Programming Forums > Application Development > C

Soulstorm · Mar 1st, 2008, 11:14 AM

I am diving into network connections, using the BSD sockets. My ultimate goal is to make a very basic IRC client, just to learn how to do network connections. My operating system is OS X, but I don't think that makes a difference, since I believe BSD Sockets exist in other platforms, too.

I am stuck.

I have created a command line tool in C and I have the following code:

 c Syntax (Toggle Plain Text) 
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>
 
#define PORT 6667 // the port client will be connecting to 
 
#define MAXDATASIZE 1000 // max number of bytes we can get at once 
 
int main(int argc, char *argv[])
{
    int sockfd, numbytes;  
    char buf[MAXDATASIZE];
    struct hostent *he;
    struct sockaddr_in their_addr; // connector's address information 
 
	he = gethostbyname("efnet.teleglobe.net");
	sockfd = socket(PF_INET, SOCK_STREAM, 0);
	if (sockfd < 0) {
		printf("error creating the socket\n");
	}
    their_addr.sin_family	= AF_INET;
	their_addr.sin_port		= htons(PORT);
	their_addr.sin_addr		= *((struct in_addr *)he->h_addr);
	memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero);
 
 
	if (connect(sockfd, (struct sockaddr *)&their_addr,
				sizeof their_addr) == -1) {
        perror("connect\n");
        exit(1);
    }
	else
		printf("You got the connection to the server!\n");
	char p[] = "join #macfilez";
	send(sockfd, p, sizeof(p), 0);
 
	recv(sockfd, buf, sizeof(buf), 0);
	buf[numbytes] = '\0';
	printf("Received: %s",buf);
	close(sockfd);
 
    return 0;
}
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>

#define PORT 6667 // the port client will be connecting to 

#define MAXDATASIZE 1000 // max number of bytes we can get at once 

int main(int argc, char *argv[])
{
    int sockfd, numbytes;  
    char buf[MAXDATASIZE];
    struct hostent *he;
    struct sockaddr_in their_addr; // connector's address information 
	
	he = gethostbyname("efnet.teleglobe.net");
	sockfd = socket(PF_INET, SOCK_STREAM, 0);
	if (sockfd < 0) {
		printf("error creating the socket\n");
	}
    their_addr.sin_family	= AF_INET;
	their_addr.sin_port		= htons(PORT);
	their_addr.sin_addr		= *((struct in_addr *)he->h_addr);
	memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero);

	
	if (connect(sockfd, (struct sockaddr *)&their_addr,
				sizeof their_addr) == -1) {
        perror("connect\n");
        exit(1);
    }
	else
		printf("You got the connection to the server!\n");
	char p[] = "join #macfilez";
	send(sockfd, p, sizeof(p), 0);
	
	recv(sockfd, buf, sizeof(buf), 0);
	buf[numbytes] = '\0';
	printf("Received: %s",buf);
	close(sockfd);
	
    return 0;
}

The server I put was a server running on IRC's EFNet network. I was supposed to receive a message, from the server, right? Not only this doesn't happen, my program also crashes without showing anything! (the debugger comes up with no explanation).

Any ideas why? Actually, I must have made a newbie mistake due to my ignorance. Please enlighten me.

Game_Ender · Mar 1st, 2008, 7:51 PM

Where is your server code?

Well you aren't check the return of "gethostbyname" which could very well be NULL. That would be a quick crash. You also don't zero out you sockaddr_in structure before using it. Here is some code I have that does a similar thing:

 cpp Syntax (Toggle Plain Text) 
    memset(&m_addr, 0, sizeof(m_addr));
    m_addr.sin_family = AF_INET;    // host byte order
    m_addr.sin_port = htons(port);  // short, network byte order
    m_addr.sin_addr = *((struct in_addr *)he->h_addr);
    memset(&m_addr, 0, sizeof(m_addr));
    m_addr.sin_family = AF_INET;    // host byte order
    m_addr.sin_port = htons(port);  // short, network byte order
    m_addr.sin_addr = *((struct in_addr *)he->h_addr);

I think you are trying to do something simular with this:

 cpp Syntax (Toggle Plain Text) 
memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero);
memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero);

Wikipedia has good article here.

