NAME

       pmAddDerived, pmAddDerivedMetric  - register a per-context derived metric name and definition

C SYNOPSIS

       #include <pcp/pmapi.h>

       char *pmAddDerived(char *name, char *expr);
       int pmAddDerivedMetric(char *name, char *expr, char **errmsg);

       cc ... -lpcp

DESCRIPTION

       Derived metrics provide a way of extending the Performance Metrics Name Space (PMNS) with new metrics de‐
       fined at the PCP client-side using expressions over the existing performance metrics.

       The  pmAddDerived  and pmAddDerivedMetric routines may be used to create per-context derived metrics, and
       can only be used after the current PMAPI context has been created with pmNewContext(3).

       Per-context derived metrics are similar in all aspects except scope to global derived metrics.  The  lat‐
       ter  are  defined across all PMAPI contexts and are created with the associated pmRegisterDerived(3), pm‐
       RegisterDerivedMetric(3) and pmLoadDerivedConfig(3) routines.

       The arguments to pmAddDerived are the name of the new derived metric and expr is an  expression  defining
       how the values of name should be computed.

       pmAddDerivedMetric  is  the exact functional equivalent to pmAddDerived except that it provides a simpli‐
       fied model of error handling, where a formatted message is returned via the errmsg parameter.

       Refer to the pmRegisterDerived(3) man page for a complete description of the syntactic  rules  for  name,
       the  syntactic  and semantic rules for expr, return values and the associated error reporting mechanisms,
       and the expression evaluation rules.

       Note that for per-context derived metrics, all syntactic and semantic checks are performed  at  the  time
       pmAddDerived  or pmAddDerivedMetric is called.  This is different to global derived metrics where the se‐
       mantic checks are delayed until the metric is used in a specific PMAPI context.

       There is no ``unregister'' method, so once registered a per-context derived metric persists for the  life
       of the PMAPI context, but it is destroyed as a side-effect of pmDestroyContext(3).

DIAGNOSTICS

       On success, pmAddDerived returns NULL.

       If a syntactic error is found at the time of calling, the value returned by pmAddDerived is a pointer in‐
       to  expr  indicating  where  the error was found.  To identify what the error was, the application should
       call pmDerivedErrStr(3) to retrieve the corresponding parser error message.

       pmAddDerivedMetric returns 0 and errmsg is undefined if the parsing is successful.

       If the given expr does not conform to the required syntax pmAddDerivedMetric returns -1 and a dynamically
       allocated error message string in errmsg.  The error message is terminated with a  newline  and  includes
       both  the  input  name and expr, along with an indicator of the position at which the error was detected.
       e.g.
                 Error: pmAddDerivedMetric("my.disk.rates", ...) syntax error
                 4rat(disk.dev.read)
                     ^

       The position indicator line may be followed by an additional diagnostic line describing the nature of the
       error, when available.

       In the case of an error, the caller is responsible for calling free(3) to release the space allocated for
       errmsg.

SEE ALSO

       PCPIntro(1), PMAPI(3), pmDerivedErrStr(3), pmDestroyContext(3), pmLoadDerivedConfig(3),  pmNewContext(3),
       pmRegisterDerived(3), pmRegisterDerivedMetric(3) and PMNS(5).

Performance Co-Pilot                                                                             PMADDDERIVED(3)