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pmLoadDerivedConfig - load derived metric definitions from files


#include <pcp/pmapi.h>

int pmLoadDerivedConfig(char *path);

cc ... -lpcp


Derived metrics may be used to extend the available metrics with new (derived) metrics using simple arithmetic expressions. The definitions of these metrics can be persisted and loaded programatically by monitor tools using pmLoadDerivedConfig.

The path parameter defines a colon separated list of files and/or directories (the syntax is the same as for the $PATH variable for sh(1)), from which derived metric specifications are to be sourced. The path components are expanded into a list of files as follows: if a component is a file, then that file is added to the list, else if a component is a directory then recursive descent is used to enumerate all files below that directory and these are added to the list. Each file in the resulting list is parsed in order, and according to the derived metrics syntax described below.

Each line of the file(s) identified by path is either a comment line (with a ``#'' in the first position of the line) or the declaration of a derived performance metric, specified as:

  • the name of the derived metric, using the same ``dot notation'' syntax that is used for PCP performance metrics, see PCPIntro(1) and PMNS(5).
  • an equals sign (``='')
  • a valid expression for a derived metric, as described in pmRegisterDerived(3).

For each line containing a derived metric definition, pmRegisterDerived(3) is called to register the new derived metric.

Once a derived metric has been declared, it may be assigned additional attributes with a line of the form:

  • the name of the derived metric,
  • a left parenthesis, an attribute type and a right parenthesis,
  • an equals sign (``=''),
  • an attribute value.

Currently, attribute type may be either oneline or helptext to designate the ``one line'' or expanded help text to be associated with the derived metric, see pmLookupText(3).

The attribute value may be either arbitrary text following the ``='' and ending at the end of the line, else a string enclosed in either single quotes (') or double quotes ("). In the latter case, the attribute value may span multiple lines, and a simple escape mechanism is supported, namely for any character ``x'', ``\x'' is replaced by ``x'' (this allows quotes to be escaped within a string, for example), and there is a special case when the ``\'' comes at the end of the line in which case the following newline is not included in the attribute value.

Outside of attribute values, white space is ignored in the lines, and blank lines are ignored altogether.

Because pmLoadDerivedConfig may process many files, each of which may contain many derived metric specifications, it is not possible to provide a specific error status on return. Hence the result from pmLoadDerivedConfig will be the number of derived metrics successfully loaded from files on the given path. Catastrophic errors such as not being able to open one of the files on the given path will cause an immediate return with a negative return value that can be passed to pmErrStr(3) to obtain the associated error message.

When errors are encountered in the derived metric specifications diagnostic messages are generated by pmRegisterDerived(3) and displayed via pmprintf(3).


# sample derived metric definitions
bad_in_pkts = +
# note the following would need to be on a single line ... = 100 * delta( /

(delta( + delta( = percentage of disk reads = '\ Percentage of disk reads compared to the total number of disk reads and disk writes.'


sh(1), PCPIntro(1), PMAPI(3), pmLookupText(3), pmRegisterDerived(3), pmprintf(3) and PMNS(5).

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