Provided by: libpcp3-dev_5.3.7-1_amd64 bug


       pmLookupDesc, pmLookupDescs - obtain descriptions for performance metrics


       #include <pcp/pmapi.h>

       int pmLookupDesc(pmID pmid, pmDesc *desc);
       int pmLookupDescs(int numpmid, pmID *pmids, pmDesc *descs);

       cc ... -lpcp


       Given  a  Performance Metrics Identifier (PMID) as pmid, the pmLookupDesc routine fills in
       the given pmDesc structure, pointed to by the parameter desc, from the current Performance
       Metrics Application Programming Interface (PMAPI) context.

       The  pmLookupDescs  variant provides equivalent functionality for numpmid metrics at once,
       with the pmids array providing the metric identifiers to lookup.  It is more efficient  as
       the  number of metrics increases, as it avoids round trip latency from multiple individual
       requests.  Note that the error protocol guarantees there is a 1:1 relationship between the
       elements of desclist and pmidlist, hence both lists contain exactly numpmid elements.  For
       this reason, the caller is expected to have  pre-allocated  a  suitably  sized  array  for

       The pmDesc structure provides all of the information required to describe and manipulate a
       performance metric via the PMAPI, and has the following declaration.

            /* Performance Metric Descriptor */
            typedef struct {
                pmID    pmid;   /* unique identifier */
                int     type;   /* base data type (see below) */
                pmInDom indom;  /* instance domain */
                int     sem;    /* semantics of value (see below) *
                pmUnits units;  /* dimension and units (see below) */
            } pmDesc;

            /* pmDesc.type -- data type of metric values */
            #define PM_TYPE_NOSUPPORT        -1    /* not impl. in this version */
            #define PM_TYPE_32               0    /* 32-bit signed integer */
            #define PM_TYPE_U32              1    /* 32-bit unsigned integer */
            #define PM_TYPE_64               2    /* 64-bit signed integer */
            #define PM_TYPE_U64              3    /* 64-bit unsigned integer */
            #define PM_TYPE_FLOAT            4    /* 32-bit floating point */
            #define PM_TYPE_DOUBLE           5    /* 64-bit floating point */
            #define PM_TYPE_STRING           6    /* array of char */
            #define PM_TYPE_AGGREGATE        7    /* arbitrary binary data */
            #define PM_TYPE_AGGREGATE_STATIC 8    /* static pointer to aggregate */
            #define PM_TYPE_EVENT            9    /* packed pmEventArray */
            #define PM_TYPE_UNKNOWN          255  /* used in pmValueBlock, not pmDesc */

            /* pmDesc.sem -- semantics/interpretation of metric values */
            #define PM_SEM_COUNTER  1  /* cumulative ctr (monotonic incr) */
            #define PM_SEM_INSTANT  3  /* instant. value continuous domain */
            #define PM_SEM_DISCRETE 4  /* instant. value discrete domain */

       The type field in the pmDesc describes various  encodings  (or  formats)  for  a  metric's

       If  a  value  is  counted in the underlying base instrumentation with less than 32 bits of
       integer precision, it is the responsibility of the Performance Metrics Domain Agent (PMDA)
       to  promote  the  value  to  a  32-bit  integer before it is exported into the Performance
       Metrics Collection Subsystem (PMCS); i.e. applications above the PMAPI never have to  deal
       with 8-bit and 16-bit counters.

       If    the    value    of    a   performance   metric   is   of   type   PM_TYPE_AGGREGATE,
       PM_TYPE_AGGREGATE_STATIC, PM_TYPE_EVENT or PM_TYPE_STRING, the interpretation of the value
       is  unknown  to  the  PMCS.  In these cases, the application using the value, and the PMDA
       providing the value must have some common understanding about how the value is  structured
       and interpreted.

       Each  value  for a performance metric is assumed to be drawn from a set of values that can
       be described in terms of their dimensionality and scale by a compact encoding as  follows.
       The  dimensionality  is  defined by a power, or index, in each of 3 orthogonal dimensions,
       namely Space, Time and Count (or  Events,  which  are  dimensionless).   For  example  I/O
       throughput might be represented as
       while  the  running total of system calls is Count, memory allocation is Space and average
       service time is
       In each dimension there are a number of common scale values that may  be  used  to  better
       encode  ranges  that  might  otherwise  exhaust  the  precision  of  a 32-bit value.  This
       information is encoded in the pmUnits structure which is embedded in the pmDesc structure.

