- Recipe (steps) for
coding a
TCP server application:
- Create a
TCP socket
int s = socket( AT_INET, SOCK_STREAM, 0 );
This socket is only used for:
- Receiving TCP connection requests from clients !!!
- Bind the
socket s to
a fixed TCP port#:
struct sockaddr_in in_addr; in_addr.sin_family = AF_INET; in_addr.sin_addr.s_addr = htonl(INADDR_ANY); in_addr.sin_port = htons(SrvPortNumber); // a fixed port# bind(s, (struct sockaddr *)&in_addr, sizeof(in_addr))Result:
- TCP messages can be received on this Internet network address
- Enable the
socket s to
receive
client's connection requests:
listen( s, #pending-connect-requests );
Result:
More details on the listen( ) function in later
- Establish
TCP connections with
clients:
int s2; // Another socket variable s2 = accept( s, &in_addr, &length );
Result:
- The socket
s2 is used to
communicate with a
client
- The socket s will still be used to establish TCP connections with (other) clients
- The socket
s2 is used to
communicate with a
client
- Create a
TCP socket
- Syntax of the
listen( ) function:
int listen(int s, int backlogLength)
s = a socket descriptor backlogLength = max. number of pending requests allowed listen returns 0 for success, -1 for error - Effect of the
listen(s, ... ) call:
- listen(s, ... )
enables the
socket s to
accept incoming
connection requests
sent by TCP client applications
Note:
- A TCP socket s does
not
accept
client's
connection requests
- You must enable the connection processing with the listen(s, ...) call
- A TCP socket s does
not
accept
client's
connection requests
- listen(s, ... )
enables the
socket s to
accept incoming
connection requests
sent by TCP client applications
- Syntax of the accept( )
function:
int accept ( int s, struct sockaddr *cli_addr, socklen_t *addrlen)
Input: s = a listen socket Output: cli_addr = network address (will contain (IPaddr, Port#) of client addrlen = length of addr Return value: a new TCP socket descriptor Note: use the returned TCP socket to send messages to client receive messages from client
- Effect of
accept( ) function:
- The
accept( ) function will
block until
a TCP connection is
successfully established:
- an incoming
connection request
is received
- a consistent TCP state has been negotiated (send/ACK numbers)
- an incoming
connection request
is received
- The TCP connection establishment will
perform
the following
in the OS kernel:
- Create a
new
descriptor entry in the
per-process descriptor table for
a TCP socket
containing:
- The state (send/ACK numbers) of
the TCP connection
- The send/receive buffer spaces for the new TCP connection
- The state (send/ACK numbers) of
the TCP connection
- Create a
new
descriptor entry in the
per-process descriptor table for
a TCP socket
containing:
- The accept( ) function
will return the
(socket) descriptor
- The server
must use
the return descriptor
to
send/receive data
to/from the client
(Because this socket contains the TCP state for reliable communication)
- The server
must use
the return descriptor
to
send/receive data
to/from the client
- The
accept( ) function will
block until
a TCP connection is
successfully established:
- Structure of a
server TCP application
written in C program code:
int s_listen; // The listen socket int s_data; // The data socket struct sockaddr_in in_addr; // Help structures to form netw address struct sockaddr_in client; int length; /* --------------------------------------- Create the TCP listen socket --------------------------------------- */ s_listen = socket( AF_INET, SOCK_STREAM, 0 ); // Create a TCP socket /* ------------------------------------------------------ Bind the socket to a network address This makes the socket identifiable by (IPaddr, Port#) ------------------------------------------------------ */ in_addr.sin_family = AF_INET; in_addr.sin_addr.s_addr = htonl(INADDR_ANY); in_addr.sin_port = htons(SrvPortNumber); if ( bind(s_listen, (struct sockaddr *)&in_addr, // Bind socket to (IP-addr, Port#) sizeof(in_addr)) == 0 ) { perror("Bind: "); exit(1); } /* --------------------------------------------------------- Enable socket s_listen to accept connect requests --------------------------------------------------------- */ listen(s_listen, 5); // Change socket to a "listen" socket /* --------------------------------------------------------- Accept TCP connection calls --------------------------------------------------------- */ while ( 1 ) { length = sizeof(struct sockaddr_in); /* ======================================================= Server program waits for client's connection requests ======================================================= */ s_data = accept( s_listen, &client, &length); // s_data = data socket // The accept( ) function will a // data socket of the TCP connection // establish with the client program /* =========================================== Now you can send and receive data using the s_data socket =========================================== */ read ( s_data, ....) to receive client's requests write( s_data, ....) to send data to client close( s_data) to disconnect }