uint16_t hostshort);
uint32_t ntohl(uint32_t netlong);
uint16_t ntohs(uint16_t netshort);
#include <netinet/in.h>
#include <arpa/inet.h>
char *inet_ntoa(struct in_addr in);
in_addr_t inet_addr(const char *cp);
套接字的创建：
#include <sys/socket.h>
int socket(int domain, int type, int protocol);
成功返回0，失败返回-1
绑定端口：
#include <sys/socket.h>
int bind(int sockfd, const struct sockaddr *addr,socklen_t addrlen);
成功返回0，失败返回-1
数据收发：
ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,struct sockaddr *src_addr, socklen_t *addrlen);//一般会阻塞
ssize_t sendto(int sockfd, const void *buf, size_t len, int flags,const struct sockaddr *dest_addr, socklen_t addrlen);
UDP接收端编程流程：
调用socket创建套接字 =》调用bind绑定端口 =》循环调用recvfrom阻塞直到收到数据 =》调用close关闭套接字.
例：
int m_sock=socket(AF_INET,SOCK_DGRAM,0);
if( -1 == m_sock)
{
perror("create receiver socket failed!");
exit(1);
}
m_local_addr.sin_family = AF_INET;
m_local_addr.sin_port = htons(5566);
m_local_addr.sin_addr.s_addr = INADDR_ANY;
if( -1 == bind(m_sock,(struct sockaddr*)&m_local_addr,sizeof(struct sockaddr)))
{
perror("bind failed!");
exit(1);
}
struct sockaddr_in remoteaddr;
int len=sizeof(struct sockaddr_in);
char buf[100];
memset(buf,0,100);
receiver.recvfrom((void*)buf,100,(struct sockaddr*)&remoteaddr,(socklen_t*)&len);
close(m_sock);
UDP发送端编程流程：
调用socket创建套接字 =》循环调用sendto发送数据 =》调用close关闭套接字.
例：
int sockfd=socket(AF_INET,SOCK_DGRAM,0);
if(-1 == sockfd)
{
perror("create socket error!");
exit(1);
}
struct sockaddr_in remoteaddr;
remoteaddr.sin_family=AF_INET;
remoteaddr.sin_port=htons(5566);
remoteaddr.sin_addr.s_addr=inet_addr("127.0.0.1");
char buf[]="helloworld!";
sendto(sockfd,buf,strlen(buf)+1,0,(struct sockaddr*)&remoteaddr,sizeof(struct sockaddr));
close(sockfd);