% File src/library/base/man/reg.finalizer.Rd
% Part of the R package, http://www.R-project.org
% Copyright 1995-2014 R Core Team
% Distributed under GPL 2 or later

\name{reg.finalizer}
\alias{reg.finalizer}
\alias{finalizer}
\title{Finalization of Objects}
\description{
  Registers an \R function to be called upon garbage collection of
  object or (optionally) at the end of an \R session.
}
\usage{
  reg.finalizer(e, f, onexit = FALSE)
}
\arguments{
  \item{e}{Object to finalize.  Must be an environment or an external pointer.}
  \item{f}{Function to call on finalization. Must accept a single argument,
    which will be the object to finalize.}
  \item{onexit}{logical: should the finalizer be run if the object is
    still uncollected at the end of the \R session?}
}
\value{
  \code{NULL}.
}
\note{
  The main purpose of this function is to allow objects that refer to
  external items (a temporary file, say) to perform cleanup actions when
  they are no longer referenced from within \R.  This only makes sense
  for objects that are never copied on assignment, hence the restriction
  to environments and external pointers.

  Note that as \R's interpreter is not re-entrant, the finalizer may
  be run in the middle of a computation.  So there are many functions
  which it is not safe to call from \code{f}: one example which caused
  trouble is \code{\link{options}}. As from \R 3.0.3 finalizers are
  marked to be run at garbage collection but only run at a relatively
  safe time thereafter.
}
\seealso{
  \code{\link{gc}} and \code{\link{Memory}} for garbage collection and
  memory management.
}
\examples{
f <- function(e) print("cleaning....")
g <- function(x){ e <- environment(); reg.finalizer(e, f) }
g()
invisible(gc()) # trigger cleanup
}
\keyword{programming}
\keyword{environment}