Writing a Computed Attribute Plug-In

This section demonstrates how to write a plug-in that generates the value of an attribute, currentTime.

Only code excerpts are shown in this chapter. The complete code example can be found where you installed Directory Server, install-path/ds6/examples/computed.c.

Initializing the Computed Attribute Plug-In

Plug-ins that compute attribute values register the functions of type slapi_compute_callback_t to compute attributes with slapi_compute_add_evaluator() during initialization with the server.

#include "slapi-plugin.h"

int
compute_init(Slapi_PBlock * pb)
{
    int rc = 0;                        /* 0 means success             */
    rc |= slapi_pblock_set(            /* Plug-in API version         */
        pb,
        SLAPI_PLUGIN_VERSION,
        SLAPI_PLUGIN_CURRENT_VERSION
    );
    rc |= slapi_pblock_set(            /* Plug-in description         */
        pb,
        SLAPI_PLUGIN_DESCRIPTION,
        (void *) &comp_desc;           /* See the code for comp_desc. */
    );
    rc |= slapi_compute_add_evaluator(compute_attrs);
    return rc;
}

Here, compute_attrs() is the function that computes the attribute value.

Computing an Attribute Value

When Directory Server receives a request for the computed attribute value, in this case currentTime, the server calls the functions that are registered to compute attributes. Directory Server provides your function the context, attribute type, and entry with which to compute the attribute.

Your function must therefore check the attribute type to see whether your function should handle the computation. If your function does not handle the attribute type in question, return -1. If your function can handle the attribute, your function must determine the value or values of the attribute. Your function must then add the computed content to the result that the server returns. The following code excerpt shows the compute_attrs() function generating a current time string for the attribute value.

#include "slapi-plugin.h"

/* Compute an attribute called currentTime                        */
static int
compute_attrs(
    computed_attr_context  * context,
    char                   * type,
    Slapi_Entry            * entry,
    slapi_compute_output_t   outfn
)
{
    /* If the attribute type is not recognized return -1.         */
    int rc = -1;

    if (slapi_attr_type_cmp("currentTime", type, 2) == 0) {
        Slapi_Attr  * time_attr;
        time_t        now;
        char          current_time[30];
        Slapi_Value * time_value;
        int           rc = 0;

        /* Construct the computed attribute.                      */
        time_attr = slapi_attr_new();
        slapi_attr_init(time_attr, type);
        now = time(NULL);
        ctime_r(&now;, current_time, 30);
        current_time[(int)strlen(current_time) - 1] = '\0';
        time_value = slapi_value_new_string(current_time);
        slapi_attr_add_value(time_attr, time_value);
        slapi_value_free(&time_value;);

        /* Add the attribute to the result returned.              */
        rc = (*outfn)(context, time_attr, entry);
        attr_done(time_attr);
        return rc;

    /* Handle other computed attributes...
    } else if (slapi_attr_type_cmp("myCompAttr", type, 2) == 0) {
    */

    }
    return rc;
}

Notice the lines of code that add the attribute to the result that is returned. The output function in this case is of type slapi_compute_output_t.

Notice also, when constructing the computed attribute value, that the compute_attrs() function removes the newline character from the string in the ctime_r() function. This removal prevents the attribute value from appearing base64-encoded when the value is displayed by the ldapsearch command.

Testing the Computed Attribute Plug-In

Before you can test the plug-in with the ldapsearch command, you must register the plug-in with Directory Server.

To Register the Plug-In

If you have not already done so, build the example plug-in library and activate both plug-in informational logging and the example plug-in.

1. Build the plug-in.

   Hint Use install-path/examples/Makefile or install-path/examples/Makefile64.

2. Configure Directory Server to log plug-in informational messages and load the plug-in.

   Hint Use the commands specified in the comments at the outset of the plug-in source file.

3. Restart Directory Server.

   $ dsadm restart instance-path

To use the Computed Attribute Plug-In

Before You Begin

Register the computed attribute plug-in with Directory Server.

Run a search on the root DSE.

$ ldapsearch -h localhost -p 1389 -b "" -s base "(objectclass=*)" currentTime version: 1 dn: currentTime: Fri Feb 17 09:30:28 2006