using C code to get same info as ifconfig

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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

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