Soulstorm · Mar 2nd, 2008, 10:34 AM

Quote:

Originally Posted by Game_Ender

Where is your server code?

Well you aren't check the return of "gethostbyname" which could very well be NULL. That would be a quick crash. You also don't zero out you sockaddr_in structure before using it. Here is some code I have that does a similar thing:

 cpp Syntax (Toggle Plain Text) 
    memset(&m_addr, 0, sizeof(m_addr));
    m_addr.sin_family = AF_INET;    // host byte order
    m_addr.sin_port = htons(port);  // short, network byte order
    m_addr.sin_addr = *((struct in_addr *)he->h_addr);
    memset(&m_addr, 0, sizeof(m_addr));
    m_addr.sin_family = AF_INET;    // host byte order
    m_addr.sin_port = htons(port);  // short, network byte order
    m_addr.sin_addr = *((struct in_addr *)he->h_addr);

I think you are trying to do something simular with this:

 cpp Syntax (Toggle Plain Text) 
memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero);
memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero);

Wikipedia has good article here.

Thanks for the reply. There is no server code. The host I connect to is an IRC network host.

The Dark · Mar 2nd, 2008, 4:07 PM

You don't set numBytes to a value before using it. This code:

(Toggle Plain Text)

buf[numbytes] = '\0';

could then try to write to anywhere.

Dameon · Mar 2nd, 2008, 4:34 PM

recv will return -1 on error, 0 upon disconnection, or otherwise the number of bytes read. You should probably use that for numbytes. This is from 'man recv'

So make sure to properly handle the return value.

I've only skimmed so that is likely not the only issue.

Game_Ender · Mar 3rd, 2008, 7:16 AM

Yes you should be checking the return of *every* socket call. I usually use wrapper function which do it for me.

I found the IRC spec and I am not sure you are following it correctly. I think you need both "CR" and "LF" which would mean messages end in "\r\n". See more here.

You should also be able to telnet into the server and test out the commands manually if you wish.

Soulstorm · Mar 3rd, 2008, 8:34 AM

Actually, this code

(Toggle Plain Text)

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>

#define PORT 6667 // the port client will be connecting to 

#define MAXDATASIZE 2000 // max number of bytes we can get at once 

int main(int argc, char *argv[])
{
    int sockfd, numbytes;
    char buf[MAXDATASIZE];
    struct hostent *he;
    struct sockaddr_in their_addr; // connector's address information
	
    he = gethostbyname("efnet.teleglobe.net");
    sockfd = socket(PF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
		printf("error creating the socket\n");
    }
    their_addr.sin_family       = AF_INET;
    their_addr.sin_port         = htons(PORT);
    their_addr.sin_addr         = *((struct in_addr *)he->h_addr);
    memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero);
	
	
    if (connect(sockfd, (struct sockaddr *)&their_addr,
				sizeof their_addr) == -1) {
		perror("connect\n");
		exit(1);
    } else {
		printf("You got the connection to the server!\n");

		fprintf(stderr,"After send, entering recv loop\n");
		numbytes=0;
		while(numbytes == 0) {
			fprintf(stderr,"In recv loop\n");
			numbytes=recv(sockfd, buf, sizeof(buf)-1, 0); //-1 b/c you will null terminate
		}
		fprintf(stderr,"After recv loop, %d bytes read.\n",numbytes);
		buf[numbytes] = '\0';
		fprintf(stderr,"After null term, %d bytes read.\n",numbytes);
		printf("Received: %s",buf);
		fflush(stdout);
		close(sockfd);
	}
	return 0;
}

makes my computer connect to the server, and then receives a message saying "NOTICE AUTH

** Processing connection to efnet.teleglobe.net"

That's all. However, I believe I was supposed to see more than this. Perhaps it's the problem that Game_Ended noticed about line endings.

Soulstorm · Mar 3rd, 2008, 2:50 PM

OK, I may need some explanation on sockets.

As far as I understand the general approach is:

Specify a host
connect to the host using connect()
give a command
and then, accept the responce from the server for this command using the recv function

However, I read upon telnet (wikipedia), but I don't quite understand what it is. And I really don't understand what game_ender said about CR and LF. I'm sorry if my questions sound newbish, but I haven't been involved with communication protocols or any kind of networking with programming and I need some basic questions answered...

