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NAME | C SYNOPSIS | DESCRIPTION | CALLBACKS | DIAGNOSTICS | SEE ALSO | COLOPHON |
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PMDAMAIN(3) Library Functions Manual PMDAMAIN(3)
pmdaMain, pmdaGetContext, pmdaSetResultCallBack,
pmdaSetCheckCallBack, pmdaSetDoneCallBack,
pmdaSetEndContextCallBack - generic PDU processing for a PMDA
#include <pcp/pmapi.h>
#include <pcp/pmda.h>
cc ... -lpcp_pmda -lpcp
void pmdaMain(pmdaInterface *dispatch);
void pmdaSetCheckCallBack(pmdaInterface *dispatch,
pmdaCheckCallBack callback);
void pmdaSetDoneCallBack(pmdaInterface *dispatch,
pmdaDoneCallBack callback);
void pmdaSetResultCallBack(pmdaInterface *dispatch,
pmdaResultCallBack callback);
void pmdaSetEndContextCallBack(pmdaInterface *dispatch,
pmdaEndContextCallBack callback);
int pmdaGetContext(void);
For Performance Metric Domain Agents (PMDA(3)) using the binary
PDU protocols to communicate with pmcd(1), the routine pmdaMain
provides a generic implementation of the PDU-driven main loop.
dispatch describes how to process each incoming PDU. It is a vec‐
tor of function pointers, one per request PDU type, as used in the
DSO interface for a PMDA, namely:
/*
* Interface Definitions for PMDA Methods
*/
typedef struct {
int domain; /* set/return performance metrics domain id here */
struct {
unsigned int pmda_interface: 8; /* PMDA DSO interface version */
unsigned int pmapi_version : 8; /* PMAPI version */
unsigned int flags : 16; /* optional feature flags */
} comm; /* set/return communication and version info */
int status; /* return initialization status here */
union {
struct { /* PMDA_INTERFACE_2 or _3 */
pmdaExt *ext;
int (*profile)(pmProfile *, pmdaExt *);
int (*fetch)(int, pmID *, pmResult **, pmdaExt *);
int (*desc)(pmID, pmDesc *, pmdaExt *);
int (*instance)(pmInDom, int, char *, pmInResult **, pmdaExt *);
int (*text)(int, int, char **, pmdaExt *);
int (*store)(pmResult *, pmdaExt *);
} two, three;
struct { /* PMDA_INTERFACE_4 or _5 */
pmdaExt *ext;
int (*profile)(pmProfile *, pmdaExt *);
int (*fetch)(int, pmID *, pmResult **, pmdaExt *);
int (*desc)(pmID, pmDesc *, pmdaExt *);
int (*instance)(pmInDom, int, char *, pmInResult **, pmdaExt *);
int (*text)(int, int, char **, pmdaExt *);
int (*store)(pmResult *, pmdaExt *);
int (*pmid)(char *, pmID *, pmdaExt *);
int (*name)(pmID, char ***, pmdaExt *);
int (*children)(char *, int, char ***, int **, pmdaExt *);
} four, five;
struct { /* PMDA_INTERFACE_6 */
pmdaExt *ext;
int (*profile)(pmProfile *, pmdaExt *);
int (*fetch)(int, pmID *, pmResult **, pmdaExt *);
int (*desc)(pmID, pmDesc *, pmdaExt *);
int (*instance)(pmInDom, int, char *, pmInResult **, pmdaExt *);
int (*text)(int, int, char **, pmdaExt *);
int (*store)(pmResult *, pmdaExt *);
int (*pmid)(char *, pmID *, pmdaExt *);
int (*name)(pmID, char ***, pmdaExt *);
int (*children)(char *, int, char ***, int **, pmdaExt *);
int (*attribute)(int, int, const char *, int, pmdaExt *);
} six;
} version;
} pmdaInterface;
This structure has been extended to incorporate the multiple in‐
terface versions that have evolved over time. For pmdaMain, dis‐
patch->domain and dispatch->status are ignored. The comm.pmda_in‐
terface field is used to determine the interface used by the PMDA.
