The ON_IOERROR procedure specifies a statement to be jumped to if an I/O error occurs in the current procedure. Normally, when an I/O error occurs, an error message is printed and program execution is stopped. If ON_IOERROR is called and an I/O related error later occurs in the same procedure activation, control is transferred to the designated statement with the error code stored in the system variable !ERROR_STATE. The text of the error message is contained in !ERROR_STATE.MSG.
The effect of ON_IOERROR can be canceled by using the label "NULL" in the call.