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2.6.2. Common Initialization

As described in Section 2.2.2, “DSO PMDA”, an initialization function is provided by a DSO PMDA and called by PMCD. Using the standard PMDA wrappers, the same function can also be used as part of the daemon PMDA initialization. This PMDA initialization function performs the following tasks:
  • Assigning callback functions to the function pointer interface of pmdaInterface
  • Assigning pointers to the metric and instance tables from pmdaExt
  • Opening the help text files
  • Assigning the domain number to the instance domains
  • Correlating metrics with their instance domains
If the PMDA uses the common data structures defined for the pcp_pmda library, most of these requirements can be handled by the default pmdaInit function; see the pmdaInit(3) man page.
Because the initialization function is the only initialization opportunity for a DSO PMDA, the common initialization function should also perform any DSO-specific functions that are required. A default implementation of this functionality is provided by the pmdaDSO function; see the pmdaDSO(3) man page. Trivial PMDA

Example 2.33, “Initialization in the Trivial PMDA” shows the trivial PMDA, which has no instances (that is, all metrics have singular values) and a single callback. This callback is for the pmdaFetch function called trivial_fetchCallBack; see the pmdaFetch(3) man page:

Example 2.33. Initialization in the Trivial PMDA

static char     *username;
static int      isDSO = 1;              /* ==0 if I am a daemon */

void trivial_init(pmdaInterface *dp)
    if (isDSO)
        pmdaDSO(dp, PMDA_INTERFACE_2, “trivial DSO”,

    if (dp->status != 0)

    pmdaSetFetchCallBack(dp, trivial_fetchCallBack);
    pmdaInit(dp, NULL, 0,
             metrictab, sizeof(metrictab)/sizeof(metrictab[0]));
The trivial PMDA can execute as either a DSO or daemon PMDA. A default installation installs it as a daemon, however, and the main routine clears isDSO and sets username accordingly.
The trivial_init routine provides the opportunity to do any extra DSO or daemon setup before calling the library pmdaInit. In the example, the help text is setup for DSO mode and the daemon is switched to run as an unprivileged user (default is root, but it is generally good form for PMDAs to run with the least privileges possible). If dp->status is non-zero after the pmdaDSO call, the PMDA will be removed by PMCD and cannot safely continue to use the pmdaInterface structure.