Controlling the Humidity with an Embedded Linux System

netsnmp_register_scalar(
    netsnmp_create_handler_registration(
        "targetHumidity",
        handle_targetHumidity,
        targetHumidity_oid,
        OID_LENGTH(targetHumidity_oid),
        HANDLER_CAN_RWRITE));


int
handle_targetHumidity(netsnmp_mib_handler           *handler,
                      netsnmp_handler_registration  *reginfo,
                      netsnmp_agent_request_info    *reqinfo,
                      netsnmp_request_info          *requests)
{
    switch (reqinfo->mode) {

    case MODE_GET:
        break;

    case MODE_SET_RESERVE1:
        break;

    case MODE_SET_COMMIT:
        break;
    }

    return SNMP_ERR_NOERROR;
}

The code shown in Listing 6 makes reference to the generated callback procedure, handle_targetHumidity, which is supplied in skeletal form only by mib2c. Not much code is needed in order to support scalar OIDs, which the humidity controller uses exclusively. Anytime a specific OID, in this case the targetHumidity OID .1.3.6.1.4.1.2200.2.3, has an operation performed, the snmpd dæmon will invoke this callback procedure with an indication of the requested operation being performed on the OID.

I rebuilt the snmpd dæmon so that the newly created humidity controller MIB structure and generated framework code could be supported. Before rebuilding the snmpd dæmon, the new MIB must be configured into the build environment. This is accomplished easily with the following command:

$ ./configure --with-mib-modules="humidityController"

Once configured, the entire net-snmp package was rebuilt with the make command. Once the snmpd dæmon was rebuilt, I tested the new MIB structure by using the net-snmp command-line interface utilities snmpset and snmpget. For example, in order to set the targetHumidity OID to 50% rH, the following command can be issued:

$ snmpset -Ovqe -v 1 -c private localhost targetHumidity.0 i 50

Note the use of relative, symbolic OIDs in the snmpset command. The actual OID .1.3.6.1.4.1.2200.2.3 could be used as well, because it's statically defined and should never change. But, I prefer symbolic references where possible, as it helps in readability. The -Ovqe switch controls the output format that results from the snmpset. Although I built the net-snmp package to support all three major versions of SNMP (1, 2 and 3), I really needed only basic version 1 support, which is why the -v 1 switch appears. The SNMP community string is indicated by the -c private switch and appears in set operations because only private communities are permitted to set OID values (this is a one-time option when the snmpd dæmon is configured).

The humidity controller MIB can be viewed with a tool included in net-snmp called mbrowse. mbrowse is a GUI that bolts onto the system MIB structure and permits manipulation of specific OIDs. Figure 4 shows a screenshot of mbrowse and the humidity controller MIB tree branch.

Figure 4. mbrowse Screenshot

Once the snmpd dæmon was complete with support for the newly added humidity controller OIDs, I was able to complete the user-mode application code. Listing 7 contains the complete user-mode application, and it is too long to print here, but it is available on the LJ FTP site (see Resources). It is very typical of an embedded application, as it perpetually reads data and then takes actions on the data. Note the use of snmpget and snmpset. The net-snmp package does include APIs for both C and Perl, but I decided it was simpler to leverage the existing snmpget and snmpset utilities.

To finish off the humidity controller, I added a Web page interface that includes a recipe that uses a tad of HTML, a smattering of JavaScript and a pinch of AJAX with server-side scripting to create an end-user browser interface. The humidity controller in a Firefox browser looks like what is shown in Figure 5. The targetHumidity (targetH) cell in the table has a JavaScript function associated such that editing is possible, and when a new value is entered, it is POSTed to Apache. Apache will invoke a Perl script to set the target humidity in the SNMP MIB. Listing 8 is an excerpt from the Perl code that shows the SNMP actions. The other cells are read-only and are refreshed periodically with values from the SNMP MIB with the help of a second Perl script, humidityController.cgi. This second Perl script pushes out only the data necessary to generate the table of values shown in Figure 5.

Figure 5. Humidity Controller and Firefox