Varidic function (printf) not found within a trigger

Hi all.

Within a trigger function I need to call a variadic function (actually
the printf found on  http://wiki.postgresql.org/wiki/Sprintf).
When the trigger is fired I get the following:

function printf(text, timestamp without time zone, timestamp without
time zone, text) does not exist
LINE 1: SELECT  printf( $l1$
                ^
HINT:  No function matches the given name and argument types. You
might need to add explicit type casts.

Is this a bug or am I missing something important?

--
Vincenzo Romano
NotOrAnd Information Technologies
NON QVIETIS MARIBVS NAVTA PERITVS