1PMDAFETCH(3) Library Functions Manual PMDAFETCH(3)
2
6 pmdaFetch, pmdaSetFetchCallBack - fill a pmResult structure with the
7 requested metric values
8
10 #include <pcp/pmapi.h>
11 #include <pcp/pmda.h>
12 int pmdaFetch(int numpmid, pmID *pmidlist, pmResult **resp,
14 pmdaExt *pmda);
15 void pmdaSetFetchCallBack(pmdaInterface *dispatch,
16 pmdaFetchCallBack callback);
17 cc ... -lpcp_pmda -lpcp
19
21 pmdaFetch is a generic callback used by a PMDA(3) to process a fetch
22 request from pmcd(1). The request from pmcd is initiated by a client
23 calling pmFetch(3).
24 This is one of the few generic callbacks in libpcp_pmda (see PMDA(3))
26 that is incomplete, requiring a further pmdaFetchCallBack method of its
27 own. The additional callback should be registered using pmdaSetFetch‐
28 CallBack and the pmdaFetchCallBack method has the following prototype:
29 int func(pmdaMetric *mdesc, unsigned int inst, pmAtomValue *avp)
30 pmdaFetch will allocate and resize the resp result structure, to store
32 values for the numpmid metrics listed in pmidlist.
33 For each instance listed in the profile (see pmdaProfile(3)) of each
35 metric listed in pmidlist, the pmdaFetchCallBack method is called to
36 fill the pmAtomValue structure identified by avp with a value for a
37 specific metric-instance pair identified by the metric descriptor mdesc
38 and the instance inst. This value is then copied into the pmResult
39 structure.
40 The pmdaFetchCallBack method should return a value less than zero for
42 an error, and the most likely cases would be PM_ERR_PMID if the metric
43 identified by mdesc is not known to the method, or PM_ERR_INST if the
44 method believes the instance inst is not known for the metric identi‐
45 fied by mdesc.
46 The success error codes depend on the version of PMDA_INTERFACE the PM‐
48 DA is using.
49 If the PMDA is using PMDA_INTERFACE_2 then on success the pmdaFetch‐
51 CallBack method should return 0.
52 If the PMDA is using PMDA_INTERFACE_3 or PMDA_INTERFACE_4 then on suc‐
54 cess the pmdaFetchCallBack method should return 1 if a value is re‐
55 turned via avp, else 0 if no values are currently available for the re‐
56 quested metric-instance pair although mdesc and inst both seem reason‐
57 able.
58 If the PMDA is using PMDA_INTERFACE_5 or later then on success the
60 pmdaFetchCallBack method should return PMDA_FETCH_STATIC (1) if the
61 value returned via avp can be ignored by pmdaFetch once it has been
62 copied into the pmResult structure, else PMDA_FETCH_DYNAMIC (2) if the
63 value returned via avp uses the either the vp or cp fields of the
64 pmAtomValue and the associated value (buffer) was allocated using one
65 of malloc(3), calloc(3), realloc(3), strdup (3) etc. and pmdaFetch
66 should release the memory by calling free(3) once a new buffer has been
67 allocated and the value copied, else PMDA_FETCH_NOVALUES (0) if no val‐
68 ues are currently available for the requested metric-instance pair al‐
69 though mdesc and inst both seem reasonable.
70 If the pmdaFetchCallBack method returns a value for an instance of a
72 metric of type PM_TYPE_STRING or PM_TYPE_AGGREGATE some special care is
73 needed – the method should either use a static buffer, set avp->cp or
74 avp->vp to the address of the buffer and return PMDA_FETCH_STATIC, or
75 use a dynamically allocated buffer, keep a static reference to the buf‐
76 fer's address, return PMDA_FETCH_STATIC and free [4m(3) or realloc [4m(3) or
77 reuse the buffer the next time the pmdaFetchCallBack method is called,
78 else use a dynamically allocated buffer and return PMDA_FETCH_DYNAMIC.
