pmDerivedErrStr man page
pmDerivedErrStr — return error message from failure to parse derived metric definition
cc ... -lpcp
When pmRegisterDerived(3) is called to add a new derived metric definition and an error occurs, pmDerivedErrStr may be called to return the associated error message.
The pmRegisterDerivedMetric interface provides a convenience wrapper around both pmRegisterDerived and pmDerivedErrStr.
PCPIntro(1), PMAPI(3) and pmRegisterDerived(3).