UDP Socket Coding Back
- UDP
- does not ensure for the arrival of datagrams
- solved by overtime or Resend
- solved by Serial Number of datagrams
- does not ensure for the order of datagrams
- does not have Flow Control
- Proerties of connected UDP
- clients should call
connect(), but there is not the process of three-way handshaking. connect() is used to mark down IP and
port
- do not need to set the address of the server when sending datagram
- can only receive datagrams from the specifical server.
- Server of UDP:
- do not accept, but bind to the port
- loop for handling datagrams from different clients.
- Client of UDP:
- do not connect, but choose a port when firstly calls
sendto()
- can receive any datagram from any host
- will probably be blocked forever
- Model of UDP Socket Coding
1. recvfrom()
- recvfrom get one datagram from buffer each time, and will be blocked when there is no datagram
- empty datagram does not mean that the opposite end has stopped to send
method
int recvfrom(int sockfd, void* buf, int len, unsigned char flags, struct socketaddr* from, socklen_t* addrlen)
parameters
- 前四個參數與
recv()相同
- from: 發送者socket地址, NULL表示不需要
- addrlen: socket地址長度, from為NULL時必須設置為NULL
return value
2. sendto()
- the socket address of receiver must be clear when calling
sendto()
- there is no sending buffer for UDP
- allow to send empty datagram, and return 0 after calling
sendto()
method
int sendto(int sockfd, const void* buf, int len, unsigned char flags, struct socketaddr* to, int tolen)
parameters
- 前四個參數與
send()相同
- to: 接收者socket地址
- tolen: socket地址長度
return value