[SOLVED] Dereferencing pointer error during compilation

zak100 · 09-29-2010, 12:21 PM

Hi,
I have got following code from a book:

Code:

#include <sys/types.h>

#include <sys/socket.h>

#include <netinet/in.h>

#include <string.h>

#include <stdlib.h>


#define PORTNUMBER  12345


main(){

	char buf[1024];

	int n, s, ns, len;

	struct sockaddr_in  name;
        char hostname[64];

	struct hostent *hp;


	if(gethostname(hostname, sizeof(hostname)) < 0) {

		perror("gethostname");

		exit(1);

	}


	if(hp= gethostbyname(hostname)) == NULL) {

		fprintf(stderr, "unknown host: %s. \n", hostname);

		exit(1);

	}



	if((s=socket(AF_INET, SOCK_STREAM, 0)) < 0) {

		perror ("socket");

		exit(1);

	}


	memset(&name, 0, sizeof(struct sockaddr_in));

	name.sin_family = AF_INET;

	name.sin_port=htons(PORTNUMBER);

	memcpy(&name.sin_addr, hp->h_addr_list[0], hp->h_length);

	len=sizeof(struct sockaddr_in);


	if(connect(s, (struct  sockaddr *) &name, len) < 0) {

		perror("connect");

		exit(1);

	}

        while((n=read(0, buf, sizeof(buf))) > 0) {

		if(send(s, buf, n, 0) < 0) {

			perror ("send");

			exit(1);

		}

	}

	close(s);

	exit(0);

}

When I am compiling this, I am getting following error:
Dereferencing pointer to incomplete type on the following line:

Code:

memcpy(&name.sin_addr, hp->h_addr_list[0], hp->h_length);

Can somebody plz help me with this?

Zulfi.

dugan · 09-29-2010, 01:27 PM

Quote:

Originally Posted by zak100

plz

This writing style is not appropriate here.

johnsfine · 09-29-2010, 01:42 PM

This line

Code:

	struct hostent *hp;

is both a forward declaration of the struct hostent and a declaration/definition of the pointer hp. But there is no definition of struct hostent.

C, unfortunately, lets you use functions such as gethostname, without declaring them. So when you fail to include the required .h file, you don't get a very clear error message for not defining hostent and even less indication of not declaring gethostname, etc.

I did a quick google search of those symbols and found pages saying the required .h file is

netdb.h

But those pages said "BSD" and didn't say "Linux" so I'm not certain netdb.h is the correct include file for you in Linux (I think it is). I'm also not certain you're asking the question for Linux, not Windows. But anyway, you need to include the right .h file and that is probably netdb.h

It is important to know that "incomplete type" means it has been declared but not defined.

Sergei Steshenko · 09-29-2010, 02:08 PM

Quote:

Originally Posted by johnsfine

...
I did a quick google search of those symbols and found pages saying the required .h file is

netdb.h

But those pages said "BSD" and didn't say "Linux" so I'm not certain netdb.h is the correct include file for you in Linux (I think it is).
...

On my Linux box:

Code:

sergei@amdam2:~> man -k hostent
endhostent (3)       - get network host entry
endhostent (3p)      - network host database functions
freehostent (3)      - get network hostnames and addresses
gethostent (3)       - get network host entry
gethostent (3p)      - network host database functions
gethostent_r (3)     - get network host entry
Net::hostent (3pm)   - by-name interface to Perl's built-in gethost*() functions
sethostent (3)       - get network host entry
sethostent (3p)      - network host database functions
sergei@amdam2:~>

- yes, it's also

Code:

#include <netdb.h>

.

Sergei Steshenko · 09-29-2010, 02:50 PM

Quote:

Originally Posted by johnsfine

...
C, unfortunately, lets you use functions such as gethostname, without declaring them.
...

Well, the real problem is "C" books for beginners. A book for beginner should teach compilation with the most strict set of compiler switches.

A good start WRT compiler switches is

-Wall -Wextra -Wformat=2 -Wmissing-prototypes

and a good place to start from reading about such switches is:

http://gcc.gnu.org/onlinedocs/gcc-4.4.4/gcc.pdf ->
3.8 Options to Request or Suppress Warnings
.

zak100 · 09-30-2010, 12:21 AM

Hi,
Actually, I dont know about the appropriate style you want. I just copied the code and pasted it within the code tags.

Thanks for help. Its working now.Its great because there is less unix/linux culture in my country.

Zulfi.

johnsfine · 09-30-2010, 07:34 AM

Quote:

Originally Posted by zak100

I dont know about the appropriate style you want.

Dugan was objecting to the way you abbreviated "please".

Many of the experts here object to posts that are full of such abbreviations. But I can't see the point of caring about a single instance in an otherwise appropriate post.

You'll notice Dugan's objection did not interfere with your getting the answer you needed, so it's best to just ignore. (I'm only explaining all this because his objection seems to have confused you).

zak100 · 07-14-2014, 12:40 AM

Hi,
Sorry for not replying it earlier. But now I am going to use this forum again so i would avoid such abbreviations. Thanks for pointing out.

Zulfi.

NevemTeve · 07-14-2014, 01:34 AM

The beginning of the program could be like this:

Code:

/* programname.c */

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

(Sorry for the on-topic)