Sockets
# Socket
**********************************************************************************************
* *
* +-----------+ *
* | IPAddress | *
* +-+-------+-+ *
* 1. \ / sockaddr_in (server) *
* '---' *
* 3. bind --> *
* *
* \ / *
* '+++' *
* | *
* | *
* | *
* | *
* | 2. socket *
* | *
* | *
* | *
* | *
* +---------+ *
* | | *
* 4. listen | | dupplicate socket *
* .-+-. .-+-. *
* / \ / \ *
* *
* 5. accept --> *
* .---. *
* / \ sockaddr_in (client) *
* +-+-------+-+ *
* | IPAddress | *
* +-----------+ *
* *
* *
**********************************************************************************************
1 . sockaddr_in
`sockaddr_in` : socket containing a internet address defined in `< netinet/in.h >`
~~~
struct sockaddr_in {
short sin_family;
u_short sin_port;
struct in_addr sin_addr;
char sin_zero[8];
};
~~~
~~~
struct sockaddr_in serv_addr, cli_addr;
~~~
~~~
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(portno);
serv_addr.sin_addr.s_addr = INADDR_ANY;
~~~
`serv_addr` is the IP address of the server machine.
There is a symbolic constant `INADDR_ANY` which gets this address.
2 . socket
~~~
sockfd = socket(AF_INET, SOCK_STREAM, 0);
~~~
- `SOCK_STREAM` : stream sockets (TCP)
- `SOCK_DGRAM` : datagram sockets (UDP)
If the socket call fails, it returns -1.
~~~
if (sockfd < 0)
error("ERROR opening socket");
~~~
3 . bind
`bind()` system call
~~~
if (bind(sockfd, (struct sockaddr *) &serv_addr,
sizeof(serv_addr)) < 0)
error("ERROR on binding");
~~~
This can fail for a number of reasons,
the most obvious being that this socket is already in use.
4 . listen
~~~
listen(sockfd,5);
~~~
5 is the size of the backlog queue, i.e. the number of connections.
5 is the maximum size permitted by most systems
5 . accept
accept() system call
~~~
clilen = sizeof(cli_addr);
newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);
if (newsockfd < 0)
error("ERROR on accept");
~~~
## Full Source Code
~~~
/* 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;
}
~~~