             * Encoding for the units (dimensions Time and Space) and scale
             * for Performance Metric Values
             * For example, a pmUnits struct of
             *      { 1, -1, 0, PM_SPACE_MBYTE, PM_TIME_SEC, 0 }
             * represents Mbytes/sec, while
             *      { 0, 1, -1, 0, PM_TIME_HOUR, 6 }
             * represents hours/million-events
            typedef struct {
                int dimSpace:4;             /* space dimension */
                int dimTime:4;              /* time dimension */
                int dimCount:4;             /* event dimension */
                unsigned int scaleSpace:4;  /* one of PM_SPACE_* below */
                unsigned int scaleTime:4;   /* one of PM_TIME_* below */
                int scaleCount:4;           /* one of PM_COUNT_* below */
            } pmUnits;                      /* dimensional units and scale of value */

            /* pmUnits.scaleSpace */
            #define PM_SPACE_BYTE   0       /* bytes */
            #define PM_SPACE_KBYTE  1       /* Kilobytes (1024) */
            #define PM_SPACE_MBYTE  2       /* Megabytes (1024^2) */
            #define PM_SPACE_GBYTE  3       /* Gigabytes (1024^3) */
            #define PM_SPACE_TBYTE  4       /* Terabytes (1024^4) */
            /* pmUnits.scaleTime */
            #define PM_TIME_NSEC    0       /* nanoseconds */
            #define PM_TIME_USEC    1       /* microseconds */
            #define PM_TIME_MSEC    2       /* milliseconds */
            #define PM_TIME_SEC     3       /* seconds */
            #define PM_TIME_MIN     4       /* minutes */
            #define PM_TIME_HOUR    5       /* hours */
             * pmUnits.scaleCount (e.g. count events, syscalls, interrupts,
             * etc.) these are simply powers of 10, and not enumerated here,
             * e.g. 6 for 10^6, or -3 for 10^-3
            #define PM_COUNT_ONE    0       /* 1 */

       Special routines (e.g.  pmExtractValue(3),  pmConvScale(3))  are  provided  to  manipulate
       values  in  conjunction with the pmUnits structure that defines the dimension and scale of
       the values for a particular performance metric.

       Below the PMAPI, the information required to complete the  pmDesc  structure,  is  fetched
       from  the  PMDAs,  and  in this way the format and scale of performance metrics may change
       dynamically, as the PMDAs and their  underlying  instrumentation  evolve  with  time.   In
       particular,  when  some  metrics  suddenly become 64-bits long, or change their units from
       Mbytes to Gbytes, well-written applications using the services provided by the PMAPI  will
       continue to function correctly.


       These routines return a negative error code to indicate failure.

              The requested PMID is not known to the PMCS

              The PMDA responsible for providing the metric is currently not available

       pmLookupDesc returns zero to indicate success.

       The  result  from  pmLookupDescs  depends  on  the  presence of any lookup failures, their
       severity and the number of metrics being looked up.

       1.  If there are no lookup failures, the return value will be numpmid.

       2.  If a fatal error is encountered, the return value will be less than  0.   For  example

       3.  If  numpmid  is  greater  than  one and non-fatal error(s) are encountered, the return
           value is the number of metric  descriptors  that  have  successfully  been  looked  up
           (greater than or equal to zero and less than or equal to numpmid).

       4.  If  numpmid is one and a non-fatal error is encountered, the return value is the error
           code (less than zero).

       When errors are  encountered,  any  metrics  that  cannot  be  looked  up  result  in  the
       corresponding descriptor element of desclist having its pmid field set to PM_ID_NULL.  The
       slightly convoluted error protocol allows  bulk  lookups,  then  probing  for  more  error
       details in the case of a specific failure.


       PMAPI(3),  pmAtomStr(3),  pmConvScale(3), pmExtractValue(3), pmGetConfig(3), pmTypeStr(3),
       pmUnitsStr(3), pcp.conf(5) and pcp.env(5).