The Socket Interface

1. The Socket Interface Chapter 29

2. The Socket API Application Program Interface A set of operations performed by an application when interacting with protocol software BSD Sockets Integrated with I/O Reading from a socket is conceptually similar to reading from a file

3. Procedures That Implement the Socket API Socket - All Close - All Bind � Servers Listen � TCP Servers Accept � TCP Servers Connect � Clients Different function in TCP and UDP (that is a play on words) Send Recv

4. Socket(protofamily, type, protocol) Creates a socket and returns an integer descriptor, cf open() Protofamily is always PF_INET There are two types: SOCK_STREAM (TCP) SOCK_DGRAM (UDP) Protocol will correspond to either TCP or to UDP (getprotobyname())

5. Close(socket) Just like closing a file.

6. Bind(socket, localaddr, addrlen) Socket has been created but not previously bound Localaddr is specified by a C struct Family, ie, address type Port number Ip address

7. Bind (continued) Specifying the IP address seems strange, and it is, except for multi-homed hosts, ie, hosts with more than one IP address, so we usually specify INADDR_ANY Bind essentially tells the OS to contact us by the socket when something arrives on the port to which the socket is bound

8. Listen(socket, queuesize) Listen registers interest in an event on the corresponding socket Queuesize specifies the number of requests we are willing to handle

9. Socket Accept(socket, caddress, caddresslen) Blocks until a client requests a connection Fills in the address of that client Returns a new socket to handle further communication

10. Connect(socket, saddress, saddresslen) Used by clients to contact servers Socket is a created socket Saddress is the IP address, port number you wish to contact Different with connection-oriented and connectionless protocols

11. Send(socket, data, length, flags) Like writing to a file whose descriptor is socket Data is the address of a buffer where you have stored what you wish to send Flags is usually zero Send assumes a connection

12. Variations on send() Sendto(socket, data, length, flags, destaddress, addresslen) Adds a struct sockaddr specifying where to send the data Sendmsg(socket, mgsstruct, flags) Fills in a struct mgsstruct with destination address and the address of the data

13. Recv(socket, buffer, length, flags) Just like reading a file Socket specifies a particular socket Buffer specifies where you want the data stored Length indicates the maximum number of bytes you are willing to receive Very important to prevent buffer overflow

14. Variations on recv Recvfrom corresponds to sendto Recvmsg corresponds to sendmsg

15. Read(socket, buffer, length) Must be used with connected sockets Really is just like reading a file Used for generality Today I�m reading a socket Tomorrow I�ll be reading a file Write is used similarly

16. Other Socket Procedures Gethostbyname is passed a string, eg, �ursa� and returns the IP address and other information about ursa Getprotobyname is passed a string, eg �tcp� and returns the corresponding number

17. Additional items of interest TCP clients and servers read data from a stream, hence in a loop UDP clients and servers use a single read statement that reads a datagram UDP servers Do not listen Do not accept Must use recvfrom and sendto UDP clients either Connect and use send Do not connect and use sendto

18. Two final, annoying things Clients do not bind!! They allow the OS to assign a free port for their communication. When filling in the destination information you usually use a struct sockaddr_in. The C functions almost uniformly expect a struct sockaddr. You end up doing a lot casts.