Transcript Socket Programming - Tatung University

Socket Programming
References:
redKlyde’s tutorial set
Winsock2 for games (gamedev.net)
Introduction (javadoc)
TCP provides a reliable, point-to-point communication
channel that client-server applications on the Internet
use to communicate with each other.
To communicate over TCP, a client program and a
server program establish a connection to one another.
Each program binds a socket to its end of the
connection. To communicate, the client and the
server each reads from and writes to the socket
bound to the connection.
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Client/Server
Normally, a server runs on a specific computer and has a socket
that is bound to a specific port number.
The server just waits, listening to the socket for a client to make
a connection request.
The client knows the hostname of the machine on which the
server is running and the port number to which the server is
connected.
To make a connection request, the client tries to rendezvous
with the server on the server's machine and port.
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Client/Server (cont)
If everything goes well, the server accepts the connection.
On the client side, if the connection is accepted, a socket is
successfully created and the client can use the socket to
communicate with the server. The client and server can now
communicate by writing to or reading from their sockets.
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Socket Definition
A socket is one endpoint of a two-way
communication link between two
programs running on the network. A
socket is bound to a port number so
that the TCP layer can identify the
application that data is destined to be
sent.
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Winsock 2.2
There are different ways to program with
WinSock.
the very basic UNIX/Berkeley type functions,
the Microsoft’s Windows specialized version of
the basic functions, or
the Object Orientated MFC version.
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Learning Objectives
Creating a socket (and VC setup)
Making the connection
Sending and receiving data
Multiple host connection

[Optional multi-thread implementation]
Integrating with game loop
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redKlyde (Tutorial #1)
VC setting: add WS2_32.lib
to project
WSAData: hold info about
our network capability
WSAStartup (…): start up
winsock
mysock = socket (AF_INET,
SOCK_STREAM,0);
 creates a TCP socket for
Internet; fails if too many
sockets are created
closesocket (mysock);
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Every winsock function will
return SOCKET_ERROR if
error occurred
WSAGetLastError() to get
the errorcode [see msdn for
details]
WSACleanup(): last call of
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Tutorial 1
Winsock v2.2
AF_INET: Socket for internet
SOCK_STREAM: TCP socket
[SOCK_DGRAM: UDP socket]
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redKlyde (Toot #2 – Server)
Host/net byte ordering
of numbers (next page)
Bind: associate a port to
socket to send/receive
data
Socket address
structure [sockaddr_in]:
port no., Internet
protocol, …
Port no: stay above
2024
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Listen (listensock,
backlog)
Clientsocket = accept
(listensock, 0,0)

A blocking call [use
debugger to verify]
Backlog: the number of
clients you can keep
waiting before calling
accept()
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[Endianness]
Big endian
Network protocol
hton, ntoh

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Little endian
Windows
Host-network
conversion
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hton & ntoh
Sizeof (short) = 4 bytes
Sizeof (long) = 8 bytes
htons (7654) = 58909
htonl (7654) = 3860660224
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In fact:
If the sending and receiving ends are of the same
endianness, there is no need to convert
1D
E6
1D
E6
7654
However, if not so, only through ntoh/hton can
guarantee the correctness of the info…
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Bind the socket to port 7654
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redKlyde (Toot#2 – client)
Which server to
connect to?
“host entry”
structure:
gethostbyname
(servername)
Socket address
[sockaddr_in]: port,
host_addr
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Connect (sock,
sockaddr_in, size)
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Tut 2(c)
Or IP
“140.129.25.194”
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redKlyde (Toot#3)
Send/recv fixed length message
Macros for earlier commands
Nbytes = recv (clientsock, buffer,buffersize,0)
Nbytes = send (clientsock,buffer,size,0);
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Tut3 (server)
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Tut3 (client)
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redKlyde (Toot#4)
How about variable
length message?
TCP concatenates all
packets into one big
chunk
Use escape character to
decipher…
Send the size of
message first, then recv
accordingly
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!? Why the size won’t
be mixed with the
coming messages?!
This is the simple
protocol we set for this
example.
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Tut4 (server)
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Tut4 (client)
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Tut5: Overview
Thread: a sequence of instructions that may
execute in parallel with other threads
Multi-threaded AP


Accept thread: accept clients connecting to the
server socket; add client socket to masterset
Main thread: polling from selected sets to process
variable-length input
Shared memory: FD_SET (set of client
sockets)
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redKlyde (Toot#5 – server)
Blocking calls: accept,
recv
Nonblocking calls: send,
connect, …
FD_SET (file descriptor
set): a list of sockets

Data member: fd_count,
fd_array
Macros for FD_SET:




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FD_SET (sock, &set)
FD_ZERO (&set)
FD_CLR (sock,&set)
FD_ISSET(sock,&set)
Select: create a list of
sockets that already
have data to read
Exception handling:



Clean disconnect: the
client closes the socket
and exits [no error]
dropped connection [an
error will occur]
Remove from FD_SET,
close the corresponding
socket
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Setting Up
Code-generation:
debug multi-threaded
Mutex: mutual exclusion algorithms used
in concurrent programming to avoid the
simultaneous use of a common resource
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Accepting
thread
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Main
Thread1
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Main
Thread2
Error check…
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Error (dropped connection)
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Error (clean disconnect)
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Client called
closesocket
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Command Summary
WSAStartup; WSACleanup
WSAGetLastError();
socket(); closesocket();
bind(); listen(); accept(); gethostbyname(),
connect()
ntohl(), ntohs(), htonl(), htons()
send(), recv()
select()
FD_SET, FD_ZERO, FD_CLR, FD_ISSET
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Networking for
Game Programmers
Reference: Glenn Fieldler’s Blog
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Topics
TCP vs. UDP
Sending & Receiving Packets (UDP)
UDP (Virtual) Connection
UDP flow control
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