/* 

An example of a TCP connection.

Writes "Hello World" to the socket.

This is designed for Linux, but may work on other unix-like systems.

This is written in ISO C99 -- should be compiled with GCC. 

*/

// standard C include files
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// unix header files
#include <unistd.h> // for write(), close()

// BSD socket header files
#include <sys/socket.h>
#include <sys/types.h>

// resolver header files
#include <netdb.h>
#include <netinet/in.h>

// hostname to ip address conversion functions
#include <arpa/inet.h>

int main(int argc, char **argv)
{
	int fd, r; // fd = file descriptor (all unix systems use this
		// to represent sockets, files, devices, etc)
		// r = result returned from API functions

	char *hostname; // our hostname, from argv

	int port; // our port, from argv

	struct hostent *hent; // host entry, used by the resolver 
			// (a resolver converts hostnames to IP addresses)

	struct sockaddr_in sin; // our socket address structure. 
			// equivalent to SockAddress in java.

	char *string = "Hello World\n"; // our "Hello World" string

	// check parameters.. needs at least 2.	
	if (argc < 3) {
		fprintf(stderr, "Syntax: <hostname> <port>\n");
		exit(1);
	}

	// grab parameters from command line.
	hostname = argv[1];
	port = atoi(argv[2]);

	// create a socket file descriptor -- this is not connected yet.
	fd = socket(AF_INET, SOCK_STREAM, 0);
	if (fd < 0) { 
		perror("socket");
		exit(1);
	}

	// setup our socket address structure
	sin.sin_family = AF_INET;
	sin.sin_port = htons(port);
	// there is a third field:
	// sin.sin_addr which contains an ip address field. 
	// we use it in the resolver below.

	// resolve hostname to ip address

	// first step: IF hostname is IP ADDRESS format, then convert
	// using inet_aton()
	if (!inet_aton(hostname, &sin.sin_addr)) {
		// IF NOT, then do the rest:

		// call gethostbyname() to resolve our hostname.
		hent = gethostbyname(hostname);
		if (!hent) {
			perror("gethostbyname");
			exit(1);	
		}

		// copy our first (0th) result to our socket address
		// field "sin.sin_addr.saddr"
		memcpy(&sin.sin_addr.s_addr, 
			hent->h_addr_list[0], 
			sizeof(sin.sin_addr.s_addr));
	}


	// to connect the socket, we supply:
	// a) file descriptor, b) socket address structure, c) size of
	// socket address structure.

	r = connect(fd, (struct sockaddr *) &sin, sizeof(sin));
	if (r < 0) {
		perror("connect");
		exit(1);
	}

	// if there were no errors, then we are connected now.
	printf("Connected\n");

	// call unix 'write' operation
	write(fd, string, strlen(string));

	// close file descriptor -- we are done with it.
	close(fd);
}