1PMDAMAIN(3) Library Functions Manual PMDAMAIN(3)
2
6 pmdaMain, pmdaGetContext, pmdaSetResultCallBack, pmdaSetCheckCallBack,
7 pmdaSetDoneCallBack, pmdaSetEndContextCallBack - generic PDU processing
8 for a PMDA
9
11 #include <pcp/pmapi.h>
12 #include <pcp/pmda.h>
13 cc ... -lpcp_pmda -lpcp
15 void pmdaMain(pmdaInterface *dispatch);
17 void pmdaSetCheckCallBack(pmdaInterface *dispatch,
18 pmdaCheckCallBack callback);
19 void pmdaSetDoneCallBack(pmdaInterface *dispatch,
20 pmdaDoneCallBack callback);
21 void pmdaSetResultCallBack(pmdaInterface *dispatch,
22 pmdaResultCallBack callback);
23 void pmdaSetEndContextCallBack(pmdaInterface *dispatch,
24 pmdaEndContextCallBack callback);
25 int pmdaGetContext(void);
26
29 For Performance Metric Domain Agents (PMDA(3)) using the binary PDU
30 protocols to communicate with pmcd(1), the routine pmdaMain provides a
31 generic implementation of the PDU-driven main loop.
32 dispatch describes how to process each incoming PDU. It is a vector of
34 function pointers, one per request PDU type, as used in the DSO inter‐
35 face for a PMDA, namely:
36 /*
38 * Interface Definitions for PMDA Methods
39 */
40 typedef struct {
41 int domain; /* set/return performance metrics domain id here */
42 struct {
43 unsigned int pmda_interface: 8; /* PMDA DSO interface version */
44 unsigned int pmapi_version : 8; /* PMAPI version */
45 unsigned int flags : 16; /* optional feature flags */
46 } comm; /* set/return communication and version info */
47 int status; /* return initialization status here */
48 union {
50 struct { /* PMDA_INTERFACE_2 or _3 */
51 pmdaExt *ext;
52 int (*profile)(pmProfile *, pmdaExt *);
53 int (*fetch)(int, pmID *, pmResult **, pmdaExt *);
54 int (*desc)(pmID, pmDesc *, pmdaExt *);
55 int (*instance)(pmInDom, int, char *, pmInResult **, pmdaExt *);
56 int (*text)(int, int, char **, pmdaExt *);
57 int (*store)(pmResult *, pmdaExt *);
58 } two, three;
59 struct { /* PMDA_INTERFACE_4 or _5 */
61 pmdaExt *ext;
62 int (*profile)(pmProfile *, pmdaExt *);
63 int (*fetch)(int, pmID *, pmResult **, pmdaExt *);
64 int (*desc)(pmID, pmDesc *, pmdaExt *);
65 int (*instance)(pmInDom, int, char *, pmInResult **, pmdaExt *);
66 int (*text)(int, int, char **, pmdaExt *);
67 int (*store)(pmResult *, pmdaExt *);
68 int (*pmid)(char *, pmID *, pmdaExt *);
69 int (*name)(pmID, char ***, pmdaExt *);
70 int (*children)(char *, int, char ***, int **, pmdaExt *);
71 } four, five;
72 struct { /* PMDA_INTERFACE_6 */
74 pmdaExt *ext;
75 int (*profile)(pmProfile *, pmdaExt *);
76 int (*fetch)(int, pmID *, pmResult **, pmdaExt *);
77 int (*desc)(pmID, pmDesc *, pmdaExt *);
78 int (*instance)(pmInDom, int, char *, pmInResult **, pmdaExt *);
79 int (*text)(int, int, char **, pmdaExt *);
80 int (*store)(pmResult *, pmdaExt *);
81 int (*pmid)(char *, pmID *, pmdaExt *);
82 int (*name)(pmID, char ***, pmdaExt *);
83 int (*children)(char *, int, char ***, int **, pmdaExt *);
84 int (*attribute)(int, int, const char *, int, pmdaExt *);
85 } six;
86 } version;
87 } pmdaInterface;
89 This structure has been extended to incorporate the multiple interface
91 versions that have evolved over time. For pmdaMain, dispatch->domain
92 and dispatch->status are ignored. The comm.pmda_interface field is
93 used to determine the interface used by the PMDA. Setting this field
94 to PMDA_INTERFACE_2 or PMDA_INTERFACE_3 will force pmdaMain to use the
95 callbacks in the version.two or version.three structure. A setting of
96 PMDA_INTERFACE_4 or PMDA_INTERFACE_5 will force pmdaMain to use the
97 callbacks in the version.four or version.five structure, and similarly
98 a PMDA_INTERFACE_6 setting forces pmdaMain to use the callbacks in the
99 version.six structure. Any other value will result in an error and
100 termination of pmdaMain.
