sysctl(3) man page examples

[Patrick Kelsey]

On Sat, Jun 27, 2015 at 6:02 PM, Sean Bruno <sbruno at ignoranthack.me> wrote:

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> sysctl(3) specifies three easy to understand examples.
>
> The first appears to depend on a FreeBSD libc() function or library that
> is missing, "printkproc()".  Is this a deprecated/deleted function from
> the past?
>

This example was committed in r71409, which was between the 4.2 and 4.3
releases.  Today, and at that time, the result of fetching a particular
kern.proc.pid is a struct kinfo_proc, not struct kinfo_kproc (which did and
does not exist).  There appears to never have been a printkproc() function
(nor print_kproc(), nor printproc(), nor print_proc()) - this seems to be a
function that is assumed to exist elsewhere in the unseen parts of the
example program.


>
> The second example works just fine.
>
> The third accesss user.cs_path which seems to be empty across all
> platforms.  I'm not sure if we should replace this example with
> something more meaningful(that is to say that its proper for
> user.cs_path to be empty) or if there is a bug causing user.cs_path to
> be empty.
>
>
This appears to be a bug that was introduced almost three years ago in
r240176. sysctl() in lib/libc/gen/sysctl.c has special handling for
USER_CS_PATH that returns the value of _PATH_STDPATH, which is
"/usr/bin:/bin:/usr/sbin:/sbin".  However, this special handling for
USER_CS_PATH was short circuited by r240176, which introduced the
requirement that __sysctl() return ENOENT in order to reach the special
USER_CS_PATH handling.  However, __sysctl() doesn't return ENOENT for
USER_CS_PATH because there is a sysctl entry for it (containing an empty
string) that is created in sys/kern/kern_mib.c, apparently so that
user.cs_path exists when enumerating the names in the sysctl tree.

-Patrick