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/impl.h>
12 #include <pcp/pmda.h>
13 int pmdaFetch(int numpmid, pmID *pmidlist, pmResult **resp,
15 pmdaExt *pmda);
16 void pmdaSetFetchCallBack(pmdaInterface *dispatch,
17 pmdaFetchCallBack callback);
18 cc ... -lpcp_pmda -lpcp
20
22 pmdaFetch is a generic callback used by a PMDA(3) to process a fetch
23 request from pmcd(1). The request from pmcd is initiated by a client
24 calling pmFetch(3).
25 This is the only generic callback in libpcp_pmda (see PMDA(3)) that is
27 incomplete, requiring a further pmdaFetchCallBack method of its own.
28 The additional callback should be registered using pmdaSetFetchCallBack
29 and the pmdaFetchCallBack method has the following prototype:
30 int func(pmdaMetric *mdesc, unsigned int inst, pmAtomValue *avp)
31 pmdaFetch will allocate and resize the resp result structure, to store
33 values for the numpmid metrics listed in pmidlist.
34 For each instance listed in the profile (see pmdaProfile(3)) of each
36 metric listed in pmidlist, the pmdaFetchCallBack method is called to
37 fill the pmAtomValue structure identified by avp with a value for a
38 specific metric-instance pair identified by the metric descriptor mdesc
39 and the instance inst. This value is then copied into the pmResult
40 structure.
41 The pmdaFetchCallBack method should return a value less than zero for
43 an error, and the most likely cases would be PM_ERR_PMID if the metric
44 identified by mdesc is not known to the method, or PM_ERR_INST if the
45 method believes the instance inst is not known for the metric identi‐
46 fied by mdesc.
47 The success error codes depend on the version of PMDA_INTERFACE the PM‐
49 DA is using.
50 If the PMDA is using PMDA_INTERFACE_2 then on success the pmdaFetch‐
52 CallBack method should return 0.
53 If the PMDA is using PMDA_INTERFACE_3 or PMDA_INTERFACE_4 then on suc‐
55 cess the pmdaFetchCallBack method should return 1 if a value is re‐
56 turned via avp, else 0 if no values are currently available for the re‐
57 quested metric-instance pair although mdesc and inst both seem reason‐
58 able.
59 If the PMDA is using PMDA_INTERFACE_5 or later then on success the
61 pmdaFetchCallBack method should return PMDA_FETCH_STATIC (1) if the
62 value returned via avp can be ignored by pmdaFetch once it has been
63 copied into the pmResult structure, else PMDA_FETCH_DYNAMIC (2) if the
64 value returned via avp uses the either the vp or cp fields of the
65 pmAtomValue and the associated value (buffer) was allocated using one
66 of malloc(3), calloc(3), realloc(3), strdup (3) etc. and pmdaFetch
67 should release the memory by calling free(3) once a new buffer has been
68 allocated and the value copied, else PMDA_FETCH_NOVALUES (0) if no val‐
69 ues are currently available for the requested metric-instance pair al‐
70 though mdesc and inst both seem reasonable.
71 If the pmdaFetchCallBack method returns a value for an instance of a
73 metric of type PM_TYPE_STRING or PM_TYPE_AGGREGATE some special care is
74 needed – the method should either use a static buffer, set avp->cp or
75 avp->vp to the address of the buffer and return PMDA_FETCH_STATIC, or
76 use a dynamically allocated buffer, keep a static reference to the buf‐
77 fer's address, return PMDA_FETCH_STATIC and free [4m(3) or realloc [4m(3) or
78 reuse the buffer the next time the pmdaFetchCallBack method is called,
79 else use a dynamically allocated buffer and return PMDA_FETCH_DYNAMIC.
