creative201347/BECE_2nd

Socket Programming

Socket creation in C: socket()

Socket call doesn't specify where data will be coming from, nor where it will be going to, it just creates the interface!

upon failure returns -1

int sockid = socket(family, type, protocol);

sockid: socket descriptor, an integer (like a file-handle)

family: integer, communication domain, e.g.,

• PF_INET, IPv4 protocols, Internet addresses (typically used)
• PF_UNIX, Local communication, File addresses

type: communication type

• SOCK_STREAM - reliable, 2-way, connection-based service
• SOCK_DGRAM - unreliable, connectionless, messages of maximum length

protocol: specifies protocol

• IPPROTO_TCP IPPROTO_UDP
• usually set to 0 (i.e., use default protocol)

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Socket close in C: close()

When finished using a socket, the socket should be closed

�status = close(sockid);

• sockid: the file descriptor (socket being closed)

• status: 0 if successful, -1 if error

• closes a connection (for stream socket)

• frees up the port used by the socket

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Specifying Addresses

Socket API defines a generic data type for addresses:

struct sockaddr{
    unsigned short sa_family; // Address family (e.g. AF_INET)
    char sa_data[14]; // Family-specific address information
}

Particular form of the sockaddr used for TCP/IP addresses:

struct in_addr{
    unsigned long s_addr;
}
struct sockaddr_in{
    unsigned short sin_family; // Internet protocol (AF_INET)
    unsigned short sin_port; // Address port (16 bits)
    struct in_addr sin_addr; // Internet address (32 bits)
    char sin_zero[8]; // Not used
}

sockaddr_in can be casted to a sockaddr

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Assign address to socket: bind()

associates and reserves a port for use by the socket

int status = bind(sockid, &addrport, size);

sockid: integer, socket descriptor

size: the size (in bytes) of the addrport structure

status: upon failure -1 is returned

addrport: struct sockaddr, the (IP) address and port of the machine for TCP/IP server, internet address is usually set to INADDR_ANY, i.e.,

chooses any incoming interface

int sockid;
struct sockaddr_in addrport;
sockid = socket(PF_INET, SOCK_STREAM, 0);
addrport.sin_family = AF_INET;
addrport.sin_port = htons(5100);
addrport.sin_addr.s_addr = htonl(INADDR_ANY);
if(bind(sockid, (struct sockaddr *) &addrport, sizeof(addrport))!= -1) {
    …}

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Assign address to socket: listen()

Instructs TCP protocol implementation to listen for connections

int status = listen(sockid, queueLimit);

sockid: integer, socket descriptor

queuelen: integer, # of active participants that can “wait” for a connection

status: 0 if listening, -1 if error

listen() is non-blocking: returns immediately

The listening socket (sockid)

• is never used for sending and receiving
• is used by the server only as a way to get new sockets

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Establish Connection: connect()

The client establishes a connection with the server by calling connect()

int status = connect(sockid, &foreignAddr, addrlen);

sockid: integer, socket to be used in connection

foreignAddr: struct sockaddr: address of the passive participant

addrlen: integer, sizeof(name)

status: 0 if successful connect, -1 otherwise

�connect() is blocking

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Incoming Connection: accept()

The server gets a socket for an incoming client connection by calling accept()

int s = accept(sockid, &clientAddr, &addrLen);

s: integer, the new socket (used for data-transfer)

sockid: integer, the orig. socket (being listened on)

clientAddr: struct sockaddr, address of the active participant

• filled in upon return

addrLen: sizeof(clientAddr): value/result parameter

• must be set appropriately before call
• adjusted upon return

accept()

• is blocking: waits for connection before returning
• dequeues the next connection on the queue for socket (sockid)

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Exchanging data with stream socket

int count = send(sockid, msg, msgLen, flags);

msg: const void[], message to be transmitted

msgLen: integer, length of message (in bytes) to transmit

flags: integer, special options, usually just 0

count: # bytes transmitted (-1 if error)

int count = recv(sockid, recvBuf, bufLen, flags);

recvBuf: void[], stores received bytes

bufLen: # bytes received

flags: integer, special options, usually just 0

count: # bytes received (-1 if error)

Calls are blocking

• returns only after data is sent / received