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NAME | C SYNOPSIS | DESCRIPTION | EXAMPLE | DIAGNOSTICS | CAVEAT | SEE ALSO | COLOPHON |
PMDAFETCH(3) Library Functions Manual PMDAFETCH(3)
pmdaFetch, pmdaSetFetchCallBack - fill a pmResult structure with the
requested metric values
#include <pcp/pmapi.h>
#include <pcp/impl.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
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 the only generic callback 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.
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 */
snprintf(avp->cp, somesize, ...);
return PMDA_FETCH_DYNAMIC;
default:
return PM_ERR_PMID;
}
return PMDA_FETCH_STATIC;
}
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.
The PMDA must be using PMDA_INTERFACE_2 or later, as specified in the
call to pmdaDSO(3) or pmdaDaemon(3).
pmcd(1), PMAPI(3), PMDA(3), pmdaDaemon(3), pmdaDSO(3), pmdaInit(3)
and pmFetch(3).
This page is part of the PCP (Performance Co-Pilot) project.
Information about the project can be found at ⟨http://www.pcp.io/⟩.
If you have a bug report for this manual page, send it to
pcp@oss.sgi.com. This page was obtained from the project's upstream
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COLOPHON (which is not part of the original manual page), send a mail
to man-pages@man7.org
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)