pmdainit(3) — Linux manual page


PMDAINIT(3)             Library Functions Manual             PMDAINIT(3)

NAME         top

       pmdaInit, pmdaRehash, pmdaSetData, pmdaExtGetData,
       pmdaExtSetData, pmdaSetFlags, pmdaSetCommFlags, pmdaExtSetFlags -
       initialize a PMDA

C SYNOPSIS         top

       #include <pcp/pmapi.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);
       void pmdaSetCommFlags(pmdaInterface *dispatch, int flags);
       void pmdaExtSetFlags(pmdaExt *pmda, int flags);

       void pmdaSetData(pmdaInterface *dispatch, void *data);
       void pmdaExtSetData(pmdaExt *pmda, void *data);
       void *pmdaExtGetData(pmdaExt *pmda);

       cc ... -lpcp_pmda -lpcp

DESCRIPTION         top

       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

EXAMPLE         top

       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,
                  { PMDA_PMID(0,0), PM_TYPE_U32, PM_INDOM_NULL, PM_SEM_INSTANT,
                    { 0,0,0,0,0,0} }, },
            /* B */
                { (void *)0,
                  { PMDA_PMID(0,1), PM_TYPE_U32, X_INDOM, PM_SEM_INSTANT,
                    { 0,0,0,0,0,0} }, },
            /* C */
                { (void *)0,
                  { PMDA_PMID(0,2), PM_TYPE_DOUBLE, Y_INDOM, PM_SEM_INSTANT,
                    { 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

METRIC LOOKUP         top

       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_EXT_FLAG_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

       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_EXT_FLAG_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.

METRIC CHANGES         top

       It should be well understood by PMDA authors that metric metadata
       for individual metrics is fixed, and ideally would not ever
       change.  In the situation where metadata is incorrect and is
       updated, such a change requires correction to logged metrics
       using pmlogrewrite(1), and as a result should be avoided whenever

       However, a PMDA may become aware of new domain metrics at
       runtime, and in this case it is ideal to export them immediately
       (without any collector system restart).  In this situation, the
       PMDA can inform all running PMAPI clients that may have already
       explored the metric namespace (for example, using
       pmTraversePMNS(3)) of the change to the metric namespace.

       This is achieved using pmdaSetFlags(pmda, PMDA_EXT_NAMES_CHANGE)
       which will result in the PMCD_NAMES_CHANGE state change
       notification being sent to each PMAPI client on next fetch.  If
       the newly discovered metrics have label metadata associated, then
       the PMDA_EXT_LABEL_CHANGE flag may also be set, which will result
       in the PMCD_LABEL_CHANGE notification being sent as well.

       pmdaExtSetFlags is equivalent to pmdaSetFlags, and is provided as
       a convenience interface in situations where the pmdaExt is more
       readily available than the pmdaInterface structure.


       Agents that make use of authentication or container attributes
       should indicate this using the pmdaSetCommFlags interface.  This
       indicates the need for these attributes to be communicated on the
       channel between the PMDA and pmcd or local context client.  Valid
       flags are PMDA_FLAG_AUTHORIZE (for authentication related
       attributes) and PMDA_FLAG_CONTAINER (for container name related

PRIVATE DATA         top

       A facility for associating private PMDA data with the pmdaExt
       structure is available.  This allows a PMDA to associate an
       arbitrary (and typically not global) pointer with the pmdaExt
       such that it can be later obtained during callbacks.  The
       interfaces for setting this pointer are pmdaSetData and
       pmdaExtSetData, and pmdaExtGetData for subsequently retrieving

CAVEAT         top

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

       The PMDA must use PMDA_INTERFACE_7 or later to issue state change
       notifications using pmdaSetFlags or pmdaExtSetFlags.

DIAGNOSTICS         top

       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.

SEE ALSO         top

       newhelp(1), pmcd(1), pmlogrewrite(1), PMAPI(3), PMDA(3),
       pmdaDaemon(3), pmdaDSO(3), pmdaFetch(3), pmdaGetOpt(3),
       pmdaText(3), pmLookupDesc(3) and pmTraversePMNS(3).

COLOPHON         top

       This page is part of the PCP (Performance Co-Pilot) project.
       Information about the project can be found at 
       ⟨⟩.  If you have a bug report for this manual
       page, send it to  This page was obtained from the
       project's upstream Git repository
       ⟨⟩ on 2023-12-22.
       (At that time, the date of the most recent commit that was found
       in the repository was 2023-12-16.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
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       (which is not part of the original manual page), send a mail to

Performance Co-Pilot               PCP                       PMDAINIT(3)

Pages that refer to this page: pmlogger(1)pmda(3)pmdacache(3)pmdaconnect(3)pmdadso(3)pmdafetch(3)pmdahelp(3)pmdalabel(3)pmdatext(3)