on.exit records the expression given as its argument as needing
to be executed when the current function exits (either naturally or as
the result of an error). This is useful for resetting graphical
parameters or performing other cleanup actions.
If no expression is provided, i.e., the call is
on.exit code is removed.
on.exit(expr = NULL, add = FALSE, after = TRUE)
an expression to be executed.
if TRUE, add
expr to be executed after any previously
set expressions (or before if
after is FALSE); otherwise (the
expr will overwrite any previously set expressions.
add is TRUE and
after is FALSE, then
expr will be added on top of the expressions that were already
registered. The resulting last in first out order is useful for freeing
or closing resources in reverse order.
expr argument passed to
on.exit is recorded without
evaluation. If it is not subsequently removed/replaced by another
on.exit call in the same function, it is evaluated in the
evaluation frame of the function when it exits (including during
standard error handling). Thus any functions or variables in the
expression will be looked for in the function and its environment at
the time of exit: to capture the current value in
substitute or similar.
on.exit expressions are set using
add = TRUE
then all expressions will be run even if one signals an error.
This is a ‘special’ primitive function: it only
evaluates the argument
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
sys.on.exit which returns the expression stored for use
on.exit() in the function in which