using C code to get same info as ifconfig

Question

Is there a way in Linux, using C code, to get the same information that "ifconfig eth0" would return? I'm interested in things like IP address, link status, and MAC address.

Here's sample output from ifconfig:

eth0      Link encap:Ethernet  HWaddr 00:0F:20:CF:8B:42           inet addr:217.149.127.10  Bcast:217.149.127.63  Mask:255.255.255.192           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1           RX packets:2472694671 errors:1 dropped:0 overruns:0 frame:0           TX packets:44641779 errors:0 dropped:0 overruns:0 carrier:0           collisions:0 txqueuelen:1000           RX bytes:1761467179 (1679.8 Mb)  TX bytes:2870928587 (2737.9 Mb)           Interrupt:28  

Answer 1

One way to get to the bottom of problems like this, particularly in cases when you don't have source, is strace.

It gives you a list of all the system calls made by any program you pass it, along with their arguments and return values. If your program just dumps some info and quits rather than running for an extended time it can be pretty straightforward to just do a man on all the system calls you see that look like they might provide the info you're looking for.

When I run

strace ifconfig 

Some of the interesting calls are:

open("/proc/net/dev", O_RDONLY)         = 6 

followed by a bunch of ioctls, corroborating @payne's answer:

ioctl(5, SIOCGIFFLAGS, {ifr_name="eth0",    ifr_flags=IFF_UP|IFF_BROADCAST|IFF_RUNNING|IFF_MULTICAST}) = 0 ioctl(5, SIOCGIFHWADDR, {ifr_name="eth0", ifr_hwaddr=84:2b:2b:b7:9e:6d}) = 0 ioctl(5, SIOCGIFMETRIC, {ifr_name="eth0", ifr_metric=0}) = 0 ioctl(5, SIOCGIFMTU, {ifr_name="eth0", ifr_mtu=1500}) = 0 

Answer 2

Yes, ifconfig itself is written in C. :) See: http://cvsweb.netbsd.org/bsdweb.cgi/src/sbin/ifconfig/ifconfig.c?rev=1.169&content-type=text/x-cvsweb-markup

Do man netdevice to see the details (on Linux). You use the ioctl() system call.