There are two types of sockets. Stream sockets and datagram sockets. Stream sockets use TCP (Transmission Control Protocol), which is a reliable, stream oriented protocol, and datagram sockets use UDP (User Datagram Protocol), which is unreliable and message oriented.
Here is an example with Stream Sockets (TCP)
The steps involved in establishing a socket on the client side are as follows:
- Create a socket with the socket() system call
- Connect the socket to the address of the server using the connect() system call
- Send and receive data. There are a number of ways to do this, but the simplest is to use the read() and write() system calls.
- Create a socket with the socket() system call
- Bind the socket to an address using the bind() system call. For a server socket on the Internet, an address consists of a port number on the host machine.
- Listen for connections with the listen() system call
- Accept a connection with the accept() system call. This call typically blocks until a client connects with the server.
- Send and receive data
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/* A simple server in the internet domain using TCP
The port number is passed as an argument */
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
void error(char *msg)
{
perror(msg);
exit(1);
}
int main(int argc, char *argv[])
{
int sockfd, newsockfd, portno, clilen;
char buffer[256];
struct sockaddr_in serv_addr, cli_addr;
int n;
if (argc < 2) {
fprintf(stderr,"ERROR, no port provided\n");
exit(1);
}
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0)
error("ERROR opening socket");
bzero((char *) &serv_addr, sizeof(serv_addr));
portno = atoi(argv[1]);
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = INADDR_ANY;
serv_addr.sin_port = htons(portno);
if (bind(sockfd, (struct sockaddr *) &serv_addr,
sizeof(serv_addr)) < 0)
error("ERROR on binding");
listen(sockfd,5);
clilen = sizeof(cli_addr);
newsockfd = accept(sockfd,
(struct sockaddr *) &cli_addr,
&clilen);
if (newsockfd < 0)
error("ERROR on accept");
bzero(buffer,256);
n = read(newsockfd,buffer,255);
if (n < 0) error("ERROR reading from socket");
printf("Here is the message: %s\n",buffer);
n = write(newsockfd,"I got your message",18);
if (n < 0) error("ERROR writing to socket");
return 0;
}
Client.c
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#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
void error(char *msg)
{
perror(msg);
exit(0);
}
int main(int argc, char *argv[])
{
int sockfd, portno, n;
struct sockaddr_in serv_addr;
struct hostent *server;
char buffer[256];
if (argc < 3) {
fprintf(stderr,"usage %s hostname port\n", argv[0]);
exit(0);
}
portno = atoi(argv[2]);
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0)
error("ERROR opening socket");
server = gethostbyname(argv[1]);
if (server == NULL) {
fprintf(stderr,"ERROR, no such host\n");
exit(0);
}
bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
bcopy((char *)server->h_addr,
(char *)&serv_addr.sin_addr.s_addr,
server->h_length);
serv_addr.sin_port = htons(portno);
if (connect(sockfd,&serv_addr,sizeof(serv_addr)) < 0)
error("ERROR connecting");
printf("Please enter the message: ");
bzero(buffer,256);
fgets(buffer,255,stdin);
n = write(sockfd,buffer,strlen(buffer));
if (n < 0)
error("ERROR writing to socket");
bzero(buffer,256);
n = read(sockfd,buffer,255);
if (n < 0)
error("ERROR reading from socket");
printf("%s\n",buffer);
return 0;
}
You might want to use a datagram socket in cases where there is only one message being sent from the client to the server, and only one message being sent back. There are several differences between a datagram socket and a stream socket.
- Datagrams are unreliable, which means that if a packet of information gets lost somewhere in the Internet, the sender is not told (and of course the receiver does not know about the existence of the message). In contrast, with a stream socket, the underlying TCP protocol will detect that a message was lost because it was not acknowledged, and it will be retransmitted without the process at either end knowing about this.
- Message boundaries are preserved in datagram sockets. If the sender sends a datagram of 100 bytes, the receiver must read all 100 bytes at once. This can be contrasted with a stream socket, where if the sender wrote a 100 byte message, the receiver could read it in two chunks of 50 bytes or 100 chunks of one byte.
- The communication is done using special system calls sendto()#### and receivefrom()#### rather than the more generic read()#### and write()####.
- There is a lot less overhead associated with a datagram socket because connections do not need to be established and broken down, and packets do not need to be acknowledged. This is why datagram sockets are often used when the service to be provided is short, such as a time-of-day service.
Code: Select all
/* Creates a datagram server. The port
number is passed as an argument. This
server runs forever */
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
void error(char *msg)
{
perror(msg);
exit(0);
}
int main(int argc, char *argv[])
{
int sock, length, fromlen, n;
struct sockaddr_in server;
struct sockaddr_in from;
char buf[1024];
if (argc < 2) {
fprintf(stderr, "ERROR, no port provided\n");
exit(0);
}
sock=socket(AF_INET, SOCK_DGRAM, 0);
if (sock < 0) error("Opening socket");
length = sizeof(server);
bzero(&server,length);
server.sin_family=AF_INET;
server.sin_addr.s_addr=INADDR_ANY;
server.sin_port=htons(atoi(argv[1]));
if (bind(sock,(struct sockaddr *)&server,length)<0)
error("binding");
fromlen = sizeof(struct sockaddr_in);
while (1) {
n = recvfrom(sock,buf,1024,0,(struct sockaddr *)&from,&fromlen);
if (n < 0) error("recvfrom");
write(1,"Received a datagram: ",21);
write(1,buf,n);
n = sendto(sock,"Got your message\n",17,
0,(struct sockaddr *)&from,fromlen);
if (n < 0) error("sendto");
}
}
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/* UDP client in the internet domain */
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
void error(char *);
int main(int argc, char *argv[])
{
int sock, length, n;
struct sockaddr_in server, from;
struct hostent *hp;
char buffer[256];
if (argc != 3) { printf("Usage: server port\n");
exit(1);
}
sock= socket(AF_INET, SOCK_DGRAM, 0);
if (sock < 0) error("socket");
server.sin_family = AF_INET;
hp = gethostbyname(argv[1]);
if (hp==0) error("Unknown host");
bcopy((char *)hp->h_addr,
(char *)&server.sin_addr,
hp->h_length);
server.sin_port = htons(atoi(argv[2]));
length=sizeof(struct sockaddr_in);
printf("Please enter the message: ");
bzero(buffer,256);
fgets(buffer,255,stdin);
n=sendto(sock,buffer,
strlen(buffer),0,&server,length);
if (n < 0) error("Sendto");
n = recvfrom(sock,buffer,256,0,&from, &length);
if (n < 0) error("recvfrom");
write(1,"Got an ack: ",12);
write(1,buffer,n);
}
void error(char *msg)
{
perror(msg);
exit(0);
}