Get the dev.cpu.0.temperature from sysctl(3)

Roland Smith rsmith at
Tue May 3 19:40:42 UTC 2011

On Tue, May 03, 2011 at 10:12:33AM +0200, David Demelier wrote:
> Hello,
> I would like to get the dev.cpu.0.temperature node from sysctlbyname(). 
> It seems this node is an opaque type but how to check it and store it to 
> the appropriate variable type ?

The best way to determine this is to read the source. I did that some time ago
to fix the temperature display in sysutils/conky. 

The sysctl dev.cpu.0.temperature returns an integer, see
/sys/dev/coretemp/coretemp.c (look for the string "temperature"), and you'll see:

         * Add the "temperature" MIB to dev.cpu.N.
        sc->sc_oid = SYSCTL_ADD_PROC(device_get_sysctl_ctx(pdev),
            OID_AUTO, "temperature",
            dev, 0, coretemp_get_temp_sysctl, "IK",
            "Current temperature");

If you look at the definition of coretemp_get_temp_sysctl in the same file:

        device_t dev = (device_t) arg1;
        int temp;

        temp = coretemp_get_temp(dev) * 10 + TZ_ZEROC;

        return (sysctl_handle_int(oidp, &temp, 0, req));

So the returned value is an 'int'. Note that TZ_ZEROC is #defined as 2732 at
the beginning of the file. The returned value is therefore the temperature
in Kelvin times ten.

On my machine, it gives e.g.:

    sysctl dev.cpu.0.temperature
	dev.cpu.0.temperature: 46.0C

If we check the 'raw' return value;

    sysctl -b dev.cpu.0.temperature|hd
    00000000  78 0c 00 00                                       |x...|

Running this value with the abovementioned algorithm in reverse through a
calculator, we get

   (0x0c78-2732)/10 = 46°C

Hope this helps.

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