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<title>avram - a virtual machine code interpreter: 3.1.4.1 Primitive types</title>

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<a name="Primitive-types"></a>
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<a name="Primitive-types-1"></a>
<h4 class="subsubsection">3.1.4.1 Primitive types</h4>

<p>A pair of functions in support of this abstraction is prototyped in
&lsquo;<tt>listfuns.h</tt>&rsquo;.  These functions will be of interest mainly to
developers wishing to implement an interface to a new library module
and make it accessible on the virtual side by way of the
<code>library</code> combinator (<a href="Library-combinator.html#Library-combinator">Library combinator</a>).
</p>
<dl>
<dt><a name="index-_002aavm_005fvalue_005fof_005flist"></a><u>Function:</u> void <b>*avm_value_of_list</b><i> (list <var>operand</var>, list *<var>message</var>, int *<var>fault</var>)</i></dt>
<dd><p>This function takes an <var>operand</var> representing a value used by a
library function in the format described above (<a href="Type-Conversions.html#Type-Conversions">Type Conversions</a>) and returns a pointer to the value.
</p>
<p>The <code>value</code> field in the <var>operand</var> normally will point to the
block of memory holding the value, and the <var>operand</var> itself will
be a list of character representations whose binary encodings spell
out the value as explained above.
</p>
<p>The <code>value</code> field need not be initialized on entry but it will be
initialized as a side effect of being computed by this function. If it
has been initialized due to a previous call with the same
<var>operand</var>, this function is a fast constant time operation.
</p>
<p>The caller should not free the pointer returned by this function
because a reference to its value will remain in the
<var>operand</var>. When the <var>operand</var> itself is freed by
<code>avm_dispose</code> (<a href="Simple-Operations.html#Simple-Operations">Simple Operations</a>), the value will go with it.
</p>
<p>If an error occurs during the evaluation of this function, the integer
referenced by <var>fault</var> will be set to a non-zero value, and the
list referenced by <var>message</var> will be assigned a representation of
a list of strings describing the error. The <var>message</var> is freshly
created and should be freed by the caller with <code>avm_dispose</code>
when no longer needed.
</p>
<p>Possible error messages are <code>&lt;'missing value'&gt;</code>, in the case of
<a name="index-missing-value"></a>
an empty <var>operand</var>, <code>&lt;'invalid value'&gt;</code> in the case of an
<a name="index-invalid-value"></a>
<var>operand</var> that is not a list of character representations, and
<code>&lt;'memory overflow'&gt;</code> if there was insufficient space to allocate
the result.
</p></dd></dl>


<dl>
<dt><a name="index-avm_005flist_005fof_005fvalue"></a><u>Function:</u> list <b>avm_list_of_value</b><i> (void *<var>contents</var>, size_t <var>size</var>, int *<var>fault</var>)</i></dt>
<dd><p>This function performs the inverse operation of
<code>avm_value_of_list</code>, taking the address of an area of
contiguously stored data and its <var>size</var> in bytes to a list
representation. The length of the list returned is equal to the number
of bytes of data, <var>size</var>, and each item of the list is a character
representation for the corresponding byte as given by <a href="Character-Table.html#Character-Table">Character Table</a>.
</p>
<p>A copy of the memory area is made so that the original is no longer
needed and may be freed by the caller. A pointer to this copy is
returned by subsequent calls to <code>avm_value_of_list</code> when the
result returned by this function is used as the <var>operand</var>
parameter.
</p>
<p>If there is insufficient memory to allocate the result, the integer
referenced by <var>fault</var> is set to a non-zero value, and a copy of
the message <code>&lt;'memory overflow'&gt;</code> represented as a list is
returned. This function could also cause a segmentation fault if it is
<a name="index-segmentation-fault-2"></a>
passed an invalid pointer or a <var>size</var> that overruns the storage
area.  However, it is acceptable to specify a <var>size</var> that is less
than the actual size of the given memory area to construct a list
representing only the first part of it. The <var>size</var> must always be
greater than zero.
</p></dd></dl>


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