1. Chapter 3
Socket API
© Bobby Hoggard, Department of Computer Science, East Carolina University
These slides may not be used or duplicated without permission

2. Socket API API: Application Program Interface
An API is simply a collection of functions to help simplify a programming task
Socket API is a standardized set of functions used to help create software that communicates over networks
Socket API is available for most modern operating systems including Microsoft's Windows, Apple's OS-X, Android, and Unix.

3. Sockets
Socket: a line of communication used by an application to talk to another application program
Socket: an endpoint in a communication flow between two application programs
Applications create sockets in order to talk to other applications
Applications can have multiple sockets
Client
Server
socket
socket

4. Using Sockets Applications create sockets for communication
When a socket is created, the OS returns a descriptor, which is an integer used to identify the socket
Applications pass the descriptor to all other API functions to identify which socket the function should work with
OS request
OS response
Application
create socket
socket #1 created
create socket
socket #2 created
socket #1
create socket
socket #3 created
data
socket #2
send data on socket 2
4 bytes sent
data
socket #3
recv data on socket 3
7 bytes sent
close socket 1
socket closed

5. Socket API Parameters Applications must supply many details:
IP address of the remote computer
Port number
Protocol to use
Functions in the API could have many parameters so that all the details can be supplied
Few functions would be needed
Socket API, instead, uses many functions
Thus, each function does not need many parameters

6. Common Socket API Functions
Used by
Description
accept
server
Accept an incoming connection
bind
Specify IP address and port number
close
either
Close the connection*
connect
client
Connect to a server
getpeername
Obtain the client's IP address
listen
Prepare socket to listen for incoming connections
recv
Receive incoming data
send
Send outgoing data
socket
Create a socket
*note: Windows uses closesocket instead of close

7. Typical Socket Function Calls
Client
Server
socket
bind
socket
listen
connect
accept
send
recv
recv
send
close
close

8. Server Uses Multiple Sockets
One socket for listening and one for communicating
Client
Server
socket 1
socket
bind
connect
listen
accept
socket 2
send
recv
recv
send
close
close

9. Creating Sockets
Please refer to the Socket API handout on the class website
int
int
int
SOCKET descriptor;
descriptor =
socket( address family, type, protocol );
Use 2 for TCP
Constant: IPPROTO_TCP
Use 1 for Stream
Constant: SOCK_STREAM
Use 6 for Internet Version 4 (IPv4)
Constant: AF_INET
For more detailed information, visit:

10. SOCKET_ERROR if unsuccessful
Connecting To A Server
Please refer to the Socket API handout on the class website
descriptor = socket( AF_INET, SOCK_STREAM, IPPROTO_TCP );
int code =
connect( socket, serverAddr, serverAddrLength );
Length in bytes of the serverAddr structure.
Use C lang sizeof command
0 = success or
SOCKET_ERROR if unsuccessful
struct SOCKADDR_IN
sin_family
sin_address
sin_port
For more detailed information, visit:

11. Starting Up A Server
Please refer to the Socket API handout on the class website
int code = bind( socket, address, lengthOfAddress );
Parameters and return value are essentially the same as the connect function.
For more detailed information, visit: and look up the function in question

12. Incoming connections above this limit will be automatically rejected
Starting Up A Server
Please refer to the Socket API handout on the class website
int code = listen( socket, queueLength );
integer
Number of pending connections that can be placed in a queue, waiting to be processed
Incoming connections above this limit will be automatically rejected
Should be >= 1

13. INVALID_SOCKET if unsuccessful
Starting Up A Server
Please refer to the Socket API handout on the class website
SOCKET commSocket = accept( socket, clientAddr, clientAddrLength );
Optional
returns the address of the connecting client
works the same as the server addresses
Returns:
a new socket or
INVALID_SOCKET if unsuccessful

14. SOCKET_ERROR if unsuccessful
Sending Data
Please refer to the Socket API handout on the class website
int bytes = send( socket, buffer, lengthOfData, flags );
Unused. Set to 0
How many BYTES to send
Returns:
# of bytes sent or
SOCKET_ERROR if unsuccessful
The address (pointer) in memory of where the data to send is located

15. Sending Data Example
Please refer to the Socket API handout on the class website
char buffer[6] = "HELLO";
int bytes = send( socket, buffer, lengthOfData, flags );
6 bytes H E L O \0

16. Sending Data Example
Please refer to the Socket API handout on the class website
int buffer[3] = { 56, 23, 17 };
int bytes = send( socket, (char*)buffer, lengthOfData, flags );
12 bytes 56 23 17
Modern C compilers define 4 bytes per int
Type cast needed because the function wants a pointer to a char(byte) array

17. SOCKET_ERROR if unsuccessful
Receiving Data
Please refer to the Socket API handout on the class website
int bytes = recv( socket, buffer, lengthOfData, flags );
Unused. Set to 0
Size of the storage buffer, in BYTES
Returns:
# of bytes received or
SOCKET_ERROR if unsuccessful
The address (pointer) in memory of where the data is to be stored

18. Receiving Data Example
Please refer to the Socket API handout on the class website
char buffer[10];
10 bytes
int bytes = recv( socket, buffer, lengthOfData, flags );
6 bytes W O R L D \0

19. Other Functions (Windows only)
int portCorrect = htons( port );
"host to network service"
converts the port (integer) from host byte ordering to the correct network byte ordering
ensures that the bytes making up the port number are in the correct order as needed by the network
Example: Port Number 5
LSB
Host: 5
Network: 5
LSB

20. Other Functions (Windows only)
inet_pton( addressFamily, addressIn, addressOut );
converts an IP address in dotted decimal notation into its 32-bit IP address form
technically: converts from a string to an integer
Example: Convert IPv4 address
AF_INET
" "
inet_pton( addressFamily, addressIn, addressOut );

21. Including the Functions
To compile a program: Socket function definitions must be included at the top of your code so that your code will know of their existance. #include <WinSock2.h> #include <Ws2tcpip.h>

22. Including the Functions
To execute a program: A) The linker must know about the location of the code for the socket functions #pragma comment(lib, "Ws2_32.lib")

23. Including the Functions
To execute a program:
B) Socket function code must be loaded into memory.
This is done after your program code starts executing through a process called dynamic linking.
WSAStartup( version, data pointer )
Example:
// Load version 2.2 of the WinSocket DLL, and store the information in wsaData structure
WSADATA wsaData;
WSAStartup( MAKEWORD(2,2), &wsaData );

24. Static Linking
Code for external functions is included with program code and is loaded into memory when the program is loaded
Every program which uses these functions has its own copy of the function code
Occupies extra memory
Occupies extra hard drive space
Hard to upgrade the library
RAM Memory
Web Browser
Client
Streaming Music App
Socket API functions
Socket API functions
Socket API functions

25. Dynamic Linking
Code for external functions is stored in a file (DLL) separate from the application program file
The program loads the DLL when needed
Programs can share the loaded function code
RAM Memory
Web Browser
Client
Streaming Music App
Socket API functions

26. Dynamic Linking
When an application ends, it needs to indicate that it is finished using the DLL code WSACleanup()
When the last application ends, the DLL code is removed from memory
RAM Memory
Web Browser
Client
Streaming Music App
Socket API functions