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NAME | C SYNOPSIS | DESCRIPTION | DIAGNOSTICS | SEE ALSO | COLOPHON |
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PMEXTRACTVALUE(3) Library Functions Manual PMEXTRACTVALUE(3)
pmExtractValue - extract a performance metric value from a
pmResult structure
#include <pcp/pmapi.h>
int pmExtractValue(int valfmt, const pmValue *ival, int itype,
pmAtomValue *oval, int otype);
cc ... -lpcp
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 ex‐
tracting values from the pmValue part of a pmResult structure, op‐
tionally 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 ac‐
cording 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 struc‐
ture, following a call to pmLookupDesc(3) for a particular perfor‐
mance 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 pmRe‐
sult 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, dou‐
ble, 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_SPTR or PM_VAL_DPTR, then the value will be ex‐
tracted 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 per‐
formed (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 rep‐
resented 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 at‐
tempting to duplicate functionality already available in the C li‐
brary 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 check‐
ing 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 con‐
structed 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.
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
PMAPI(3), pmAtomStr(3), pmConvScale(3), pmFetch(3),
pmFetchGroup(3), pmLookupDesc(3), pmPrintValue(3), pmTypeStr(3),
pmUnitsStr(3) and pmUnpackEventRecords(3).
This page is part of the PCP (Performance Co-Pilot) project. In‐
formation about the project can be found at ⟨http://www.pcp.io/⟩.
If you have a bug report for this manual page, send it to
pcp@groups.io. This page was obtained from the project's upstream
Git repository ⟨https://github.com/performancecopilot/pcp.git⟩ on
2025-02-02. (At that time, the date of the most recent commit
that was found in the repository was 2025-01-30.) 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 man-pages@man7.org
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)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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