Are these available as symbolic constrants ? No, it seems.

Then it is time to add them to sys/sysctl.h and get rid of the numbers in the definition of sysctls from kern_sysctl.c and e.g. libc sysctlnametonib.

spaces around '*'

just a note: int oid[CTL_MAXNAME] is correct (an OID can have CTL_MAXNAME levels in the kernel) but it is useless because the "undocumented interface" can handle up to "CTL__MAXNAME - 2" levels (except 0.3 name2oid), CTL_MAXNAME + 2 for qoid does not solve (unfortunately this false solution is quite widespread)

I would wait to document the magical nodes yet, https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=240390

here the problem (from the comment above), len + 2 could be 26 > CTL_MAXNAME

I couldn't find any obvious CTL_MAXNAME - 2 (apart of libjail, TBC) so if there is a truncation, it's probably because one of the in-kernel functions doesn't use CTL_MAXNAME + 2. I'll take a look at this but it's out of scope for this review anyway.

I matched the names with the names functions responsible as used by kern/kern_sysctl.c Hope that helps.

Spaces around '+'.

I suggest to print the rest of the input mib in numeric format.

print numeric value as well

Why not print the mib in <num>.<num>... format, esp. since you already fetched it ? The pointer as useless as it can be.

The types of the second and fifth args are size_t, not int. It practically matters on 64bit big-endian hosts.

Spaces around '+' and '-', there and till the end of the chunk.