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NAME | C SYNOPSIS | DESCRIPTION | EXAMPLE | CAVEAT | DIAGNOSTICS | SEE ALSO | COLOPHON |
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PMDAFETCH(3) Library Functions Manual PMDAFETCH(3)
pmdaFetch, pmdaSetFetchCallBack - fill a pmResult structure with
the requested metric values
#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
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 PM‐
DA_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 allo‐
cated 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 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;
}
The PMDA must be using PMDA_INTERFACE_2 or later, as specified in
the call to pmdaDSO(3) or pmdaDaemon(3).
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_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 pm‐
daMetric table is illegal.
pmdaFetch will return -errno if an error occurred while allocating
the pmResult structure or copying the value from the pmAtomValue.
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. 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
pcp@groups.io. This page was obtained from the project's upstream
Git repository ⟨https://github.com/performancecopilot/pcp.git⟩ on
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-08-11.) 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 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)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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