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 pmAtom
Value 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 */
void *vp; /* void 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 par
ticular 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
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 and
PM_TYPE_AGGREGATE 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 horizon
tally. 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 conver
sions 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(3S) and sprintf(3S).
| 32 | U32 | 64 | U64 | FLOAT | DBLE | STRING | AGGR |
=======|=====|=======|=====|=======|=======|======|========|======|
32 | Y | S | Y | S | P | P | N | N |
U32 | T | Y | Y | Y | P | P | N | N |
64 | T | T,S | Y | S | P | P | N | N |
U64 | T | T | T | Y | P | P | N | N |
FLOAT | P,T | P,T,S | P,T | P,T,S | Y | Y | N | N |
DBLE | P,T | P,T,S | P,T | P,T,S | P | Y | N | N |
STRING | N | N | N | N | N | N | Y | N |
AGGR | N | N | N | N | N | N | N | Y |
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 pmExtract
Value(3) using malloc(3C), and it is the caller's respon
sibility 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
the type field in a pmDesc structure, into a canonical
type for local processing.
SEE ALSO
PMAPI(3), pmAtomStr(3), pmConvScale(3), pmFetch(3),
pmLookupDesc(3), pmPrintValue(3), pmTypeStr(3) and pmU
nitsStr(3).
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
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