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<h4 class="subsection">3.1.2 Recoverable Operations</h4>

<p>The functions in this section are similar to the ones in the previous
section except with regard to error handling. Whereas the other ones
cause an error message to be printed and the process to exit in the
event of an overflow, these return to the caller, whose responsibility
it is to take appropriate action. The functions in both sections are
declared in <samp><span class="file">lists.h</span></samp>, and should be preceded by a call to
<code>avm_initialize_lists</code>.

<div class="defun">
&mdash; Function: list <b>avm_recoverable_join</b> (<var>list left, list right</var>)<var><a name="index-avm_005frecoverable_005fjoin-418"></a></var><br>
<blockquote><p>This function is similar to <code>avm_join</code>, but will return a
<code>NULL</code> pointer if memory that was needed can not be allocated. A
<code>NULL</code> pointer would never be the result of a join under normal
circumstances, so the overflow can be detected by the caller. Regardless
of whether overflow occurs, the arguments are deallocated by this function
and should not be referenced thereafter. 
</p></blockquote></div>

<div class="defun">
&mdash; Function: void <b>avm_recoverable_enqueue</b> (<var>list *front, list *back, list operand, int *fault</var>)<var><a name="index-avm_005frecoverable_005fenqueue-419"></a></var><br>
<blockquote><p>This version of the enqueue function will dispose of the <var>operand</var> if there
isn't room to append another item and set <code>*</code><var>fault</var> to a non-zero
value. Other than that, it does the same as <code>avm_enqueue</code>. 
</p></blockquote></div>

<div class="defun">
&mdash; Function: counter <b>avm_recoverable_length</b> (<var>list operand</var>)<var><a name="index-avm_005frecoverable_005flength-420"></a></var><br>
<blockquote><p>This function checks for arithmetic overflow when calculating the length
of a list, and returns a zero value if overflow occurs. The caller can
detect the error by noting that zero is not the length of any list other
than <code>NULL</code>. This kind of overflow is impossible unless the host
does not have long enough integers for its address space. 
</p></blockquote></div>

<div class="defun">
&mdash; Function: counter <b>avm_recoverable_area</b> (<var>list operand, int *fault</var>)<var><a name="index-avm_005frecoverable_005farea-421"></a></var><br>
<blockquote><p>This function is similar to <code>avm_area</code>, except that it reacts
differently to arithmetic overflow. The <code>fault</code> parameter should be
the address of an integer known to the caller, which will be set to a
non-zero value if overflow occurs. In that event, the value of zero will
also be returned for the area. Note that it is possible for non-empty
lists to have an area of zero, so this condition alone is not indicative
of an error. 
</p></blockquote></div>

<div class="defun">
&mdash; Function: list <b>avm_recoverable_natural</b> (<var>counter number</var>)<var><a name="index-avm_005frecoverable_005fnatural-422"></a></var><br>
<blockquote><p>This function returns the <code>list</code> representation of a native
unsigned long integer, provided that there is enough memory, similarly to the
<code>avm_natural</code> function. Unlike that function, this one will return
a value of <code>NULL</code> rather than exiting the program in the event of a
memory overflow. The overflow can be detected by the caller insofar as a
<code>NULL</code> <code>list</code> does not represent any number other than zero. 
</p></blockquote></div>

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