Re: do we have a generic string-number sysctl mapping library ?

On Friday, June 27, 2014 5:14:59 am Luigi Rizzo wrote:
> Hi,
> I have frequently found myself using sysctls to control some kernel
> feature where a string would be a better (and sometimes the only)
> option than using a numeric value, yet the internal representation
> should be numeric for speed and robustness.
> Examples are the kern.timecounter, the default scheduler in dummynet,
> and now in netmap the selection between native and emulated mode.
> I am sure many of you can come up with other cases.
> 
> I wonder if we have some support for that already in the sysctl code,
> or i should build a generic one next time i need to do that.
> 
> Feel free to criticise the approach and suggest better ones.
> Right now i am using sysctls because i have a set of macros
> and wrapper functions that let me convert them to sysfs
> entries when building kernel code on linux, so I have a
> portable solutions.
> 
> For the details, I'd like to have a mechanism that requires the
> kernel programmer supply a (possibly extensible) table of
> supported values, and matching constants to be used within
> the kernel. A single declaration should generate entries
> to get/set the current value as well as list options.
> We can discuss frills (such as wildcards, multiple values,etc).

I am not aware of such a beast.  Even just supporting a simple table to map 
labels to indices would be nice and would handle many cases.  I.e. if you
had something like:

struct sysctl_table vals[] = {
   "foo",  (void *)1,
   "bar",  (void *)2,
   NULL, NULL
};

static int myval;

static int
my_sysctl(SYSCTL_HANDLER_ARGS)
{
    void *val;
    int error;

    val = (void *)myval;
    error = sysctl_handle_table(oidp, vals, &val, req);
    if (error || req->newptr == NULL)
       return (error);
    myval = (intptr_t)val;
    return (0);
}

sysctl_handle_table() would use the initial value to find a suitable string 
from the table for the "old" string, etc.  Using void * for the value would 
let you store arbitrary data, etc.

-- 
John Baldwin