pmdafetch(3) — Linux manual page


PMDAFETCH(3)            Library Functions Manual            PMDAFETCH(3)

NAME         top

       pmdaFetch, pmdaSetFetchCallBack - fill a pmResult structure with
       the requested metric values

C SYNOPSIS         top

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

       int pmdaFetch(int numpmid, pmID *pmidlist, pmResult **resp,
               pmdaExt *pmda);
       void pmdaSetFetchCallBack(pmdaInterface *dispatch,
               pmdaFetchCallBack callback);

       cc ... -lpcp_pmda -lpcp

DESCRIPTION         top

       pmdaFetch is a generic callback used by a PMDA(3) to process a
       fetch request from pmcd(1).  The request from pmcd is initiated
       by a client calling pmFetch(3).

       This is one of the few generic callbacks in libpcp_pmda (see
       PMDA(3)) that is incomplete, requiring a further
       pmdaFetchCallBack method of its own.  The additional callback
       should be registered using pmdaSetFetchCallBack and the
       pmdaFetchCallBack method has the following prototype:
       int func(pmdaMetric *mdesc, unsigned int inst, pmAtomValue *avp)

       pmdaFetch will allocate and resize the resp result structure, to
       store values for the numpmid metrics listed in pmidlist.

       For each instance listed in the profile (see pmdaProfile(3)) of
       each metric listed in pmidlist, the pmdaFetchCallBack method is
       called to fill the pmAtomValue structure identified by avp with a
       value for a specific metric-instance pair identified by the
       metric descriptor mdesc and the instance inst.  This value is
       then copied into the pmResult structure.

       The pmdaFetchCallBack method should return a value less than zero
       for an error, and the most likely cases would be PM_ERR_PMID if
       the metric identified by mdesc is not known to the method, or
       PM_ERR_INST if the method believes the instance inst is not known
       for the metric identified by mdesc.

       The success error codes depend on the version of PMDA_INTERFACE
       the PMDA is using.

       If the PMDA is using PMDA_INTERFACE_2 then on success the
       pmdaFetchCallBack method should return 0.

       If the PMDA is using PMDA_INTERFACE_3 or PMDA_INTERFACE_4 then on
       success the pmdaFetchCallBack method should return 1 if a value
       is returned via avp, else 0 if no values are currently available
       for the requested metric-instance pair although mdesc and inst
       both seem reasonable.

       If the PMDA is using PMDA_INTERFACE_5 or later then on success
       the pmdaFetchCallBack method should return PMDA_FETCH_STATIC (1)
       if the value returned via avp can be ignored by pmdaFetch once it
       has been copied into the pmResult structure, else
       PMDA_FETCH_DYNAMIC (2) if the value returned via avp uses the
       either the vp or cp fields of the pmAtomValue and the associated
       value (buffer) was allocated using one of malloc(3), calloc(3),
       realloc(3), strdup(3) etc. and pmdaFetch should release the
       memory by calling free(3) once a new buffer has been allocated
       and the value copied, else PMDA_FETCH_NOVALUES (0) if no values
       are currently available for the requested metric-instance pair
       although mdesc and inst both seem reasonable.

       If the pmdaFetchCallBack method returns a value for an instance
       of a metric of type PM_TYPE_STRING or PM_TYPE_AGGREGATE some
       special care is needed – the method should either use a static
       buffer, set avp->cp or avp->vp to the address of the buffer and
       return PMDA_FETCH_STATIC, or use a dynamically allocated buffer,
       keep a static reference to the buffer's address, return
       PMDA_FETCH_STATIC and free(3) or realloc(3) or reuse the buffer
       the next time the pmdaFetchCallBack method is called, else use a
       dynamically allocated buffer and return PMDA_FETCH_DYNAMIC.

