Unix Socket Programming
Client Server Architecture
In the client server architecture, a machine(refered as client) makes a
request to connect to another machine (called as server) for providing
some service. The services running on the server run on known
ports(application identifiers) and the client needs to know the address
of the server machine and this port in order to connect to the server.
On the other hand, the server does not need to know about the address or
the port of the client at the time of connection initiation. The first
packet which the client sends as a request to the server contains these
informations about the client which are further used by the server to
send any information. Client acts as the active device which makes the
first move to establish the connection whereas the server passively
waits for such requests from some client.
Illustration of Client Server Model
What is a Socket ?
In unix, whenever there is a need for inter process communication within
the same machine, we use mechanism like signals or pipes(named or
unnamed). Similarly, when we desire a communication between two
applications possibly running on different machines, we need
sockets.
Sockets are treated as another entry in the unix open file table. So
all the system calls which can be used for any IO in unix can be used on
socket. The server and client applications use various system calls to
conenct which use the basic construct called
socket. A socket is one end of the communication channel between two applications running on different machines.
Steps followed by client to establish the connection:
- Create a socket
- Connect the socket to the address of the server
- Send/Receive data
- Close the socket
Steps followed by server to establish the connection:
- Create a socket
- Bind the socket to the port number known to all clients
- Listen for the connection request
- Accept connection request
- Send/Receive data