Provided by: libpcp3-dev_4.3.1-1_amd64 bug

NAME

       pmFetch - get performance metric values

C SYNOPSIS

       #include <pcp/pmapi.h>

       int pmFetch(int numpmid, pmID *pmidlist, pmResult **result);

       cc ... -lpcp

DESCRIPTION

       Given   a  list  of  Performance  Metric  Identifiers  (PMID)s,  e.g.  as  constructed  by
       pmLookupName(3), via pmidlist and numpmid, fetch the values for these performance metrics.

       The call to pmFetch is executed in the context of a source of  metrics,  instance  profile
       and  collection  time,  previously  established  by  calls  to the appropriate context and
       profile functions,  namely  some  of  pmNewContext(3),  pmDupContext(3),  pmUseContext(3),
       pmAddProfile(3), pmDelProfile(3) and pmSetMode(3).

       The  principal result from pmFetch is returned in the argument result as a tree, using the
       following component data structures;

            typedef struct {
                  unsigned int vtype : 8;        /* value type (same as pmDesc.type) */
                  unsigned int vlen : 24;        /* bytes for vtype/vlen + vbuf */
                  char         vbuf[1];          /* one or more values */
            } pmValueBlock;

            typedef struct {
                  int      inst;                 /* instance identifier */
                  union {
                        pmValueBlock *pval;      /* pointer to value-block */
                        int          lval;       /* integer value insitu */
                  } value;
            } pmValue;

            typedef struct {
                  pmID      pmid;                /* metric identifier */
                  int       numval;              /* number of values or error code */
                  int       valfmt;              /* value style, insitu or ptr */
                  pmValue   vlist[1];            /* set of instances/values */
            } pmValueSet;

            /* Result returned by pmFetch() */
            typedef struct {
                  struct timeval timestamp;      /* time stamped by collector */
                  int            numpmid;        /* number of PMIDs */
                  pmValueSet     *vset[1];       /* set of value sets */
            } pmResult;

       To accommodate metrics with multiple value instances, the numval field indicates how  many
       values are returned for each requested PMID.  The field valfmt in the pmValueSet structure
       indicates if the values for this metric are stored insitu in the lval field, i.e. a 32-bit
       integer  quantity  (either  int, unsigned int, long or unsigned long) or if the values are
       held in associated pmValueBlock structures.  The pmValueBlock structure is always used for
       floating  point values (float or double) and also accommodates arbitrary sized binary data
       such as `string-valued' metrics and metrics with aggregated or complex  data  types.   The
       maximum  length  of  a  pmValueBlock buffer is PM_VAL_VLEN_MAX bytes.  If the pmValueBlock
       format is used, the vtype field indicates the data type of the value.  This field has  the
       same interpretation as the type field in the pmDesc structure, see pmLookupDesc(3).

       Note  that the insitu value may be a signed or unsigned 32 bit integer, signed or unsigned
       32 bit long value (on 32 bit platforms), In the special cases described below, it may also
       be  a 32 bit floating point value.  If the application needs to know the type of an insitu
       value, which is almost always the case, it is necessary to fetch the  descriptor  for  the
       metric  and interpret the type field, as described in detail in pmLookupDesc(3).  When the
       pmResult is received from a PCP1.x pmcd, insitu values may also be 32 bit  floating  point
       values  (of type PM_TYPE_FLOAT).  In all cases, it is good practice to use pmLookupDesc(3)
       to fetch the descriptor for the metric and interpret the type field  therein.   Note  also
       that the PMAPI(3) will automatically translate from the PCP2.0 format to the PCP1.x format
       when a PCP1.x client requests 32 bit floating point values from a  PCP2.0  pmcd,  but  the
       reverse  translation  does  not  occur  (because  the  PCP2.0  pmcd  cannot  automatically
       distinguish between arbitrary 32 bit floating point values and 32 bit integers).

       If one value (i.e. associated with a  particular  instance)  for  a  requested  metric  is
       `unavailable'  (at  the  requested time), then there is no associated pmValue structure in
       the result.  If there are no available values for a metric, then numval will be  zero  and
       the   associated   pmValue[]  instance  will  be  empty  (valfmt  is  undefined  in  these
       circumstances, however pmid will be correctly set to  the  PMID  of  the  metric  with  no
       values).

       As  an  extension of this protocol, if the Performance Metrics Collection System (PMCS) is
       able to provide a reason why no values are available for  a  particular  metric,  this  is
       encoded  as  a standard error code in the corresponding numval.  Since the error codes are
       all negative, values for a requested metric are `unavailable' if numval is less  than,  or
       equal  to,  zero.   A  performance  metric's  value  may  be `unavailable' for a number of
       reasons; the following list is illustrative but not exhaustive: of the  software  for  the
       associated Performance Metric Domain

       +      Collection   is  not  currently  activated  in  the  software  for  the  associated
              Performance Metric Domain

       +      The associated PMID is not known

       +      The current system configuration does not include the associated hardware component
              and/or  the  associated software module, e.g. a disk is not installed, or off-line,
              or Oracle is not installed

       +      The metric is one for which an instance profile is required, and none was  provided
              (there  are  a  small  number of metrics in this category, typically ones with very
              large, and/or very dynamic instance domains, and/or expensive metric  instantiation
              methods).

