pmExtractValue(3) — Linux manual page


PMEXTRACTVALUE(3)         Library Functions Manual         PMEXTRACTVALUE(3)

NAME         top

       pmExtractValue  -  extract a performance metric value from a pmResult

C SYNOPSIS         top

       #include <pcp/pmapi.h>

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

       cc ... -lpcp

DESCRIPTION         top

       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

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

       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_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

       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

       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.

SEE ALSO         top

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

DIAGNOSTICS         top


              Impossible conversion, marked by N in above table


              High-order truncation occurred


              Conversion of negative value to unsigned type attempted

COLOPHON         top

       This page is part of the PCP (Performance Co-Pilot) project.
       Information about the project can be found at ⟨⟩.
       If you have a bug report for this manual page, send it to  This page was obtained from the project's upstream
       Git repository ⟨⟩ on
       2020-09-18.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-09-18.)  If you discover any
       rendering problems in this HTML version of the page, or you believe
       there is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

Performance Co-Pilot                 PCP                   PMEXTRACTVALUE(3)

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