NAME
pmdaMain, pmdaMainLoop, pmdaSetResultCallBack, pmdaSetCheckCallBack,
pmdaSetDoneCallBack, pmdaMainLoopFreeResultCallback - generic PDU
processing for a PMDA
C SYNOPSIS
#include <pcp/pmapi.h>
#include <pcp/impl.h>
#include <pcp/pmda.h>
cc ... -lpcp_pmda -lpcp
PMDA Interface 2 or higher
void pmdaMain(pmdaInterface *dispatch);
void pmdaSetResultCallBack(pmdaInterface *dispatch, pmdaResultCallBack
callback);
void pmdaSetCheckCallBack(pmdaInterface *dispatch, pmdaCheckCallBack
callback);
void pmdaSetDoneCallBack(pmdaInterface *dispatch, pmdaDoneCallBack
callback);
PMDA Interface 1
void pmdaMainLoop(char *name, int infd, int outfd, _pmPMDA *dispatch,
int (*check)(void), void (*done)(void));
void pmdaMainLoopFreeResultCallback(void (*callback)(pmResult *res));
DESCRIPTION
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.
pmdaMainLoop and pmdaMainLoopFreeResultCallback implemented an earlier
version of this functionality. Both functions will be removed in a
later release and should not be used.
The arguments that are now encapsulated in pmdaExt that were passed to
pmdaMainLoop include the name of the PMDA (used only in error messages)
and the file descriptors infd and outfd used to receive and send PDUs
to the pmcd(1) process.
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; /* usage TBD */
} comm; /* set/return communication and version info */
int status; /* return initialization status here */
union {
struct { /* PMDA_INTERFACE_1 */
int (*profile)(__pmProfile *);
int (*fetch)(int, pmID *, pmResult **);
int (*desc)(pmID, pmDesc *);
int (*instance)(pmInDom, int, char *, __pmInResult **);
int (*text)(int, int, char **);
int (*control)(pmResult *, int, int, int);
int (*store)(pmResult *);
} one;
struct { /* PMDA_INTERFACE_2 or _3 */
pmdaExt *e_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;
struct { /* PMDA_INTERFACE_4 */
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;
} 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_1 will force pmdaMain to use the callbacks in the
version.one structure. A setting of PMDA_INTERFACE_2 or
PMDA_INTERFACE_3 will force pmdaMain to use the callbacks in the
version.two structure. A setting of PMDA_INTERFACE_4 will force
pmdaMain to use the callbacks in the version.four 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.
pmdaMainLoop provided two additional callbacks, check and done. 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 callback done would typically be used to monitor termination
conditions, or do PMDA housekeeping required after PDU processing.
These callbacks should now be set with pmdaSetCheckCallBack and
pmdaSetDoneCallBack when using pmdaMain.
One further 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. The routine pmdaMainLoopFreeResultCallback has been
replaced by pmdaSetResultCallBack.
pmdaMain executes as a continuous loop, returning only when an end of
file is encountered on the PDU input file descriptor.
SEE ALSO
pmcd(1), pmdbg(1), PMAPI(3) and PMDA(3).
DIAGNOSTICS
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) debug control variable (pmdebug) has the
DBG_TRACE_LIBPMDA flag set then each PDU that is received is reported
in the PMDA’s log file.