Socket Programming Lec 2Rishi KantReview of Socket programming• Decide which type of socket – stream or datagram. Based on type create socket using socket() function• For datagram - Define the IP and port you wish to connect from, and connect to in 2 separate sockaddr_in structures• For stream – Define the IP and port you wish to listen on (server) or connect to (client) in a sockaddr_in structure• For datagrams and stream servers, bind the sockaddr_in structure to the socket allocated using the bind() functionReview of Socket programming• For servers, use the listen() function to signal the OS to monitor incoming connections and use the accept() function to wait for a client• For clients, use the connect() function to initiate the 3-way handshake and set up the stream connection to the server• For streams, use the send() and recv() functions to transmit and receive bytes• For datagram, use the sendto() and recvfrom() functions to transmit and receive packetsExample from book (pg 33)#include <stdio.h>#include <sys/types.h>#include <sys/socket.h>#include <netinet/in.h>#include <netdb.h>#define PORT 5432#define MAX_LINE 256int main (int argc, char **argv) {FILE *fp;struct hostent *hp;struct sockaddr_in sin;char *host;char buf[MAX_LINE];int s;int len;if (argc == 2) {host = argv[1];} else {fprintf (stderr, "usage: simplex-talk host\n");exit(1);}hp = gethostbyname (host);if (!hp) {fprintf (stderr, "simplex-talk: unknown host: %s\n", host);exit (1);}Example from book (pg 33)bzero ( (char *) &sin, sizeof (sin));sin.sin_family = AF_INET;bcopy (hp->h_addr, (char *) &sin.sin_addr, hp->h_length);sin.sin_port = htons (PORT);if ((s = socket(AF_INET, SOCK_STREAM, 0)) < 0) {perror ("simple-talk: socket");exit (1);}if (connect (s, (struct sockaddr *) &sin, sizeof (sin)) < 0) {perror ("simplex-talk: connect");close (s);exit (1);}while (fgets (buf, sizeof(buf), stdin)) {buf[MAX_LINE-1] = '\0';len = strlen (buf) + 1;send (s, buf, len, 0);}}Example from book (pg 33)#include <stdio.h>#include <sys/types.h>#include <sys/socket.h>#include <netinet/in.h>#include <netdb.h>#define PORT 5432#define MAX_PENDING 5#define MAX_LINE 256int main () {FILE *fp;struct sockaddr_in sin, client;char buf[MAX_LINE];int s, new_s;int len;bzero ( (char *) &sin, sizeof (sin));sin.sin_family = AF_INET;sin.sin_addr.s_addr = INADDR_ANY;sin.sin_port = htons (PORT);if ((s = socket(AF_INET, SOCK_STREAM, 0)) < 0) {perror ("simple-talk: socket");exit (1);}if (bind (s, (struct sockaddr *) &sin, sizeof (sin)) < 0) {}Example from book (pg 33)perror ("simplex-talk: bind");close (s);exit (1);}listen (s, MAX_PENDING);while (1) {if ((new_s = accept (s, (struct sockaddr *) &client, &len)) < 0) {perror ("simplex-talk: accept");exit (1);}while (len = recv (new_s, buf, sizeof (buf), 0)) {fputs (buf, stdout);}close (new_s);}How to compile your programgcc –o <output file> <list of source files> -lsocket –lnsle.g.gcc –o project proj.c –lsocket -lnslPitfalls• Forgetting to convert from host to network byte order and back [htons, htonl etc]• Forgetting to check if a function generated an error by checking return value• Forgetting to check the number of bytes transmitted/received by send()/recv()• Forgetting to use the addressof (&) operator• Forgetting to include the proper header files• Forgetting to flush output streams [fflush()]• Forgetting to set the initial value of length before passing it to accept() or recvfrom() – problem I faced, but not shown in any of the examplesPolling streams• Read operations are blocking calls, so we need a way to check when a stream is ready to be read from• Accomplished by using the select() function• Very good tutorial: http://www.ecst.csuchico.edu/~beej/guide/net/html/Polling streams• Include files:#include <sys/time.h>#include <sys/types.h>#include <unistd.h> • FD_ZERO(fd_set *set) -- clears a file descriptor set• FD_SET(int fd, fd_set *set) -- adds fd to the set• FD_CLR(int fd, fd_set *set) -- removes fd from the set• FD_ISSET(int fd, fd_set *set) -- tests to see if fd is in the set• int select(int numfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout)Java Sockets• http://java.sun.com/docs/books/tutorial/networking/sockets/index.html• Socket and ServerSocket classes for TCP• DatagramSocket for UDP• Clients use the Socket and servers use ServerSocket• Simple example:// Server linesServerSocket svr = new ServerSocket (2000);Socket s = Svr.accept();// Client linesSocket s = new Socket (“localhost”, 2000);//
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