How to Write a Program in C to detect the Ethernet Cable?

christyyim · 03-11-2009, 04:40 AM

I wish to write a C program to detect whether the Ethernet cable is plugged or unplugged. I found out by using a command "nm-tool" in Linux terminal will show me whether a Ethernet cable is plugged or not. If Ethernet cable is plugged, in the device part of eth0, the Hardware Link of Wired Settings will indicate a "yes" and "no" if no Ethernet cable.

Hence, in my previous code, I use one function called popen to read the state as shown below:

PHP Code:



f1 = popen( "nm-tool | grep Link", "r" );         
            while ( fgets( cmd_line, sizeof cmd_line, f1 )) 
            { 
                printf( "%s", cmd_line ); 
              } 
               pclose( f1 ); 
 
            if( strstr( cmd_line, "no" ) == NULL ) 
            { 
                printf( "LAN cable is plugged\n"); 
            }

However, now my project wish to not use the NetworkManager (where the "nm-tool" command comes from). And this gives me trouble to detect the Ethernet cable. So is there any other method for me to detect the Ethernet cable in C programming?

Any advice is welcomed and appreciated. Thanks for help.

Christyyim

Maligree · 03-11-2009, 04:48 AM

Have you heard of ifplugd? It's a deamon that sets up the appropriate interface when a cable is plugged in. Check its source and see how it's done?

christyyim · 03-11-2009, 05:13 AM

The reason I cannot use NetworkManager is that I need to set a static IPv6 address permanently to eth0. Once I set the static IP permanently, the NetworkManager will be disabled which make me cannot use the "nm-tool" command because it will give me error. Therefore, I wonder will the ifplugd daemon will give me the same problem? Sorry because I did not state clearly in the previous post.

Thanks.

Christyyim

gnashley · 03-11-2009, 05:21 AM

There is a program out there that does exactly what you want -it's called 'ethwireck'
(Ethernet Wire Check). It returns true if the ethernet port is connected to a live wire and false if not. It's very handy for use in init scripts where you are connecting using dhcp and don't want to waste time waitning for a timeout if the connection is not even live.

ddaemonunics · 03-11-2009, 05:51 AM

ethtool can help you to see if the link is present or not
also ip link show can tell if the CARRIER is present or not

christyyim · 03-11-2009, 10:26 AM

First of all, thanks for all replies.

I had tried ethwireck, but it cannot detect my ethernet cable. It gives me the error as follow:

Code:

christyyim@christyyim-laptop:~/Desktop/ethwireck-1.2$ ./ethwireck -v
ETHTOOL_GLINK failed: Operation not permitted
SIOCGMIIPHY on eth0 failed: Operation not permitted
SIOCDEVPRIVATE on eth0 failed: Operation not supported
eth0: could not determine status

Besides that, I had tried ethtool in the terminal, but I need to type sudo to get the information:

PHP Code:



sudo ethtool eth0

and this will require my password and i cannot use the "popen" command in my C program in this case. And for your information, I am a beginner in programming

, quite a big task for me to port the source code from ethtool into my program since when I tried just now, I cannot link some of their header files and libraries, keep giving me a lot of error

So I wonder is there other method for me to detect the Ethernet cable?

Thanks in advance.

Christyyim

ddaemonunics · 03-12-2009, 01:30 AM

Well for sudo....modify /etc/sudoers so that you don't have to be prompted for a password for that command and the user that is running as....this is if you don't want to port your application on other systems

jf.argentino · 03-12-2009, 02:51 AM

If you have the "/sys" pseudo file system, just get the content of "/sys/class/net/eth0/carrier", 0 means you're unplugged

christyyim · 03-13-2009, 12:16 AM

yah, thanks jf.argentino. You solved my problem. Thanks.