упрощенная выборка и преобразование значений показателей производительности (simplified performance metrics value fetch and conversion)
Диагностика (Diagnostic)
The fetchgroup API offers several options for collecting
diagnostics. Negative integer error codes may be returned from
each function for serious conditions.
In addition, each output pmAtomValue may have a corresponding
integer variable, where pmFetchGroup can store per-metric per-
instance error codes.
As an alternative, per-metric per-instance error conditions are
also signalled by setting the corresponding pmAtomValue to a
sentinel value. If unambiguous and precise error detection is
not required, this may be sufficient. The sentinel value is
negative one for all integers (including unsigned integers - i.e.
all bits are set), NaN for floating point types, a NULL pointer
for strings, and 0.0s for the timestamp. The fetchgroup API
guarantees that once an output pmAtomValue is registered (during
a successful pmExtendFetchGroup_* call), it will be cleared to
the sentinel value or to a valid converted metric value, from the
time of registration until the pmDestroyFetchGroup call.