EXAMPLE         top

       The following code fragments are for a hypothetical PMDA has with
       metrics (A, B, C and D) and an instance domain (X) with two
       instances (X1 and X2).  The instance domain and metrics
       description tables (see pmdaInit(3)) could be defined as:

            static pmdaInstid _X[] = {
                { 0, "X1" }, { 1, "X2" }
            static pmdaIndom indomtab[] = {
            #define X_INDOM 0
                { 0, 2, _X },
            static pmdaMetric metrictab[] = {
            /* A */
                { (void *)0,
                  { PMDA_PMID(0,0), PM_TYPE_32, PM_INDOM_NULL,
                    PM_SEM_INSTANT, {0,0,0,0,0,0} }, },
            /* B */
                { (void *)0,
                  { PMDA_PMID(0,1), PM_TYPE_DOUBLE, X_INDOM,
                    PM_SEM_INSTANT, {0,1,0,0,PM_TIME_SEC,0} }, },
            /* C */
                { (void *)0,
                  { PMDA_PMID(0,2), PM_TYPE_STRING, PM_INDOM_NULL,
                    PM_SEM_INSTANT, {0,0,0,0,0,0} }, },
            /* D */
                { (void *)0,
                  { PMDA_PMID(0,3), PM_TYPE_STRING, PM_INDOM_NULL,
                    PM_SEM_INSTANT, {0,0,0,0,0,0} }, },

       A pmdaFetchCallBack method to be called from pmdaFetch could be
       defined as:

            myFetchCallBack(pmdaMetric *mdesc, unsigned int inst, pmAtomValue *avp)
              static char sbuf[20]; // reuse this buffer
              char        *dbuf;    // malloc'd
              switch (pmID_item(mdesc->m_desc.pmid)) {
                case 0:
                  /* assign some value for metric A */;
                  avp->l = ...
                case 1:
                  switch (inst) {
                      case 0:
                       /* assign a value for metric B, instance X1 */;
                          avp->d = ...
                      case 1:
                       /* assign a value for metric B, instance X2 */;
                          avp->d = ...
                          return PM_ERR_INST;
                case 2:
                  /* place value for metric C in dbuf[] */
                  memcpy(dbuf, ...);
                  avp->cp = dbuf;
                case 3:
                  avp->cp = (char *)malloc(somesize);
                  /* place value in avp->cp */
                  pmsprintf(avp->cp, somesize, ...);
                  return PMDA_FETCH_DYNAMIC;
                  return PM_ERR_PMID;
              return PMDA_FETCH_STATIC;

CAVEAT         top

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

DIAGNOSTICS         top

       The following error messages indicate that there is discrepancy
       between the namespace, pmdaMetric and pmdaIndom tables passed to
       pmdaInit(3), and the registered fetch callback:

       pmdaFetch: Requested metric metric is not defined
              A requested metric metric is not listed in the pmdaMetric
              table.  The namespace for this PMDA(3) may contain
              additional metrics.

       pmdaFetch: PMID pmid not handled by fetch callback
              The pmdaFetchCallBack method has returned PM_ERR_PMID.
              This indicates that a metric may be listed in the
              pmdaMetric table, but is not supported by the callback

       pmdaFetch: Instance inst of PMID pmid not handled by fetch
              The pmdaFetchCallBack method has returned PM_ERR_INST.
              This indicates that an instance of metric is listed in the
              pmdaIndom table, but is not supported by the callback

       pmdaFetch: Fetch callback error:
              The pmdaFetchCallBack method returned a result other than

       pmdaFetch: Descriptor type (type) for metric pmid is bad
              The data type type specified for the metric pmid in the
              pmdaMetric table is illegal.

       pmdaFetch will return -errno if an error occurred while
       allocating the pmResult structure or copying the value from the

SEE ALSO         top

       pmcd(1), PMAPI(3), PMDA(3), pmdaDaemon(3), pmdaDSO(3),
       pmdaInit(3) and pmFetch(3).

COLOPHON         top

       This page is part of the PCP (Performance Co-Pilot) project.
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       ⟨⟩ on 2023-12-22.
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Performance Co-Pilot               PCP                      PMDAFETCH(3)

Pages that refer to this page: pmda(3)pmdacache(3)pmdadaemon(3)pmdadso(3)pmdainit(3)pmdamain(3)pmdaprofile(3)