C program to check if network connection available or not

steffin · 06-03-2011, 12:53 AM

Hi,

I am a newbie with Linux and I am writing a simple C program which has to find whether the system is connected to network/internet or not..

Can please someone help?

jschiwal · 06-03-2011, 04:15 AM

Moved: This thread is more suitable in Programming and has been moved accordingly to help your question get the exposure it deserves.

markush · 06-03-2011, 05:06 AM

Hello steffin, welcome to LQ,

do you have any requirements how your program should check for the connection?

Note that it is a (more or less) big difference between "connected to the network" and "connected to the internet".

If you want to check for a network-connection, your program may scan the output of the ifconfig-command (which can be done with a simple shellscript and does not require C).

How about your experience with C and programming in general?

What are your ideas yet, did you yet write any code for your program?

Note that you'll find the manpages for the C-librarie in section 3, so the command

Code:

apropos network | grep -i \(3\)

will give you an overview about all network-related library-functions and may give you an idea where to search.

Markus

steffin · 06-03-2011, 05:25 AM

Thanks markush for the reply..

What I want is to check whether the system is connected to the network or not..
I want the C program to get the status of the network connectivity..

This is the code I used to get the IP address of the system..Its working in Ubuntu environment..But failing in the platform environment which I actually want because the iocntl functionality is not working...

Quote:

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <net/if.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/ioctl.h>

int main(void)
{
int fd;
struct if_nameindex *curif, *ifs;
struct ifreq req;

if((fd = socket(PF_INET, SOCK_DGRAM, 0)) != -1) {
ifs = if_nameindex();
if(ifs) {
for(curif = ifs; curif && curif->if_name; curif++) {
strncpy(req.ifr_name, curif->if_name, IFNAMSIZ);
req.ifr_name[IFNAMSIZ] = 0;
if (ioctl(fd, SIOCGIFADDR, &req) < 0)
perror("ioctl"); //Getting Error here
else
printf("%s: [%s]\n", curif->if_name,
inet_ntoa(((struct sockaddr_in*) &req.ifr_addr)->sin_addr));
}
if_freenameindex(ifs);
if(close(fd)!=0)
perror("close");
} else
perror("if_nameindex");
} else
perror("socket");
return 0;
}

I dont know why..Can you specify any Linux APIs which we can use in the C program?

Heraton · 06-03-2011, 05:21 PM

Hi!

I compiled and ran your code on two different machines: Linux Mint 10 (32 bit) and openSUSE 11.3 (64 bit). I did not find anything but one thing:

The interface list you receive is containing all the interfaces. And this includes the interfaces which are down (via ifconfig or because of not being connected to a switch) or do not have an IPv4 configuration. In both cases the ioctl-call will fail, because there is no ip configuration to receive:

Quote:

ioctl: Cannot assign requested address

As far as i know this is the way ioctl is supposed to behave. ioctl simply tells you, it could not assign a configuration to the struct you told it.

regards, Heraton

steffin · 06-05-2011, 09:34 PM

Thanks for running the code.

I was getting the same error,but not in Ubuntu.Dont know why!!

do you have any ideas abt modifying the code or abt any other code that can get the ip address/the connectivity status to the network.

steffin · 06-06-2011, 05:28 AM

Can someone give a solution for the question above?

Heraton · 06-06-2011, 12:59 PM

Quote:

Originally Posted by steffin

I was getting the same error,but not in Ubuntu.Dont know why!!

Did you double check that there are interfaces on your Ubuntu machine which should cause errors? If all the interfaces have an IP configuration, i see no reason to expect errors... If in doubt, just check via ifconfig that there is no ipv4 configuration before starting your program...

If you have double checked this already, what does your program print for the IP configuration of those interfaces without configuration?

regards, Heraton

steffin · 06-07-2011, 04:14 AM

Quote:

Originally Posted by Heraton

Did you double check that there are interfaces on your Ubuntu machine which should cause errors? If all the interfaces have an IP configuration, i see no reason to expect errors... If in doubt, just check via ifconfig that there is no ipv4 configuration before starting your program...

If you have double checked this already, what does your program print for the IP configuration of those interfaces without configuration?

regards, Heraton

I got this problem solved.I ran some other program and found the IP address of the device.

But the problem is how do I find whether the network connection is available using this available IP address?

markush · 06-07-2011, 07:37 AM

You can ping another host, but if this works depends on the firewall of the other system and yours.
If you're using DHCP and have a valid IP-configuration as well as valid DNS-settings (look at /etc/resolv.conf) the network should be available.

Markus

Heraton · 06-08-2011, 01:38 PM

I still don't understand what exactly your problem was and i would be interested to know what kind of program solved your problem. Anyway, i did some research on the connectivity topic and found some interesting threads...

Network Manager

ifconfig source code

I'm aware you're not going to build your custom kernel, but the ifconfig source might be worth a look. I just took a quick glimpse at it, and must admit i am kind of amazed how easy it is to find your way into it...

From what i've seen, i would start to delve into it in net-tools.../lib/interface.c. I would expect the check for whether the interface is up or not somewhere around line 780 (function ife_print_long) but that's still guessing. Anyway, shouldn't be hard to find.

regards, Heraton