# on.exit

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##### Function Exit Code

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(), then the current on.exit code is removed.

Keywords
programming
##### Usage
on.exit(expr = NULL, add = FALSE, after = TRUE)
##### Arguments
expr

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 default) expr will overwrite any previously set expressions.

after

if 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.

##### Details

The 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 expr use substitute or similar.

If multiple 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 add.

##### Value

Invisible NULL.

##### References

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 by on.exit() in the function in which sys.on.exit() is evaluated.
library(base) # NOT RUN { require(graphics) opar <- par(mai = c(1,1,1,1)) on.exit(par(opar)) # }