Game_Ender · Mar 3rd, 2008, 8:59 PM

From what I understand telnet is a program that lets you connect to another server on a given port, and then send and receive plan text over a TCP connection. Since the IRC protocol is plain text you can talk to a server "by hand".

Now the issue is that TCP is a stream oriented protcol (its just like reading from a file), you have no idea how many bytes someone has sent you. So you have to build something into your communications protocol to be able tell when messages start in end. In the IRC protocol they want you to put "\r\n" at the end of every message. That is how the server knows you are done sending. Here is an example:

 c Syntax (Toggle Plain Text) 
/* Bad */
char p[] = "join #macfilez";
/* Good */
char p[] = "join #macfilez\r\n";
/* Bad */
char p[] = "join #macfilez";
/* Good */
char p[] = "join #macfilez\r\n";

Game_Ender · Mar 3rd, 2008, 9:00 PM

You should really make a dumb server which just prints whatever it is sent so that you at least know what are you sending is correct.

Mar 1st, 2008, 11:14 AM	#1
Soulstorm Hobbyist Programmer Join Date: Jan 2006 Location: Menidi, Athens, Greece Posts: 234 Rep Power: 3	Simple BSD Sockets Problem I am diving into network connections, using the BSD sockets. My ultimate goal is to make a very basic IRC client, just to learn how to do network connections. My operating system is OS X, but I don't think that makes a difference, since I believe BSD Sockets exist in other platforms, too. I am stuck. I have created a command line tool in C and I have the following code: c Syntax (Toggle Plain Text) #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <errno.h> #include <string.h> #include <netdb.h> #include <sys/types.h> #include <netinet/in.h> #include <sys/socket.h> #define PORT 6667 // the port client will be connecting to #define MAXDATASIZE 1000 // max number of bytes we can get at once int main(int argc, char argv[]) { int sockfd, numbytes; char buf[MAXDATASIZE]; struct hostent he; struct sockaddr_in their_addr; // connector's address information he = gethostbyname("efnet.teleglobe.net"); sockfd = socket(PF_INET, SOCK_STREAM, 0); if (sockfd < 0) { printf("error creating the socket\n"); } their_addr.sin_family = AF_INET; their_addr.sin_port = htons(PORT); their_addr.sin_addr = ((struct in_addr )he->h_addr); memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero); if (connect(sockfd, (struct sockaddr )&their_addr, sizeof their_addr) == -1) { perror("connect\n"); exit(1); } else printf("You got the connection to the server!\n"); char p[] = "join #macfilez"; send(sockfd, p, sizeof(p), 0); recv(sockfd, buf, sizeof(buf), 0); buf[numbytes] = '\0'; printf("Received: %s",buf); close(sockfd); return 0; } #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <errno.h> #include <string.h> #include <netdb.h> #include <sys/types.h> #include <netinet/in.h> #include <sys/socket.h> #define PORT 6667 // the port client will be connecting to #define MAXDATASIZE 1000 // max number of bytes we can get at once int main(int argc, char argv[]) { int sockfd, numbytes; char buf[MAXDATASIZE]; struct hostent he; struct sockaddr_in their_addr; // connector's address information he = gethostbyname("efnet.teleglobe.net"); sockfd = socket(PF_INET, SOCK_STREAM, 0); if (sockfd < 0) { printf("error creating the socket\n"); } their_addr.sin_family = AF_INET; their_addr.sin_port = htons(PORT); their_addr.sin_addr = ((struct in_addr )he->h_addr); memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero); if (connect(sockfd, (struct sockaddr )&their_addr, sizeof their_addr) == -1) { perror("connect\n"); exit(1); } else printf("You got the connection to the server!\n"); char p[] = "join #macfilez"; send(sockfd, p, sizeof(p), 0); recv(sockfd, buf, sizeof(buf), 0); buf[numbytes] = '\0'; printf("Received: %s",buf); close(sockfd); return 0; } The server I put was a server running on IRC's EFNet network. I was supposed to receive a message, from the server, right? Not only this doesn't happen, my program also crashes without showing anything! (the debugger comes up with no explanation). Any ideas why? Actually, I must have made a newbie mistake due to my ignorance. Please enlighten me. __________________ Project::Soulstorm (personal homepage) Last edited by Soulstorm; Mar 1st, 2008 at 11:27 AM.

