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

       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

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;

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 callback
                      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 method.

       pmdaFetch: Fetch callback error:
                      The pmdaFetchCallBack method returned a result other
                      than PMDA_FETCH_NOVALUES, PMDA_FETCH_STATIC,
                      PMDA_FETCH_DYNAMIC, PM_ERR_PMID or PM_ERR_INST.

       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 pmAtomValue.

CAVEAT         top

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

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.
       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
       2020-09-18.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-09-18.)  If you discover any
       rendering problems in this HTML version of the page, or you believe
       there is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

Performance Co-Pilot                 PCP                        PMDAFETCH(3)

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