79
81 The following code fragments are for a hypothetical PMDA has with met‐
82 rics (A, B, C and D) and an instance domain (X) with two instances (X1
83 and X2). The instance domain and metrics description tables (see pm‐
84 daInit(3)) could be defined as:
85 static pmdaInstid _X[] = {
87 { 0, "X1" }, { 1, "X2" }
88 };
89 static pmdaIndom indomtab[] = {
90 #define X_INDOM 0
91 { 0, 2, _X },
92 };
93 static pmdaMetric metrictab[] = {
94 /* A */
95 { (void *)0,
96 { PMDA_PMID(0,0), PM_TYPE_32, PM_INDOM_NULL,
97 PM_SEM_INSTANT, {0,0,0,0,0,0} }, },
98 /* B */
99 { (void *)0,
100 { PMDA_PMID(0,1), PM_TYPE_DOUBLE, X_INDOM,
101 PM_SEM_INSTANT, {0,1,0,0,PM_TIME_SEC,0} }, },
102 /* C */
103 { (void *)0,
104 { PMDA_PMID(0,2), PM_TYPE_STRING, PM_INDOM_NULL,
105 PM_SEM_INSTANT, {0,0,0,0,0,0} }, },
106 /* D */
107 { (void *)0,
108 { PMDA_PMID(0,3), PM_TYPE_STRING, PM_INDOM_NULL,
109 PM_SEM_INSTANT, {0,0,0,0,0,0} }, },
110 };
111 A pmdaFetchCallBack method to be called from pmdaFetch could be defined
113 as:
114 int
116 myFetchCallBack(pmdaMetric *mdesc, unsigned int inst, pmAtomValue *avp)
117 {
118 static char sbuf[20]; // reuse this buffer
119 char *dbuf; // malloc'd
120 switch (pmID_item(mdesc->m_desc.pmid)) {
121 case 0:
122 /* assign some value for metric A */;
123 avp->l = ...
124 break;
125 case 1:
126 switch (inst) {
127 case 0:
128 /* assign a value for metric B, instance X1 */;
129 avp->d = ...
130 break;
131 case 1:
132 /* assign a value for metric B, instance X2 */;
133 avp->d = ...
134 break;
135 default:
136 return PM_ERR_INST;
137 }
138 case 2:
139 /* place value for metric C in dbuf[] */
140 memcpy(dbuf, ...);
141 avp->cp = dbuf;
142 break;
143 case 3:
144 avp->cp = (char *)malloc(somesize);
145 /* place value in avp->cp */
146 pmsprintf(avp->cp, somesize, ...);
147 return PMDA_FETCH_DYNAMIC;
148 default:
149 return PM_ERR_PMID;
150 }
151 return PMDA_FETCH_STATIC;
152 }
153
155 The PMDA must be using PMDA_INTERFACE_2 or later, as specified in the
156 call to pmdaDSO(3) or pmdaDaemon(3).
157
159 The following error messages indicate that there is discrepancy between
160 the namespace, pmdaMetric and pmdaIndom tables passed to pmdaInit(3),
161 and the registered fetch callback:
162 pmdaFetch: Requested metric metric is not defined
164 A requested metric metric is not listed in the pmdaMet‐
165 ric table. The namespace for this PMDA(3) may contain
166 additional metrics.
167 pmdaFetch: PMID pmid not handled by fetch callback
169 The pmdaFetchCallBack method has returned PM_ERR_PMID.
170 This indicates that a metric may be listed in the pmda‐
171 Metric table, but is not supported by the callback meth‐
172 od.
173 pmdaFetch: Instance inst of PMID pmid not handled by fetch callback
175 The pmdaFetchCallBack method has returned PM_ERR_INST.
176 This indicates that an instance of metric is listed in
177 the pmdaIndom table, but is not supported by the call‐
178 back method.
179 pmdaFetch: Fetch callback error:
181 The pmdaFetchCallBack method returned a result other
182 than PMDA_FETCH_NOVALUES, PMDA_FETCH_STATIC, PM‐
183 DA_FETCH_DYNAMIC, PM_ERR_PMID or PM_ERR_INST.
184 pmdaFetch: Descriptor type (type) for metric pmid is bad
186 The data type type specified for the metric pmid in the
187 pmdaMetric table is illegal.
188 pmdaFetch will return -errno if an error occurred while allocating the
190 pmResult structure or copying the value from the pmAtomValue.
191
193 pmcd(1), PMAPI(3), PMDA(3), pmdaDaemon(3), pmdaDSO(3), pmdaInit(3) and
194 pmFetch(3).
195Performance Co-Pilot PCP PMDAFETCH(3)