I don't think this file is supposed to be here?

We don't normally have a LICENSE file. Just put the appropriate license into the C and include files.

Please move the examples into the examples directory in share.

This (and SRCS/INCS/INCSDIR) should be indented with tabs.

This should be indented with a tab.

superfluous space after "do".

s/accomodating/accommodating/

Superfluous space after "do".

Superfluous space after "do".

Superfluous space after "do".

This will likely be easier to read if the conditions are inverted, i.e. check for error, not success.

Something like:

if ((desc = realloc(desc, descrlen)) == NULL) {
    h->error.errtype = OTHER;
    h->error.errcode = ENOMEM;
    return (-1);
}

ifr.ifr_buffer.buffer = descr;
ifr.ifr_buffer.length = descrlen;

if (libifc_ioctlwrap(h, AF_LOCAL, SIOCGIFHESCR, &ifr) != 0)
    return (-1);

if (ifr.ifr_buffer.length > descrlen) {
    descrlen = ifr.ifr_buffer.length;
    continue;
}

...

The maximum length is configured through the net.ifdescr_maxlen sysctl.
If the name is longer the ioctl() will return ENAMETOOLONG.

I don't think you need to check for this before the ioctl() call. Just handle the error.

It's safe to call free(NULL). You don't need this check.
Also, I don't think it can be NULL at this point. You can trust the kernel to not remove the buffer for you.

The kernel doesn't truncate, and if the name is longer than it likes you'll get an error.

The maximum number of address families (looking at /usr/include/sys/socket.h) seems to be low enough that this shouldn't be required. AF_MAX is 42 (a pleasing number).

Just allocating a fixed array of AF_MAX will waste very little memory, simplify things, and get rid of the lookup loop in libifc_socket().

Chop all the x-mas trees. :)

I agree that allocating a static amount of memory (sizeof(...)*AF_MAX) should incur an insignificant amount of wasted memory, and that the reduction in code complexity is desired.

It does however not remove the need for the lookup loop, as this is required to find the proper socket and create it if it doesn't exist.

Yes, but if you allocate fds[AF_MAX] you can index based on address family and the lookup becomes a simple array access, rather than an iteration of the array.
(so fds[AF_INET], rather than a for loop).

You may want to initialise the sockets array elements to -1, because '0' can be a valid file descriptor.

So != -1 here.

You could assign directly to h->sockets[addressfamily] here.

I think you missed a rename here.