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       pmLookupDesc, pmReceiveDesc, pmRequestDesc - obtain a description for a
       performance metric


       #include <pcp/pmapi.h>

       int pmLookupDesc(pmID pmid, pmDesc *desc)
       int pmRequestDesc(int ctx, pmID pmid)
       int pmReceiveDesc(int ctx, pmDesc *desc)

       cc ... -lpcp


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

       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_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 value.

       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 (or
       indeed 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 */
                int scaleSpace:4;  /* one of PM_SPACE_* below */
                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.

       pmRequestDesc  and  pmReceiveDesc  are  used by applications which must
       communicate with PMCD  asynchronously.  These  functions  take  explict
       context  handle  ctx which must refer to a host context (i.e. a context
       created by  passing  PM_CONTEXT_HOST  to  pmNewContext).  pmRequestDesc
       sends  request  to  PMCD and returns  without waiting for the response,
       pmReceiveDesc reads reply from PMCD. It is the  responsibility  of  the
       application   to   make   sure   the  data  are  ready  before  calling
       pmReceiveDesc to avoid blocking.


       PMAPI(3),     pmAtomStr(3),     pmConvScale(3),      pmExtractValue(3),
       pmGetConfig(3),    pmTypesStr(3),    pmUnitsStr(3),   pcp.conf(4)   and


              The requested PMID is not known to the PMCS

              The PMDA responsible for providing the metric is  currently  not

              Context is currently in use by another asynchronous call.