
on.exit Function Exit Code Description 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.  Usage 
on.exit(expr = NULL, add = FALSE, after = TRUE)
 Arguments  
expr  an expression to be executed. 
 
add  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 arguments add and after.  Value Invisible NULL.  References Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.  See Also sys.on.exit which returns the expression stored for use by on.exit() in the function in which sys.on.exit() is evaluated.  Examples 
require(graphics)

opar <- par(mai = c(1,1,1,1))
on.exit(par(opar))



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