101 Note that the use of dispatch as the interface between the pmcd(1) and
103 the methods of the PMDA allows each PMDA to be implemented as though it
104 were a DSO, with pmdaMain providing a convenient wrapper that may be
105 used to convert from the DSO interface to the binary PDU (daemon PMDA)
106 interface.
107 pmdaMain executes as a continuous loop, returning only when an end of
109 file is encountered on the PDU input file descriptor.
110
112 In addition to the individual PDU processing callbacks - pmdaPro‐
113 file(3), pmdaFetch(3), pmdaDesc(3), pmdaInstance(3), pmdaText(3), pmda‐
114 Store(3), pmdaPMID(3), pmdaName(3), pmdaChildren(3), and pmdaAt‐
115 tribute(3) there are other callbacks that can affect or inform all PDU
116 processing within a PMDA, namely check, done and end. These callbacks
117 should be set with pmdaSetCheckCallBack, pmdaSetDoneCallBack and pm‐
118 daSetEndContextCallBack.
119 If not null, check is called after each PDU is received (but before it
121 was processed), and done is called after each PDU is sent. If check
122 returns a value less than zero (typically PM_ERR_AGAIN), the PDU pro‐
123 cessing is skipped and in most cases the function value is returned as
124 an error PDU to pmcd(1) - this may be used for PMDAs that require some
125 sort of deferred connection or reconnect protocols for the underlying
126 sources of performance metrics, e.g. a DBMS. The error indication from
127 check is not passed back to pmcd(1) in the cases where no acknowledg‐
128 ment is expected, e.g. for a PDU_PROFILE.
129 The end callback allows a PMDA to keep track of state for individual
131 clients that are requesting it to perform actions (PDU processing).
132 Using pmdaGetContext a PMDA can determine, at any point, an integer
133 identifier that uniquely identifies the client tools at the remote end
134 of PMCD (for local context modes, this identifier is always zero).
135 This becomes very important for handling event metrics, where each
136 event must be propagated once only to each interested client. It also
137 underlies the mechanism whereby connection information is passed to the
138 PMDA, such as the the credentials (user and group identifiers) for the
139 client tool.
140 One final callback mechanism is provided for handling the pmResult
142 built for a PDU_RESULT in response to a PDU_FETCH request. By default,
143 pmdaMain will free the pmResult once the result has been sent to the
144 pmcd(1). For some PMDAs this is inappropriate, e.g. the pmResult is
145 statically allocated, or contains a hybrid of pinned PDU buffer infor‐
146 mation and dynamically allocated information. pmdaSetResultCallBack
147 may be used to define an alternative callback from pmdaMain.
148
150 These messages may be appended to the PMDA's log file:
151 PMDA interface version interface not supported
153 The interface version is not supported by pm‐
154 daMain.
155 Unrecognized pdu type The PMDA received a PDU from pmcd that it does
157 not recognize. This may indicate that the pmcd
158 process is using a more advanced interface
159 than pmdaMain.
160 If the PMAPI(3) debugging control options have the ``libpmda'' option
162 set then each PDU that is received is reported in the PMDA's log file.
163
165 pmcd(1), PMAPI(3), PMDA(3), pmdaProfile(3), pmdaFetch(3), pmdaDesc(3),
166 pmdaInstance(3), pmdaText(3), pmdaStore(3), pmdaPMID(3), pmdaName(3),
167 pmdaChildren(3), and pmdaAttribute(3).
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