       +      If the current context involves fetching metrics from an archive log, values may be
              unavailable in the  region  around  a  <mark>  record  (see  pmlogextract(1))  that
              indicate a temporal discontinuity in the time-series of metric values.

       In  general,  we  may  not  be  able  to  differentiate  between the various cases, and if
       differentiation is not possible, numval will simply be zero.

       The argument definition and the result specifications have been constructed to ensure that
       for each PMID in the requested pmidlist there is exactly one pmValueSet in the result, and
       further the PMIDs appear in exactly the same sequence in both pmidlist and  result.   This
       makes  the  number  and  order  of entries in result completely deterministic, and greatly
       simplifies the application programming logic after the call to pmFetch.

       The result structure returned by pmFetch is dynamically allocated using a  combination  of
       malloc(3)  calls  and  specialized  allocation  strategies, and should be released when no
       longer required by calling pmFreeResult(3) - under  no  circumstances  should  free(3)  be
       called directly to release this space.

       As  common  error  conditions  are  encoded in the result data structure, we'd expect only
       cataclysmic events to cause an error value to be returned.  One example would  be  if  the
       metrics  source  context  was a remote host, and that host or the PMCS on that host became
       unreachable.  Otherwise the value returned by the pmFetch function will be non-negative.

       If the current context involves fetching metrics  from  a  Performance  Metrics  Collector
       Daemon  (PMCD),  then  the  return  value may be used to encode out-of-band changes in the
       state of the PMCD and the associated Performance Metrics Daemon Agents (PMDAs), as a  bit-
       wise ``or'' of the following values:

       PMCD_RESTART_AGENT  An attempt has been made to restart at least one failed PMDA.

       PMCD_ADD_AGENT      At least one PMDA has been started.

       PMCD_DROP_AGENT     PMCD has noticed the termination of at least one PMDA.

       PMCD_AGENT_CHANGE   A convenience macro for any of the three PMDA changes.

       PMCD_LABEL_CHANGE   PMCD  has  been informed of changes to global (context) labels, or new
                           metrics have appeared which have associated labels.

       PMCD_NAMES_CHANGE   PMCD has been informed that the namespace has been modified, such that
                           new metrics have appeared or existing metrics have been removed.

       The  default  is  to  return  zero  to  indicate  no change in state, however the pmResult
       returned by pmFetch has the same interpretation independent of the return value being zero
       or greater than zero.

SEE ALSO

       pmcd(1),  pmAddProfile(3),  PMAPI(3), pmDelProfile(3), pmDupContext(3), pmExtractValue(3),
       pmFetchArchive(3),  pmFreeResult(3),  pmGetInDom(3),  pmLookupDesc(3),  pmLookupLabels(3),
       pmLookupName(3), pmNewContext(3), pmSetMode(3), pmUseContext(3) and pmWhichContext(3).

       Note  that  pmFetch  is the most primitive method of fetching metric values from the PMCS.
       See the pmFetchGroup(3) API for a higher level method that insulates  the  user  from  the
       intricacies  of  looking  up  metric  names  and  metadata,  setting up instance profiles,
       pmResult traversal, conversions, and scaling.

DIAGNOSTICS

       As mentioned above, pmFetch returns error codes insitu in  the  argument  result.   If  no
       result  is  returned,  e.g.  due to IPC failure using the current PMAPI context, or end of
       file on an archive log, then pmFetch will return  a  negative  error  code  which  may  be
       examined using pmErrStr(3).

       PM_ERR_EOL
              When  fetching  records  from  an  archive  log, pmFetch returns this error code to
              indicate the end of the log has been passed (or the  start  of  the  log  has  been
              passed,  if  the direction of traversal is backwards in time).  If the ``mode'' for
              the current PMAPI context (see pmSetMode(3)) is PM_MODE_INTERP then the time origin
              is  advanced, even when this error code is returned.  In this way applications that
              position the time outside the range defined by the records in the archive, and then
              commence  to  pmFetch  will eventually see valid results once the time origin moves
              inside the temporal span of the archive.

ENVIRONMENT

       Many of the performance metrics exported from PCP agents have  the  semantics  of  counter
       meaning  they  are expected to be monotonically increasing.  Under some circumstances, one
       value of these metrics may be smaller than the previously fetched value.  This can  happen
       when  a  counter  of  finite  precision overflows, or when the PCP agent has been reset or
       restarted, or when the PCP agent is exporting values from some underlying  instrumentation
       that is subject to some asynchronous discontinuity.
       The  environment variable PCP_COUNTER_WRAP may be set to indicate that all such cases of a
       decreasing ``counter'' should be treated as a counter overflow, and hence the  values  are
       assumed  to  have  wrapped  once  in  the  interval  between  consecutive  samples.   This
       ``wrapping'' behavior was the default in earlier PCP versions, but  by  default  has  been
       disabled in PCP version 1.3 and later.