pmdaInit(3) initialize a PMDA

Other Alias

pmdaRehash, pmdaSetFlags


#include <pcp/pmapi.h>
#include <pcp/impl.h>
#include <pcp/pmda.h>

void pmdaInit(pmdaInterface *dispatch, pmdaIndom *indoms, int nindoms, pmdaMetric *metrics, int nmetrics);

void pmdaRehash(pmdaExt *pmda, pmdaMetric *metrics, int nmetrics);

void pmdaSetFlags(pmdaInterface *dispatch, int flags);

cc ... -lpcp_pmda -lpcp


pmdaInit initializes a PMDA so that it is ready to receive PDUs from pmcd(1). The function expects as arguments the instance domain table (indoms) and the metric description table (metrics) that are initialized by the PMDA. The arguments nindoms and nmetrics should be set to the number of instances and metrics in the tables, respectively.

Much of the pmdaInterface structure can be automatically initialized with pmdaDaemon(3), pmdaGetOpt(3) and pmdaDSO(3). pmdaInit completes the PMDA initialization phase with three operations. The first operation adds the domain and instance numbers to the instance and metric tables. Singular metrics (metrics without an instance domain) should have the instance domain PM_INDOM_NULL set in the indom field of the pmDesc structure (see pmLookupDesc(3)). Metrics with an instance domain should set this field to be the serial number of the instance domain in the indoms table.

The instance domain table may be made empty by setting indoms to NULL and nindoms to 0. This allows the caller to provide custom Fetch and Instance callback functions. The metric table may be made empty by setting metrics to NULL and nmetrics to 0. This allows the caller to provide custom Fetch and Descriptor callback functions.


For example, a PMDA has three metrics: A, B and C, and two instance domains X and Y, with two instances in each instance domain. The instance domain and metrics description tables could be defined as:

static pmdaInstid _X[] = {
    { 0, "X1" }, { 1, "X2" }
static pmdaInstid _Y[] = {
    { 0, "Y1" }, { 1, "Y2" }
static pmdaIndom indomtab[] = {
#define X_INDOM 0
    { X_INDOM, 2, _X },
#define Y_INDOM 3
    { Y_INDOM, 2, _Y }
static pmdaMetric metrictab[] = {
/* A */
    { (void *)0, 
        { 0,0,0,0,0,0} }, },
/* B */
    { (void *)0, 
        { 0,0,0,0,0,0} }, },
/* C */
    { (void *)0, 
        { 0,1,0,0,PM_TIME_SEC,0} }, }

The metric description table defines metric A with no instance domain, metric B with instance domain X and metric C with instance domain Y. Metric C has units of seconds, while the other metrics have no units (simple counters). pmdaInit will take these structures and assign the PMDA(3) domain number to the it_indom field of each instance domain. This identifier also replaces the indom field of all metrics which have that instance domain, so that they are correctly associated.

The second stage opens the help text file, if one was specified with the -h command line option (see pmdaGetOpt(3)) or as a helptext argument to pmdaDSO(3) or pmdaDaemon(3).

The final stage involves preparing the metric table lookup strategy.


When fetch and descriptor requests are made of the PMDA, each requested PMID must be mapped to a metric table entry. There are currently three strategies for performing this mapping - direct, linear and hashed. Each has its own set of tradeoffs and an appropriate strategy should be selected for each PMDA.

If all of the metric PMID item numbers correspond to the position in the metrics table, then direct mapping is used. This is the most efficient of the lookup functions as it involves a direct array index (no additional memory is required nor any additional processing overhead). If the PMID numbering requirement is met by the PMDA, it is ideal. This strategy can be explicitly requested by calling pmdaSetFlags(pmda, PMDA_FLAG_EXT_DIRECT) before calling pmdaInit. In this case, if the direct mapping is not possible (e.g. due to an oversight on the part of the PMDA developer), a warning is logged and the linear strategy is used instead.

The second strategy (linear search) is the default, when a direct mapping cannot be established. This provides greater flexibility in the PMID numbering scheme, as the PMDA item numbers do not have to be unique (hence, the PMID cluster numbers can be used more freely, which is often extremely convenient for the PMDA developer). However, lookup involves a linear walk from the start of the metric table until a matching PMID is found, for each requested PMID in a request.

The third strategy (hash lookup) can be requested by calling pmdaSetFlags(pmda, PMDA_FLAG_EXT_HASHED) before calling pmdaInit. This strategy is most useful for PMDAs with large numbers of metrics (many hundreds, or thousands). Such PMDAs will almost always use the cluster numbering scheme, so the direct lookup scheme becomes inappropriate. They may also be prepared to sacrifice a small amount of additional memory for a hash table, mapping PMID to metric table offsets, to speed up lookups in their vast metric tables.

This final strategy can also be used by PMDAs serving up dynamically numbered metrics. For this case, the pmdaRehash function should be used to replace the metric table when new metrics become available, or existing metrics are removed. The PMID hash mapping will be recomputed at the same time that the new metric table is installed.


pmdaInit will set dispatch->status to a value less than zero if there is an error that would prevent the PMDA(3) from successfully running. pmcd(1) will terminate the connection to the PMDA(3) if this occurs.

pmdaInit may issue any of these messages:

PMDA interface version interface not supported
The interface version is not supported by pmdaInit.
Using pmdaFetch() but fetch call back not set
The fetch callback, pmdaFetch(3), requires an additional callback to be provided using pmdaSetFetchCallBack(3).
Illegal instance domain inst for metric pmid
The instance domain inst that was specified for metric pmid is not within the range of the instance domain table.
No help text path specified
The help text callback, pmdaText(3), requires a help text file for the metrics to have been opened, however no path to the help text was specified as a command line option, or as an argument to pmdaDSO(3) or pmdaDaemon(3). This message is only a warning.
Direct mapping for metrics disabled @ num
The unit numbers of the metrics did not correspond to the index in the metric description table. The direct mapping failed for metric number num in the metrics table. This is less efficient but is not fatal and the message is only a warning.
Hashed mapping for metrics disabled @ num
A memory allocation failure occurred while building the hash table to index the metric description table. This is a non-fatal warning message - a fallback to linear searching will be automatically performed should this situation arise.


The PMDA must be using PMDA_INTERFACE_2 or later, as specified in the call to pmdaDSO(3) or pmdaDaemon(3).