80
82 The following code fragments are for a hypothetical PMDA has with met‐
83 rics (A, B, C and D) and an instance domain (X) with two instances (X1
84 and X2). The instance domain and metrics description tables (see pm‐
85 daInit(3)) could be defined as:
86 static pmdaInstid _X[] = {
88 { 0, "X1" }, { 1, "X2" }
89 };
90 static pmdaIndom indomtab[] = {
91 #define X_INDOM 0
92 { 0, 2, _X },
93 };
94 static pmdaMetric metrictab[] = {
95 /* A */
96 { (void *)0,
97 { PMDA_PMID(0,0), PM_TYPE_32, PM_INDOM_NULL,
98 PM_SEM_INSTANT, {0,0,0,0,0,0} }, },
99 /* B */
100 { (void *)0,
101 { PMDA_PMID(0,1), PM_TYPE_DOUBLE, X_INDOM,
102 PM_SEM_INSTANT, {0,1,0,0,PM_TIME_SEC,0} }, },
103 /* C */
104 { (void *)0,
105 { PMDA_PMID(0,2), PM_TYPE_STRING, PM_INDOM_NULL,
106 PM_SEM_INSTANT, {0,0,0,0,0,0} }, },
107 /* D */
108 { (void *)0,
109 { PMDA_PMID(0,3), PM_TYPE_STRING, PM_INDOM_NULL,
110 PM_SEM_INSTANT, {0,0,0,0,0,0} }, },
111 };
112 A pmdaFetchCallBack method to be called from pmdaFetch could be defined
114 as:
115 int
117 myFetchCallBack(pmdaMetric *mdesc, unsigned int inst, pmAtomValue *avp)
118 {
119 static char sbuf[20]; // reuse this buffer
120 char *dbuf; // malloc'd
121 switch (pmid_item(mdesc->m_desc.pmid)) {
122 case 0:
123 /* assign some value for metric A */;
124 avp->l = ...
125 break;
126 case 1:
127 switch (inst) {
128 case 0:
129 /* assign a value for metric B, instance X1 */;
130 avp->d = ...
131 break;
132 case 1:
133 /* assign a value for metric B, instance X2 */;
134 avp->d = ...
135 break;
136 default:
137 return PM_ERR_INST;
138 }
139 case 2:
140 /* place value for metric C in dbuf[] */
141 memcpy(dbuf, ...);
142 avp->cp = dbuf;
143 break;
144 case 3:
145 avp->cp = (char *)malloc(somesize);
146 /* place value in avp->cp */
147 snprintf(avp->cp, somesize, ...);
148 return PMDA_FETCH_DYNAMIC;
149 default:
150 return PM_ERR_PMID;
151 }
152 return PMDA_FETCH_STATIC;
153 }
154
156 The following error messages indicate that there is discrepancy between
157 the namespace, pmdaMetric and pmdaIndom tables passed to pmdaInit(3),
158 and the registered fetch callback:
159 pmdaFetch: Requested metric metric is not defined
161 A requested metric metric is not listed in the pmdaMet‐
162 ric table. The namespace for this PMDA(3) may contain
163 additional metrics.
164 pmdaFetch: PMID pmid not handled by fetch callback
166 The pmdaFetchCallBack method has returned PM_ERR_PMID.
167 This indicates that a metric may be listed in the pmda‐
168 Metric table, but is not supported by the callback
169 method.
170 pmdaFetch: Instance inst of PMID pmid not handled by fetch callback
172 The pmdaFetchCallBack method has returned PM_ERR_INST.
173 This indicates that an instance of metric is listed in
174 the pmdaIndom table, but is not supported by the call‐
175 back method.
176 pmdaFetch: Fetch callback error:
178 The pmdaFetchCallBack method returned a result other
179 than PMDA_FETCH_NOVALUES, PMDA_FETCH_STATIC, PM‐
180 DA_FETCH_DYNAMIC, PM_ERR_PMID or PM_ERR_INST.
181 pmdaFetch: Descriptor type (type) for metric pmid is bad
183 The data type type specified for the metric pmid in the
184 pmdaMetric table is illegal.
185 pmdaFetch will return -errno if an error occurred while allocating the
187 pmResult structure or copying the value from the pmAtomValue.
188
190 The PMDA must be using PMDA_INTERFACE_2 or later, as specified in the
191 call to pmdaDSO(3) or pmdaDaemon(3).
192
194 pmcd(1), PMAPI(3), PMDA(3), pmdaDaemon(3), pmdaDSO(3), pmdaInit(3) and
195 pmFetch(3).
196Performance Co-Pilot PCP PMDAFETCH(3)