pmextractvalue(3) — Linux manual page


PMEXTRACTVALUE(3)       Library Functions Manual       PMEXTRACTVALUE(3)

NAME         top

       pmExtractValue - extract a performance metric value from a
       pmResult structure

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

       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

       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,
       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

DIAGNOSTICS         top


              Impossible conversion, marked by N in above table


              High-order truncation occurred


              Conversion of negative value to unsigned type attempted

SEE ALSO         top

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

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 2023-12-22.
       (At that time, the date of the most recent commit that was found
       in the repository was 2023-12-16.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
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       (which is not part of the original manual page), send a mail to

Performance Co-Pilot               PCP                 PMEXTRACTVALUE(3)

Pages that refer to this page: pcpintro(3)pmapi(3)pmatomstr(3)pmconvscale(3)pmextractvalue(3)pmfetch(3)pmfetchgroup(3)pmlookupdesc(3)pmprintvalue(3)pmsemstr(3)pmtypestr(3)pmunitsstr(3)