Setting this field to PMDA_INTERFACE_2 or PMDA_INTERFACE_3 will
force pmdaMain to use the callbacks in the version.two or ver‐
sion.three structure. A setting of PMDA_INTERFACE_4 or PMDA_IN‐
TERFACE_5 will force pmdaMain to use the callbacks in the ver‐
sion.four or version.five structure, and similarly a PMDA_INTER‐
FACE_6 setting forces pmdaMain to use the callbacks in the ver‐
sion.six structure. Any other value will result in an error and
termination of pmdaMain.
Note that the use of dispatch as the interface between the pmcd(1)
and the methods of the PMDA allows each PMDA to be implemented as
though it were a DSO, with pmdaMain providing a convenient wrapper
that may be used to convert from the DSO interface to the binary
PDU (daemon PMDA) interface.
pmdaMain executes as a continuous loop, returning only when an end
of file is encountered on the PDU input file descriptor.
In addition to the individual PDU processing callbacks -
pmdaProfile(3), pmdaFetch(3), pmdaDesc(3), pmdaInstance(3),
pmdaText(3), pmdaStore(3), pmdaPMID(3), pmdaName(3),
pmdaChildren(3), and pmdaAttribute(3) there are other callbacks
that can affect or inform all PDU processing within a PMDA, namely
check, done and end. These callbacks should be set with pm‐
daSetCheckCallBack, pmdaSetDoneCallBack and pmdaSetEndContextCall‐
Back.
If not null, check is called after each PDU is received (but be‐
fore it was processed), and done is called after each PDU is sent.
If check returns a value less than zero (typically PM_ERR_AGAIN),
the PDU processing is skipped and in most cases the function value
is returned as an error PDU to pmcd(1) - this may be used for PM‐
DAs that require some sort of deferred connection or reconnect
protocols for the underlying sources of performance metrics, e.g.
a DBMS. The error indication from check is not passed back to
pmcd(1) in the cases where no acknowledgment is expected, e.g. for
a PDU_PROFILE.
The end callback allows a PMDA to keep track of state for individ‐
ual clients that are requesting it to perform actions (PDU pro‐
cessing). Using pmdaGetContext a PMDA can determine, at any
point, an integer identifier that uniquely identifies the client
tools at the remote end of PMCD (for local context modes, this
identifier is always zero). This becomes very important for han‐
dling event metrics, where each event must be propagated once only
to each interested client. It also underlies the mechanism where‐
by connection information is passed to the PMDA, such as the the
credentials (user and group identifiers) for the client tool.
One final callback mechanism is provided for handling the pmResult
built for a PDU_RESULT in response to a PDU_FETCH request. By de‐
fault, pmdaMain will free the pmResult once the result has been
sent to the pmcd(1). For some PMDAs this is inappropriate, e.g.
the pmResult is statically allocated, or contains a hybrid of
pinned PDU buffer information and dynamically allocated informa‐
tion. pmdaSetResultCallBack may be used to define an alternative
callback from pmdaMain.
These messages may be appended to the PMDA's log file:
PMDA interface version interface not supported
The interface version is not supported by pmdaMain.
Unrecognized pdu type
The PMDA received a PDU from pmcd that it does not recog‐
nize. This may indicate that the pmcd process is using a
more advanced interface than pmdaMain.
If the PMAPI(3) debugging control options have the ``libpmda'' op‐
tion set then each PDU that is received is reported in the PMDA's
log file.
pmcd(1), PMAPI(3), PMDA(3), pmdaProfile(3), pmdaFetch(3),
pmdaDesc(3), pmdaInstance(3), pmdaText(3), pmdaStore(3),
pmdaPMID(3), pmdaName(3), pmdaChildren(3), and pmdaAttribute(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
2025-02-02. (At that time, the date of the most recent commit
that was found in the repository was 2025-01-30.) 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 PMDAMAIN(3)
Pages that refer to this page: pmda(3), pmdaattribute(3), pmdachildren(3), pmdaeventclient(3), pmdaname(3), pmdapmid(3)
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HTML rendering created 2025-02-02 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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