C Network Programming - recv() help

Applications Programming
1

So I'm making a program that gets the IP Address of an inputed website, then sends the IP Address to an inputed host. This program has no real meaning, but just a learning experiment. So I sucessfully made the program, then I wanted to try out recv(), and it produces some weird output; here is the source:

#include <stdio.h>
#include <string.h>
#include <stdlib.h> /* exit() */
#include <errno.h> /* herror() */
#include <netdb.h> /* gethostbyname() */
#include <sys/types.h> /* bind() accept() */
#include <sys/socket.h> /* bind() accept() */

#define PORT 3490

main(int argc, char *argv[]) {
	int sockfd, n_sockfd, sin_size, len;
	char *host_addr, *recv_msg;
	struct hostent *host;
	struct sockaddr_in my_addr;
	struct sockaddr_in their_addr;

	if (argc != 3) {
		fprintf(stderr, "usage: %s [hostname] [ip address]\n",
		        argv[0]);
		exit(1);
	}
	if ((host = gethostbyname(argv[1])) == NULL) {
		herror("gethostbyname");
		exit(1);
	}
	if ((sockfd = socket(PF_INET, SOCK_STREAM, 0)) == -1) {
		perror("socket");
		exit(1);
	}

	my_addr.sin_family = AF_INET;
	my_addr.sin_port = htons(PORT);
	my_addr.sin_addr.s_addr = inet_addr(argv[2]);
	memset(my_addr.sin_zero, '\0', sizeof my_addr.sin_zero);

	if ((bind(sockfd, (struct sockaddr *)&my_addr,
	     sizeof(struct sockaddr))) == -1) {
		perror("bind");
		exit(1);
	}
	if ((listen(sockfd, 10)) == -1) {
		perror("socket");
		exit(1);
	}
	sin_size = sizeof(struct sockaddr_in);
	if ((n_sockfd = accept(sockfd, (struct sockaddr *)&their_addr,
	    &sin_size)) == -1) {
		perror("accept");
		exit(1);
	}
	close(sockfd);

	host_addr = inet_ntoa(*((struct in_addr *)host->h_addr));

	len = strlen(host_addr);
	send(n_sockfd, host_addr, len, 0);
	len = strlen("\n");
	send(n_sockfd, "\n", len, 0);

	recv(n_sockfd, recv_msg, 10, 0);
	printf("%s\n", recv_msg);

	close(n_sockfd);
}

And here is my command line stuff:

The program in action:

~/c/network/testing $ ./hostinfo google.com 127.0.0.1
hello
 +FVfv
                       ~/c/network/testing $

And the same computer interacting with it:

~ $ telnet 127.0.0.1 3490
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
64.233.167.99
hello
Connection closed by foreign host.
~ $

How what is with the +FVfv?

Thanks for reading,
Octal

2

You are checking the return code from the socket call. Do the same with recv() and send(). If recv fails, the buffer is untouched and you just print whatever garbage was in it.

3

I actually figured it out:

bytes = recv(n_sockfd, recv_msg, 10, 0);
recv_msg[bytes] = '\0';
printf("%s\n", recv_msg);

And bytes was obviously an int that I had to declare.