Mar 1st, 2008, 7:51 PM	#2
Game_Ender Professional Programmer Join Date: May 2006 Location: Maryland, USA Posts: 306 Rep Power: 3	Re: Simple BSD Sockets Problem Where is your server code? Well you aren't check the return of "gethostbyname" which could very well be NULL. That would be a quick crash. You also don't zero out you sockaddr_in structure before using it. Here is some code I have that does a similar thing: cpp Syntax (Toggle Plain Text) memset(&m_addr, 0, sizeof(m_addr)); m_addr.sin_family = AF_INET; // host byte order m_addr.sin_port = htons(port); // short, network byte order m_addr.sin_addr = ((struct in_addr )he->h_addr); memset(&m_addr, 0, sizeof(m_addr)); m_addr.sin_family = AF_INET; // host byte order m_addr.sin_port = htons(port); // short, network byte order m_addr.sin_addr = ((struct in_addr )he->h_addr); I think you are trying to do something simular with this: cpp Syntax (Toggle Plain Text) memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero); memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero); Wikipedia has good article here. __________________ Robotics @ Maryland AUV Team - Software Lead

Mar 2nd, 2008, 4:07 PM	#4
The Dark Expert Programmer Join Date: Jun 2005 Posts: 816 Rep Power: 4	Re: Simple BSD Sockets Problem You don't set numBytes to a value before using it. This code: (Toggle Plain Text) buf[numbytes] = '\0'; buf[numbytes] = '\0'; could then try to write to anywhere.

Mar 2nd, 2008, 4:34 PM	#5
Dameon Troll Join Date: Apr 2005 Location: Texas Posts: 732 Rep Power: 4	Re: Simple BSD Sockets Problem recv will return -1 on error, 0 upon disconnection, or otherwise the number of bytes read. You should probably use that for numbytes. This is from 'man recv' So make sure to properly handle the return value. I've only skimmed so that is likely not the only issue. __________________ MD5(sig) = bcef75433db02e9ad9bf81d6f7c5c270

Mar 3rd, 2008, 7:16 AM	#6
Game_Ender Professional Programmer Join Date: May 2006 Location: Maryland, USA Posts: 306 Rep Power: 3	Re: Simple BSD Sockets Problem Yes you should be checking the return of every socket call. I usually use wrapper function which do it for me. I found the IRC spec and I am not sure you are following it correctly. I think you need both "CR" and "LF" which would mean messages end in "\r\n". See more here. You should also be able to telnet into the server and test out the commands manually if you wish. __________________ Robotics @ Maryland AUV Team - Software Lead

