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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
extracting values from the pmValue part of a pmResult structure,
optionally converting the data type, and making the result avail‐
able 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 per‐
formance 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 pm‐
Result structure that provides the ival parameter, and if valfmt
specifies PM_VAL_INSITU, then the following types are not al‐
lowed, 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 respective‐
ly). If valfmt specifies PM_VAL_SPTR or PM_VAL_DPTR, then the
value will be extracted from the associated pmValueBlock struc‐
ture, and the __int32_t, __uint32_t and float options (itype be‐
ing 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 con‐
version 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 at‐
tempting 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 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
[email protected]. This page was obtained from the project's
upstream Git repository
⟨https://github.com/performancecopilot/pcp.git⟩ on 2024-06-14.
(At that time, the date of the most recent commit that was found
in the repository was 2024-06-14.) 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
[email protected]
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 2024-06-26 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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