PMDAFETCH(3)

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

C SYNOPSIS
#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
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

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:

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;
}

DIAGNOSTICS
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 method.

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

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