pmdafetch(3) — Linux manual page

NAME | C SYNOPSIS | DESCRIPTION | EXAMPLE | CAVEAT | DIAGNOSTICS | SEE ALSO | COLOPHON

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  pmdaFetchCall‐
       Back method of its own.  The additional callback should be regis‐
       tered 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  met‐
       ric  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_DY‐
       NAMIC (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 spe‐
       cial 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 PM‐
       DA_FETCH_STATIC and free(3) or realloc(3) or reuse the buffer the
       next time the pmdaFetchCallBack method is called, else use a  dy‐
       namically 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 in‐
       stances (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:

            int
            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 = ...
                  break;
                case 1:
                  switch (inst) {
                      case 0:
                       /* assign a value for metric B, instance X1 */;
                          avp->d = ...
                          break;
                      case 1:
                       /* assign a value for metric B, instance X2 */;
                          avp->d = ...
                          break;
                      default:
                          return PM_ERR_INST;
                  }
                case 2:
                  /* place value for metric C in dbuf[] */
                  memcpy(dbuf, ...);
                  avp->cp = dbuf;
                  break;
                case 3:
                  avp->cp = (char *)malloc(somesize);
                  /* place value in avp->cp */
                  pmsprintf(avp->cp, somesize, ...);
                  return PMDA_FETCH_DYNAMIC;
                default:
                  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 addi‐
              tional 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 pmdaMet‐
              ric table, but is not supported by the callback method.

       pmdaFetch: Instance inst of PMID pmid not handled by fetch call‐
       back
              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_DYNAM‐
              IC, 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 allocat‐
       ing the pmResult structure or copying the value from the pmAtom‐
       Value.

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.  In‐
       formation about the project can be found at ⟨http://www.pcp.io/⟩.
       If you have a bug report for this manual page, send it to
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       ⟨https://github.com/performancecopilot/pcp.git⟩ on 2024-06-14.
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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)