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<title>avram - a virtual machine code interpreter: 3.1.4 Type Conversions</title>

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<a name="Type-Conversions"></a>
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<a name="Type-Conversions-1"></a>
<h3 class="subsection">3.1.4 Type Conversions</h3>

<p>External library functions accessed by the <code>library</code> combinator
as explained in <a href="Library-combinator.html#Library-combinator">Library combinator</a> may operate on data other
than the <code>list</code> type usually used by <code>avram</code>, such as
floating point numbers and arrays, but a virtual code application must
be able to represent the arguments and results of these functions in
order to use them. As a matter of convention, a data structure
occupying <var>size</var> bytes of contiguous storage on the host machine
appears as a list of length <var>size</var> to a virtual code application,
in which each item corresponds to a byte, and is represented according
to <a href="Character-Table.html#Character-Table">Character Table</a>.
</p>
<p>In principle, a virtual code application invoking a library function
to operate on a contiguous block of data, such as an IEEE double
precision number, for example, would construct a list of eight
character representations (one for each byte in a double precision
number), and pass this list as an argument to the library
function. The virtual machine would transparently convert this
representation to the native floating point format, evaluate the
function, and convert the result back to a list.  In practice, high
level language features beyond the scope of this document would
insulate the programmer from some of the details on the application
side as well.
</p>
<p>To save the time of repeatedly converting between the list
representation and the contiguous native binary representation, the
structure referenced by a <code>list</code> pointer contains a <code>value</code>
<a name="index-value-field"></a>
field which is a <code>void</code> pointer to a block of memory of
unspecified type, and serves as a persistent cache of the value
represented by the list. This field normally should be managed by the
API rather than being accessed directly by client modules, but see the
code in &lsquo;<tt>mpfr.c</tt>&rsquo; for an example of a situation in which it&rsquo;s
appropriate to break this rule. (Generally these situations involve
library functions operating on non-contiguous data.)
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="Primitive-types.html#Primitive-types">3.1.4.1 Primitive types</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="One-dimensional-arrays.html#One-dimensional-arrays">3.1.4.2 One dimensional arrays</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="Two-dimensional-arrays.html#Two-dimensional-arrays">3.1.4.3 Two dimensional arrays</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="Related-utility-functions.html#Related-utility-functions">3.1.4.4 Related utility functions</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
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