pmdamain(3) — Linux manual page

NAME | C SYNOPSIS | DESCRIPTION | CALLBACKS | DIAGNOSTICS | SEE ALSO | COLOPHON

PMDAMAIN(3)             Library Functions Manual             PMDAMAIN(3)

NAME         top

       pmdaMain, pmdaGetContext, pmdaSetResultCallBack,
       pmdaSetCheckCallBack, pmdaSetDoneCallBack,
       pmdaSetEndContextCallBack - generic PDU processing for a PMDA

C SYNOPSIS         top

       #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);

DESCRIPTION         top

       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
       vector 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
       interface versions that have evolved over time.  For pmdaMain,
       dispatch->domain and dispatch->status are ignored.  The
       comm.pmda_interface 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 version.three structure.  A setting of
       PMDA_INTERFACE_4 or PMDA_INTERFACE_5 will force pmdaMain to use
       the callbacks in the version.four or version.five structure, and
       similarly a PMDA_INTERFACE_6 setting forces pmdaMain to use the
       callbacks in the version.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.

CALLBACKS         top

       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
       pmdaSetCheckCallBack, pmdaSetDoneCallBack and
       pmdaSetEndContextCallBack.

       If not null, check is called after each PDU is received (but
       before 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 PMDAs 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
       individual clients that are requesting it to perform actions (PDU
       processing).  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
       handling event metrics, where each event must be propagated once
       only to each interested client.  It also underlies the mechanism
       whereby 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 default, 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 information.  pmdaSetResultCallBack may be
       used to define an alternative callback from pmdaMain.

DIAGNOSTICS         top

       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
              recognize. 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''
       option set then each PDU that is received is reported in the
       PMDA's log file.

SEE ALSO         top

       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).

COLOPHON         top

       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@groups.io.  This page was obtained from the
       project's upstream Git repository
       ⟨https://github.com/performancecopilot/pcp.git⟩ on 2020-12-18.
       (At that time, the date of the most recent commit that was found
       in the repository was 2020-12-18.)  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)