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

[christyyim]

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:

 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

[the_ultimate_samurai]

perhaps ifupdown or ifconfig?

[christyyim]

thanks the_ultimate_samurai.

ifconfig can do the task by using command

ifconfig eth0|sed -n '/running/I p'

if there is a cable, it will show "UP BROADCASTRUNNINGMULTICAT", else there will be nothing.

another alternative is to get the content of /sys/class/net/eth0/carrier, "0" means cable is unplugged and "1" means cable is plugged.

[blinky]

how about ethtool

eg. ethtool eth0 | grep "Link detected"

should give:

Link detected: yes

or

Link detected: no

[blinky]

Also take a look at files under /sys/class/net/

for eth0 subdirectory /sys/class/net/eth0/
(files carrier, dormant, operstate) change depending on link conditions:

if up + cable	if up + NO cable	if down + cable
=============	==========			==================
carrier 1	carrier 0		cat: carrier: Invalid argument

dormant 0	dormant 0		cat: dormant: Invalid argument

operstate up	operstate down		operstate down

[christyyim]

Thanks blinky.

I had tried "ethtool eth0" before, but in my case, I had to use "sudo" for this command, so not suitable for my case.

And checked the file "/sys/class/net/eth0/carrier" so far work for me.

Thanks for your information