noble (3) pmExtractValue.3.gz

Provided by: libpcp3-dev_6.2.0-1.1build4_amd64 bug

NAME

       pmExtractValue - extract a performance metric value from a pmResult structure

C SYNOPSIS

       #include <pcp/pmapi.h>

       int pmExtractValue(int valfmt, const pmValue *ival, int itype, pmAtomValue *oval, int otype);

       cc ... -lpcp

DESCRIPTION

       The  pmValue  structure  is  embedded  within  the  pmResult structure that is used to return one or more
       performance metrics; see pmFetch(3).

       All performance metric values may be encoded in a pmAtomValue union, defined as follows;

            typedef union {
                __int32_t    l;     /* 32-bit signed */
                __uint32_t   ul;    /* 32-bit unsigned */
                __int64_t    ll;    /* 64-bit signed */
                __uint64_t   ull;   /* 64-bit unsigned */
                float        f;     /* 32-bit floating point */
                double       d;     /* 64-bit floating point */
                char         *cp;   /* char ptr */
                pmValueBlock *vbp;  /* pmValueBlock ptr */
            } pmAtomValue;

       The routine pmExtractValue provides a convenient mechanism for extracting values from the pmValue part of
       a  pmResult  structure,  optionally  converting  the  data  type,  and making the result available to the
       application programmer.

       itype defines the data type of the input value held in ival according to the storage  format  defined  by
       valfmt (see pmFetch(3)).  otype defines the data type of the result to be placed in oval.

       The  value  for itype is typically extracted from a pmDesc structure, following a call to pmLookupDesc(3)
       for a particular performance metric.

       The otype value should be one of the defined PM_TYPE_...  values, that have a 1:1 correspondence with the
       fields in the pmAtomValue union.

       Normally  the  valfmt  parameter would be plucked from the same pmResult structure that provides the ival
       parameter, and if valfmt specifies PM_VAL_INSITU, then the following types  are  not  allowed,  as  these
       cannot  be  encoded in 32-bits; __int64_t, __uint64_t, double, char * and void * (the corresponding itype
       values are PM_TYPE_64, PM_TYPE_U64, PM_TYPE_DOUBLE, PM_TYPE_STRING, PM_TYPE_AGGREGATE  and  PM_TYPE_EVENT
       respectively).   If  valfmt  specifies  PM_VAL_PTR,  then the value will be extracted from the associated
       pmValueBlock structure, and  the  __int32_t,  __uint32_t  and  float  options  (itype  being  PM_TYPE_32,
       PM_TYPE_U32 and PM_TYPE_FLOAT respectively) are not allowed, as PM_VAL_INSITU is the appropriate encoding
       for these.

       The following table defines the various possibilities for the type conversion -- the input  type  (itype)
       is shown vertically, and the output type (otype) is shown horizontally.  Y means the conversion is always
       acceptable, N means the conversion can never be performed (the function returns PM_ERR_CONV), P means the
       conversion  may  lose  accuracy  (but  no error status is returned), T means the result may be subject to
       high-order truncation (in which case the function returns PM_ERR_TRUNC) and S means the conversion may be
       impossible  due  to  the sign of the input value (in which case the function returns PM_ERR_SIGN).  If an
       error occurs, the value represented by oval will be zero (or NULL).

       Note that although some of the conversions involving the types PM_TYPE_STRING and  PM_TYPE_AGGREGATE  are
       indeed  possible,  but  are  marked  N - the rationale is that pmExtractValue should not be attempting to
       duplicate functionality already available in the C library via sscanf(3) and sprintf(3).

       No conversion involving the type PM_TYPE_EVENT is supported.

             | 32  |  U32  | 64  |  U64  | FLOAT | DBLE | STRNG | AGGR | EVENT
       ======|=====|=======|=====|=======|=======|======|=======|======|=======
       32    |  Y  |   S   |  Y  |   S   |   P   |  P   |   N   |  N   |   N
       U32   |  T  |   Y   |  Y  |   Y   |   P   |  P   |   N   |  N   |   N
       64    |  T  |  T,S  |  Y  |   S   |   P   |  P   |   N   |  N   |   N
       U64   |  T  |   T   |  T  |   Y   |   P   |  P   |   N   |  N   |   N
       FLOAT | P,T | P,T,S | P,T | P,T,S |   Y   |  Y   |   N   |  N   |   N
       DBLE  | P,T | P,T,S | P,T | P,T,S |   P   |  Y   |   N   |  N   |   N
       STRNG |  N  |   N   |  N  |   N   |   N   |  N   |   Y   |  N   |   N
       AGGR  |  N  |   N   |  N  |   N   |   N   |  N   |   N   |  Y   |   N
       EVENT |  N  |   N   |  N  |   N   |   N   |  N   |   N   |  N   |   N

       In the cases where multiple conversion errors could occur, the first encountered error will be  notified,
       and the order of checking is not defined.

       If   the   output  conversion  is  to  one  of  the  pointer  types,  i.e.  otype  is  PM_TYPE_STRING  or
       PM_TYPE_AGGREGATE, then the value buffer will have been allocated by pmExtractValue(3)  using  malloc(3),
       and it is the caller's responsibility to free the space when it is no longer required.

       Although  this  function  appears  rather  complex,  it has been constructed to assist the development of
       performance tools that wish to convert values, whose type is only known via the type field  in  a  pmDesc
       structure,  into  a  canonical  type  for local processing.  See the pmFetchGroup functions for a simpler
       alternative.

DIAGNOSTICS

       PM_ERR_CONV

              Impossible conversion, marked by N in above table

       PM_ERR_TRUNC

              High-order truncation occurred

       PM_ERR_SIGN

              Conversion of negative value to unsigned type attempted

SEE ALSO

       PMAPI(3), pmAtomStr(3), pmConvScale(3), pmFetch(3),  pmFetchGroup(3),  pmLookupDesc(3),  pmPrintValue(3),
       pmTypeStr(3), pmUnitsStr(3) and pmUnpackEventRecords(3).