Win32 Sockets
Jim Fawcett
CSE 687 – Object Oriented Design
Spring 2015
References
Socket Routines, MSDN help
Network Programming for Microsoft Windows, Jones
& Ohlund, Microsoft Press, 1999 (a later edition is in
print)
C# Network Programming, Richard Blum, Sybex,
2003
http://tangentsoft.net/wskfaq
What are Sockets?
Sockets provide a common interface to the
various protocols supported by networks.
They allow you to establish connections
between machines to send and receive
data.
Sockets support the simultaneous
connection of multiple clients to a single
server machine.
Network Protocols
Socket applications can adopt communication styles supported
by a specific underlying protocol, e.g.:
Protocol Name Message Type Connection Type Reliable Packet Ordered
IP MSAFD TCP stream connection yes yes
MSAFD UDP message connectionless no no
RSVP TCP stream connection yes yes
RSVP UDP message connectionless no no
NetBios Sequential Packets message connection yes yes
Datagrams message connectionless no no
We will focus on sockets using TCP/IP, that is, reliable, packet
ordered, connection-oriented communication with streams.
TCP Protocol
TCP/IP stands for "Transmission Control Protocol / Internet Protocol.
TCP/IP is the most important of several protocols used on the internet. Some
others are: HyperText Transport Protocol (HTTP), File Transfer Protocol (FTP),
Simple Mail Transfer Protocol (SMTP), and Telnet, a protocol for logging into a
remote computer. Sockets provide a standard interface for a variety of network
protocols. TCP/IP is, by far, the most commonly used protocol for sockets. Here
are the main features of TCP/IP:
IP is a routable protocol.
That means that TCP/IP messages can be passed between networks in a
Wide Area Network (WAN) cluster.
Each device using TCP/IP must have an IP address.
This address is a 32 bit word, organized into four 8-bit fields, called octets.
Part of the IP address identifies the network and the rest identifies a specific
host on the network.
IP addresses are organized into three classes.
Each class has a different allocation of octets to these two identifiers. This
allows the internet to define many networks, each containing up to 256
devices (mostly computers), and a few networks, each containing many
more devices.
A single machine can run mulitple communictaions sessions using
TCP/IP.
That is, you can run a web browser while using Telnet and FTP,
simultaneously.
TCP/IP based Sockets
Connection-oriented means that two communicating
machines must first connect.
All data sent will be received in the same order as sent.
Note that IP packets may arrive in a different order than that
sent.
This occurs because all packets in a communication do not
necessarily travel the same route between sender and receiver.
Streams mean that, as far as sockets are concerned, the
only recognized structure is bytes of data.
Socket Logical Structure
Socket
recv buffer bytes
recv buffer
bytes recv buffer
Socket
Creating Sockets
Socket connections are based on:
Domains – network connection or IPC pipe
AF_INET for IPv4 protocol
AF_INET6 for IPv6 protocol
Type – stream, datagram, raw IP packets, …
SOCK_STREAM TCP packets
SOCK_DGRAM UDP packets
Protocol – TCP, UDP, …
0 default, e.g., TCP for SOCK_STREAM
Example:
HANDLE sock = socket(AF_INET,SOCK_STREAM,0);
Connecting Sockets
Socket addresses
struct SOCKADDR_IN {
sin_family // AF_INET
sin_address.s_addr // inet_addr(“127.0.0.1”);
sin_port // htons(8000);
} addr;
Bind server listener to port:
int err = bind(sock, (SOCKADDR_IN*)&addr,sizeof(addr));
Connect client to server:
HANDLE connect(sock, (SOCKADDR_IN*)&addr,sizeof(addr))
Client / Server Processing
Server Client
socket() socket()
bind()
listen()
accept() connect()
recv() send()
send() recv()
close() close()
Accessing Sockets Library
#include <winsock2.h>
Link with wsock32.lib
To build a server for multiple clients you will need to use threads,
e.g.:
#include <process.h>
and use the Project Settings:
C/C++ language\category=code generation\debug multithreaded
Project Setting #1
Project Setting #2
Sockets API
WSAStartup - loads WS2_32.dll
WSACleanup - unloads dll
socket - create socket object
connect - connect client to server
bind - bind server socket to address/port
listen - request server to listen for connection requests
accept - server accepts a client connection
send - send data to remote socket
recv - collect data from remote socket
Shutdown - close connection
closesocket - closes socket handle
Sequence of Server Calls
WSAStartup
socket (create listener socket)
bind
listen
accept
create new socket so listener can continue listening
create new thread for socket
send and recv
closesocket (on new socket)
terminate thread
shutdown
closesocket (on listener socket)
WSACleanup
WSAStartup
wVersionRequested = MAKEWORD(1,1);
WSAData wData;
lpWSAData = &wData
int WSAStartup(
WORD wVersionRequested,
LPWSADATA lpWSAData
)
Loads WS2_32.dll
TCP/IP socket
af = AF_INET
type = SOCK_STREAM
protocol = IPPROTO_IP
SOCKET socket(int af, int type, int protocol)
Creates a socket object and returns handle to socket.
Bind socket
Struct sockaddr_in local;
… define fields of local …
name = (sockaddr*)&local
namelen = sizeof(local)
int bind(
SOCKET s,
const struct sockaddr *name,
int namelen
)
Bind listener socket to network card and port
Listen for incoming requests
int listen(SOCKET s, int backlog)
backlog is the number of incoming connections queued (pending)
for acceptance
Puts socket in listening mode, waiting for requests for service
from remote clients.