Mar 3rd, 2008, 8:34 AM	#7
Soulstorm Hobbyist Programmer Join Date: Jan 2006 Location: Menidi, Athens, Greece Posts: 234 Rep Power: 3	Re: Simple BSD Sockets Problem Actually, this code (Toggle Plain Text) #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <errno.h> #include <string.h> #include <netdb.h> #include <sys/types.h> #include <netinet/in.h> #include <sys/socket.h> #define PORT 6667 // the port client will be connecting to #define MAXDATASIZE 2000 // max number of bytes we can get at once int main(int argc, char argv[]) { int sockfd, numbytes; char buf[MAXDATASIZE]; struct hostent he; struct sockaddr_in their_addr; // connector's address information he = gethostbyname("efnet.teleglobe.net"); sockfd = socket(PF_INET, SOCK_STREAM, 0); if (sockfd < 0) { printf("error creating the socket\n"); } their_addr.sin_family = AF_INET; their_addr.sin_port = htons(PORT); their_addr.sin_addr = ((struct in_addr )he->h_addr); memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero); if (connect(sockfd, (struct sockaddr )&their_addr, sizeof their_addr) == -1) { perror("connect\n"); exit(1); } else { printf("You got the connection to the server!\n"); fprintf(stderr,"After send, entering recv loop\n"); numbytes=0; while(numbytes == 0) { fprintf(stderr,"In recv loop\n"); numbytes=recv(sockfd, buf, sizeof(buf)-1, 0); //-1 b/c you will null terminate } fprintf(stderr,"After recv loop, %d bytes read.\n",numbytes); buf[numbytes] = '\0'; fprintf(stderr,"After null term, %d bytes read.\n",numbytes); printf("Received: %s",buf); fflush(stdout); close(sockfd); } return 0; } #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <errno.h> #include <string.h> #include <netdb.h> #include <sys/types.h> #include <netinet/in.h> #include <sys/socket.h> #define PORT 6667 // the port client will be connecting to #define MAXDATASIZE 2000 // max number of bytes we can get at once int main(int argc, char argv[]) { int sockfd, numbytes; char buf[MAXDATASIZE]; struct hostent he; struct sockaddr_in their_addr; // connector's address information he = gethostbyname("efnet.teleglobe.net"); sockfd = socket(PF_INET, SOCK_STREAM, 0); if (sockfd < 0) { printf("error creating the socket\n"); } their_addr.sin_family = AF_INET; their_addr.sin_port = htons(PORT); their_addr.sin_addr = ((struct in_addr )he->h_addr); memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero); if (connect(sockfd, (struct sockaddr )&their_addr, sizeof their_addr) == -1) { perror("connect\n"); exit(1); } else { printf("You got the connection to the server!\n"); fprintf(stderr,"After send, entering recv loop\n"); numbytes=0; while(numbytes == 0) { fprintf(stderr,"In recv loop\n"); numbytes=recv(sockfd, buf, sizeof(buf)-1, 0); //-1 b/c you will null terminate } fprintf(stderr,"After recv loop, %d bytes read.\n",numbytes); buf[numbytes] = '\0'; fprintf(stderr,"After null term, %d bytes read.\n",numbytes); printf("Received: %s",buf); fflush(stdout); close(sockfd); } return 0; } makes my computer connect to the server, and then receives a message saying "NOTICE AUTH ** Processing connection to efnet.teleglobe.net" That's all. However, I believe I was supposed to see more than this. Perhaps it's the problem that Game_Ended noticed about line endings. __________________ Project::Soulstorm (personal homepage)

Mar 3rd, 2008, 2:50 PM	#8
Soulstorm Hobbyist Programmer Join Date: Jan 2006 Location: Menidi, Athens, Greece Posts: 234 Rep Power: 3	Re: Simple BSD Sockets Problem OK, I may need some explanation on sockets. As far as I understand the general approach is: Specify a host connect to the host using connect() give a command and then, accept the responce from the server for this command using the recv function However, I read upon telnet (wikipedia), but I don't quite understand what it is. And I really don't understand what game_ender said about CR and LF. I'm sorry if my questions sound newbish, but I haven't been involved with communication protocols or any kind of networking with programming and I need some basic questions answered... __________________ Project::Soulstorm (personal homepage)

Mar 3rd, 2008, 8:59 PM	#9
Game_Ender Professional Programmer Join Date: May 2006 Location: Maryland, USA Posts: 306 Rep Power: 3	Re: Simple BSD Sockets Problem From what I understand telnet is a program that lets you connect to another server on a given port, and then send and receive plan text over a TCP connection. Since the IRC protocol is plain text you can talk to a server "by hand". Now the issue is that TCP is a stream oriented protcol (its just like reading from a file), you have no idea how many bytes someone has sent you. So you have to build something into your communications protocol to be able tell when messages start in end. In the IRC protocol they want you to put "\r\n" at the end of every message. That is how the server knows you are done sending. Here is an example: c Syntax (Toggle Plain Text) /* Bad / char p[] = "join #macfilez"; / Good / char p[] = "join #macfilez\r\n"; / Bad / char p[] = "join #macfilez"; / Good */ char p[] = "join #macfilez\r\n"; __________________ Robotics @ Maryland AUV Team - Software Lead

Mar 3rd, 2008, 9:00 PM	#10
Game_Ender Professional Programmer Join Date: May 2006 Location: Maryland, USA Posts: 306 Rep Power: 3	Re: Simple BSD Sockets Problem You should really make a dumb server which just prints whatever it is sent so that you at least know what are you sending is correct. __________________ Robotics @ Maryland AUV Team - Software Lead

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Simple BSD Sockets Problem