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<h4 class="subsection">A.1.12 Exception and Error Handling in Oct-Files</h4>
<p>Another important feature of Octave is its ability to react to the user
typing <kbd>Control-C</kbd> even during calculations. This ability is based on the
C++ exception handler, where memory allocated by the C++ new/delete
methods are automatically released when the exception is treated. When
writing an oct-file, to allow Octave to treat the user typing <kbd>Control-C</kbd>,
the <code>OCTAVE_QUIT</code><!-- /@w --> macro is supplied. For example:
<pre class="example"> for (octave_idx_type i = 0; i < a.nelem (); i++)
{
OCTAVE_QUIT;
b.elem(i) = 2. * a.elem(i);
}
</pre>
<p>The presence of the <code>OCTAVE_QUIT</code><!-- /@w --> macro in the inner loop allows
Octave to treat the user request with the <kbd>Control-C</kbd>. Without this macro,
the user must either wait for the function to return before the interrupt is
processed, or press <kbd>Control-C</kbd> three times to force Octave to exit.
<p>The <code>OCTAVE_QUIT</code><!-- /@w --> macro does impose a very small speed penalty, and so
for loops that are known to be small it might not make sense to include
<code>OCTAVE_QUIT</code><!-- /@w -->.
<p>When creating an oct-file that uses an external libraries, the function
might spend a significant portion of its time in the external
library. It is not generally possible to use the <code>OCTAVE_QUIT</code><!-- /@w --> macro
in this case. The alternative in this case is
<pre class="example"> BEGIN_INTERRUPT_IMMEDIATELY_IN_FOREIGN_CODE;
... some code that calls a "foreign" function ...
END_INTERRUPT_IMMEDIATELY_IN_FOREIGN_CODE;
</pre>
<p>The disadvantage of this is that if the foreign code allocates any
memory internally, then this memory might be lost during an interrupt,
without being deallocated. Therefore, ideally Octave itself should
allocate any memory that is needed by the foreign code, with either the
fortran_vec method or the <code>OCTAVE_LOCAL_BUFFER</code><!-- /@w --> macro.
<p>The Octave unwind_protect mechanism (<a href="The-unwind_005fprotect-Statement.html#The-unwind_005fprotect-Statement">The unwind_protect Statement</a>)
can also be used in oct-files. In conjunction with the exception
handling of Octave, it is important to enforce that certain code is run
to allow variables, etc. to be restored even if an exception occurs. An
example of the use of this mechanism is
<pre class="example"><pre class="verbatim"> #include <octave/oct.h>
#include <octave/unwind-prot.h>
void
err_hand (const char *fmt, ...)
{
// Do nothing!!
}
DEFUN_DLD (unwinddemo, args, nargout, "Unwind Demo")
{
int nargin = args.length();
octave_value retval;
if (nargin < 2)
print_usage ();
else
{
NDArray a = args(0).array_value ();
NDArray b = args(1).array_value ();
if (! error_state)
{
unwind_protect::begin_frame ("Funwinddemo");
unwind_protect_ptr (current_liboctave_warning_handler);
set_liboctave_warning_handler(err_hand);
retval = octave_value (quotient (a, b));
unwind_protect::run_frame ("Funwinddemo");
}
}
return retval;
}
</pre>
</pre>
<p>As can be seen in the example:
<pre class="example"> unwinddemo (1, 0)
⇒ Inf
1 / 0
⇒ warning: division by zero
Inf
</pre>
<p>The division by zero (and in fact all warnings) is disabled in the
<code>unwinddemo</code> function.
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