Accept Incoming Connection
SOCKET accept(
SOCKET s,
struct sockaddr *addr,
int *addrLen
)
Accepts a pending request for service and returns a socket
bound to a new port for communication with new client.
Usually server will spawn a new thread to manage the socket
returned by accept.
Client/Server Configuration
Client
Client
Socket Server Main Thread
Socket Receiver Thread
data Server
port Socket
use socket
data
Thread
Create
listener listener
port socket
recv
int recv(
SOCKET s,
char *buff,
int len,
int flags
)
Receive data in buff up to len bytes.
Returns actual number of bytes read.
flags variable should normally be zero.
send
int send(
SOCKET s,
char *buff,
int len,
int flags
)
Send data in buff up to len bytes.
Returns actual number of bytes sent.
flags variable should normally be zero.
shutdown
int shutdown(SOCKET s, int how)
how = SD_SEND or SD_RECEIVE or SD_BOTH
Disables new sends, receives, or both, respectively. Sends a FIN to
server causing thread for this client to terminate (server will continue
to listen for new clients).
closesocket
int closesocket(SOCKET s)
Closes socket handle s, returning heap
allocation for that data structure back to
system.
WSACleanup
int WSACleanup( )
Unloads W2_32.dll if no other users. Must call this once for
each call to WSAStartup.
Sequence of Client Calls
WSAStartup
socket
address resolution - set address and port of
intended receiver
connect - send and recv
shutdown
closesocket
WSACleanup
TCP Addresses
struct sockaddr_in{
short sin_family;
unsigned short sin_port;
struct in_addr sin_addr;
char sin_zero[8];
} SOCKADDR_IN;
TCP/IP Address fields
sin_family AF_INET
sin_port at or above 1024
sin_addr inet_addr(“127.0.0.1”);
sin_zero padding
Setting sin_addr.s_addr = INADDR_ANY allows a server
application to listen for client activity on every network
interface on a host computer.
connect
int connect(
SOCKET s,
const struct sockaddr *name,
int namelen
)
Connects client socket to a specific machine and port.
Special Functions
htons – converts short from host to
network byte order
htonl – converts long from host to network
byte order
ntohs – converts short from network to host
byte order
ntohl – converts long from network to host
byte order
A Word of Caution
With stream oriented sockets, send does not
guarantee transfer of all bytes requested in a single
call.
That’s why send returns an int, the number of bytes
actually send.
It’s up to you to ensure that all the bytes are actually
sent
See my code example – socks.cpp
Non-Blocking Communication
Process #1 Process #2
sender receiver receiver thread
function sending processing thread
interprocess
data to
communication
Process #2
FIFO queue
function receiving
data from
Process #1
Store and Forward Architecture
Process #1 - Main Thread Process #2 - Main Thread
Socket Thread Socket Thread
Interprocess
socket socket
Communication
SendQ RecvQ
SendQ is used to hold messages RecvQ is used to quickly remove
in the event that communication messages from the socket
with the remote receiver fails. connection so that the socket
buffer never fills (that would block
Messages are held until the sender).
communication is re-established.
Messaging System Architecture
client/server
model
send and receive
Client messages and data
Server
MessageMgr
Model
thread-safe asynchronous messages thread-safe
MessageMgr MessageMgr
queues and data transfers queues
- develops interface for clients
and servers
- implements protocols for
message and data transfer
- uses sockets interface to
effect transfers
- queues messages at receiver
- handles socket with one thread,
parses messages and handles
queue with another
sockets model
bidirectional - server listens, spawns a
sockets byte stream
sockets thread for each client
Talk Protocol
The hardest part of a client/server socket
communication design is to control the active
participant
If single-threaded client and server both talk at the same
time, their socket buffers will fill up and they both will block,
e.g., deadlock.
If they both listen at the same time, again there is deadlock.
Often the best approach is to use separate send and
receive threads
State Chart - Socket
Bilateral Communication Protocol
Server’s Client Handler
/receive done /extract token
receiving sending
/send token
/extract message /send message
/send token
/send token
sending receiving
/send done
/extract token
/send message /extract message
Each connection channel
contains one “sending” token. Client
Message Length
Another vexing issue is that the receiver may not
know how long a sent message is.
so the receiver doesn’t know how many bytes to pull from
the stream to compose a message.
Often, the communication design will arrange to use
message delimiters, fixed length messages, or message
headers that carry the message length as a parameter.
MessageFramingWithThreadsAndQs – only in C# right now
SocketBlocks SocketCommunicator
Message Framing
Sockets only understand arrays of bytes
Don’t know about strings, messages, or objects
In order to send messages you simply build the
message string, probably with XML
string msg = “<msg>message text goes here</msg>”
Then send(sock,msg,strlen(msg),flags)
Receiving messages requires more work
Read socket one byte at a time and append to message
string:
recv(sock,&ch,1,flags); msg.append(ch);
Search string msg from the back for </
Then collect the msg>
They’re Everywhere
Virtually every network and internet
communication method uses sockets, often in
a way that is invisible to an application
designer.
Browser/server
ftp
SOAP
Network applications
What we didn’t talk about
udp protocol
socket select(…) function
non-blocking sockets
Sockets
The End
