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

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

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

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

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

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

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

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

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

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

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

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

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

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

approach.</p>

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

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

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

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

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

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

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

and potentially significantly increased code size.</p>

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

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

places. </p>