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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-1171C47F-D7F3-5A49-B191-DB6CB7C84DC8" xml:lang="en"><title>Where

    13 to put trap harnesses?</title><shortdesc>Describes the criteria that should be taken into account when deciding

    14 where to put a trap harness.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    15 <p>Trap harnesses can be nested. If a function leaves, control goes to the

    16 most recent trap harness in the call stack. This allows independent sub-modules

    17 to do their own exception handling. Effective trap programming requires that

    18 the unit of recovery be correctly identified. </p>

    19 <p>The most basic option is to rely on the top-level trap harness provided

    20 as part of the application framework for all GUI programs. If a leave occurs,

    21 and it is not handled by any explicitly coded harness, the framework displays

    22 an error message corresponding to the leave code.</p>

    23 <p>For some applications, the unit of recovery may be the processing of any

    24 user command. In a particular application, the unit of recovery may be associated

    25 with part of the processing of a particular command — a much finer-grain

    26 approach.</p>

    27 <p>Coding a coarse-grain unit of recovery has the advantage of only one trap

    28 harness and recovery code, but the disadvantage that recovery code may be

    29 general and complex, and the danger that a small error leads to catastrophic

    30 results for a user (e.g. if not enough memory to apply bold formatting resulted

    31 in termination of the word processor with loss of data!).</p>

    32 <p>Coding too fine-grain units of recovery results in many trap harnesses,

    33 lots of recovery code which may require individual attention in each case,

    34 and potentially significantly increased code size.</p>

    35 <p>The correct choice is application-specific. For large applications, there

    36 may be a single course-grain unit of recovery, with other harnesses in particular

    37 places. </p>

    38 </conbody></concept>