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<H1><A NAME="SECTION04770000000000000000">Error Handling in the PBLAS</A></H1>
<P>
<A NAME="6333"> </A>
<P>
If a PBLAS routine is called with an invalid value for
any of its arguments, it must report the fact
and terminate the execution of the program. In the
model implementation, each routine, on detecting an
error, calls a common error-handling PBLAS routine<A NAME="6334"> </A><A NAME="6335"> </A>,
passing to it the current BLACS
context, the name of the routine and the number of
the first argument that is in error. If an error is
detected in the <IMG WIDTH=20 HEIGHT=32 ALIGN=MIDDLE ALT="tex2html_wrap_inline19432" SRC="img736.gif"> entry of a descriptor array,
which is the <IMG WIDTH=17 HEIGHT=15 ALIGN=BOTTOM ALT="tex2html_wrap_inline19434" SRC="img737.gif"> argument in the parameter list, the
number passed to the PBLAS error-handler routine has been arbitrarily
chosen to be <IMG WIDTH=76 HEIGHT=25 ALIGN=MIDDLE ALT="tex2html_wrap_inline19471" SRC="img738.gif">. This allows the user to
distinguish an error on a descriptor entry from an
error on a scalar argument. For efficiency purposes,
the PBLAS routines performs only a local validity
check of their argument list. If an error is detected
in at least one process of the current context, the
program execution is stopped.
<P>
A global validity check of the input arguments passed
to a PBLAS routine must be performed in the higher-level
calling procedure. To understand the need and cost of
global checking, as well as the reason why this type
of checking is not performed in the PBLAS, consider
the following example. The value of a global input
argument is legal but differs from one process to
another. The results are unpredictable. In order to
detect this kind of error situation, a synchronization
point would be necessary, which may result in a
significant performance degradation. Since every
process must call the same routine to perform the
desired operation successfully, it is natural and
safe to restrict somewhat the amount of checking
operations performed in the PBLAS routines.
<P>
Specialized implementations may call system-specific
exception-handling facilities, either via an auxiliary
routine for error handling or directly from the routine.
In addition, the testing programs can take advantage
of this exception handling mechanism by simulating
specific erroneous input argument lists and then
verifying that particular errors are correctly
detected.
<P>
For complete details on the specification of all routines, please
refer to [<A HREF="node189.html#lawn100">26</A>]. Appendix <A HREF="node184.html#chapqrefpblas">D.2</A> contains the
Quick Reference Guide to the PBLAS. An <EM>html</EM> version of this
Quick Reference Guide, along with the leading comments from each
of the routines, is available on the ScaLAPACK homepage.
<P>
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<P><ADDRESS>
<I>Susan Blackford <BR>
Tue May 13 09:21:01 EDT 1997</I>
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