pmdaGetContext(3)
NAME
pmdaMain, pmdaGetContext, pmdaSetResultCallBack, pmdaSetCheckCallBack, pmdaSetDoneCallBack, pmdaSetEndContextCallBack - generic PDU processing for a PMDA
C SYNOPSIS
#include <pcp/pmapi.h>
#include <pcp/impl.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
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
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
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.
SEE ALSO
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).
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
