Socket Programming: a PrimerWhy does one need sockets?So what exactly does a socket do?Types of SocketsHow should one define a socket?How to create a socket?How to define address?Bind a SocketHow to convert addresses?Using a Datagram SocketUsing a Stream SocketUsing a Stream Socket (contd)A Quick Summary: DatagramA Quick Summary: StreamSample Codes: Datagram ClientSample Code: Datagram ClientSample Code: Datagram ServerSlide 18Sample Code: Stream ClientSlide 20Sample Code: Stream ServerSlide 22Further ReadingSocket Programming: a PrimerSocket to me!Feb. 23, 2001 EE122, UCB 2Why does one need sockets? applicationnetwork protocolsocketsnetworkFeb. 23, 2001 EE122, UCB 3So what exactly does a socket do?•It is an API between applications and network protocol software•Functions it provides:–Define an “end-point” for communication–Initiate and accept a connection–Send and receive data–Terminate a connection gracefully•Supports multiple protocol families–Examples: Unix inter-process communication, TCP/IP–Only Internet sockets will be covered in this lectureFeb. 23, 2001 EE122, UCB 4Types of Sockets•Two different types of sockets:–stream vs. datagram •Stream socket: (a.k.a. connection-oriented socket)–It provides reliable, connected networking service –Error free; no out-of-order packets (uses TCP)–applications: telnet, http, …•Datagram socket: (a.k.a. connectionless socket)–It provides unreliable, best-effort networking service–Packets may be lost; may arrive out of order (uses UDP)–applications: streaming audio/video, …Feb. 23, 2001 EE122, UCB 5How should one define a socket?•To define an end-point of communication, one needs to specify–the family of protocol it uses (Internet vs. others)–addressing information (IP address + port number)–the type of service it provides (stream vs. datagram)•Done in three steps–create a socket–define address and port number–associate address with the socketFeb. 23, 2001 EE122, UCB 6How to create a socket?# include <sys/types.h># include <sys/socket.h>int sock;sock = socket (AF_INET, SOCK_STREAM, 0); /* for stream */ sock = socket (AF_INET, SOCK_DGRAM, 0); /* for datagram */ •Notice that the socket descriptor is just a regular int !•So it has the same usage as a file descriptor in Unix…Feb. 23, 2001 EE122, UCB 7How to define address?struct sockaddr { u_short sa_family; char sa_data[14];}struct sockaddr_in { short sin_family; u_short sin_port; struct in_addr sin_addr; char sin_zero[8];}struct in_addr { u_long s_addr;}WARNING: Don’t forget to convert byte orders!htons, htonl, ntohs, ntohlFeb. 23, 2001 EE122, UCB 8Bind a Socket•bind( ): associate a socket descriptor with an address•putting everything togetherint bind (int sockfd, struct sockaddr *addr, int len); int sockfd;struct sockaddr_in addr;sockfd=socket(AF_INET,SOCKE_STREAM, 0);addr.sin_family=AF_INET;addr.sin_port=htons(5000); /* 0: randomly assigned by OS */addr.sin_addr.s_addr=htonl(INADDR_ANY); /* local address */bzero(&(addr.sin_zero),8); /* pad zeros */bind(sockfd,(struct sockaddr *)&addr, sizeof(struct sockaddr));Feb. 23, 2001 EE122, UCB 9How to convert addresses?•You also need to define address for the other end•If you know its IP addressaddr.sin_addr.s_addr=inet_addr(“128.32.138.240”);•If you know its name only:–need to perform a DNS lookupstruct hostent *gethostbyname(char *name)struct hostent { char *h_name; char **h_aliases; int h_addrtype; int h_length; char **h_addr_list;#define h_addr h_addr_list[0]}struct hostent *h;struct in_addr *inad;h=gethostbyname(“cory.eecs”);inad=(struct in_addr *) h->h_addr;addr.sin-addr.s_addr=inad;Feb. 23, 2001 EE122, UCB 10Using a Datagram Socket•Sending dataint sendto(int sockfd, void *msg, int msg_len, u_short flags, struct sockaddr *dest, int dest_len);•Receiving dataint recvfrom(int sockfd, void *msg, int msg_len, u_short flags, struct sockaddr *src, int src_len);Feb. 23, 2001 EE122, UCB 11Using a Stream Socket•establishing a connectionint listen(int sockfd, int backlog);int connect(int sockfd, struct sockaddr *addr, int addr_len);int accept(int sockfd, void *addr, int *addrlen );listen ()connect( )accept ()clientserverFeb. 23, 2001 EE122, UCB 12Using a Stream Socket (contd)•Sending dataint send(int sockfd, void *msg, int msg_len, u_short flags);•Receiving dataint recv(int sockfd, void *msg, int msg_len, u_short flags);•Notice that no address is required!Feb. 23, 2001 EE122, UCB 13A Quick Summary: Datagramsocket() tocreate socketbind() to areceiving portrecvfrom()sendto ()socket() tocreate socketbind() toany portrecvfrom()sendto ()serverclientFeb. 23, 2001 EE122, UCB 14A Quick Summary: Streamsocket()serverbind() to areceiving portlisten ()to socketbind() toany portconnect ()To serverclientaccept ()connectionsocket()send ()recv ()send ()recv ()Feb. 23, 2001 EE122, UCB 15Sample Codes: Datagram Client#include <sys/types.h>#include <sys/socket.h>#include <netinet/in.h>#include <netdb.h>#define Bfsize 1024main(int argc, char *argv[]) { int sock; struct sockaddr_in client, server; struct hostent *host, *gethostname(); sock=socket(AF_INET, SOCK_DGRAM, 0); /* open socket */ client.sin_family=AF_INET; client.sin_addr.s_addr=htonl(INADDR_ANY); /* local addr */ client.sin_port=htons(0); /* any port # */ bind(sock,(struct sockaddr *)&client,sizeof(client));Feb. 23, 2001 EE122, UCB 16Sample Code: Datagram Client host=gethostbyname(argv[1]); /* get host name */ memcpy((char *)&server.sin_addr, (char *)host->h_addr, host->h_length); server.sin_family=AF_INET; server.sin_port=htons(atoi(argv[2])); sendto(sock,msg,sizeof(msg),0,(struct sockaddr *)&server, sizeof(server)); close(sock);}Don’t forget error handling when calling these functions in your programs!Feb. 23, 2001 EE122, UCB 17Sample Code: Datagram Server#include <sys/types.h>#include <sys/socket.h>#include <netinet/in.h>#include <netdb.h>#define Bfsize 1024main(int argc, char *argv[]) { int sock, length, count; struct sockaddr_in server, client; char buffer[Bfsize]; sock=socket(AF_INET, SOCK_DGRAM,0); server.sin_family=AF_INET; server.sin_addr.s_addr=htonl(INADDR_ANY); server.sin_port=htons(atoi(argv[1])); /* listening port */ bind(sock,(struct sockaddr *)&server,sizeof(server));Feb. 23, 2001 EE122, UCB 18Sample Code: Datagram Server count=recvfrom(sock, buffer, Bfsize, 0,
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