pmExtractValue (3) - Linux Manuals

pmExtractValue: extract a performance metric value from a pmResult structure

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 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 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 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, PM_TYPE_STRING, PM_TYPE_AGGREGATE and 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 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 alternative.

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