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<title>Values From Inferior In Guile (Debugging with GDB)</title>
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<a name="Values-From-Inferior-In-Guile"></a>
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<p>
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Next: <a href="Arithmetic-In-Guile.html#Arithmetic-In-Guile" accesskey="n" rel="next">Arithmetic In Guile</a>, Previous: <a href="Guile-Exception-Handling.html#Guile-Exception-Handling" accesskey="p" rel="prev">Guile Exception Handling</a>, Up: <a href="Guile-API.html#Guile-API" accesskey="u" rel="up">Guile API</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Values-From-Inferior-In-Guile-1"></a>
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<h4 class="subsubsection">23.3.3.5 Values From Inferior In Guile</h4>
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<a name="index-values-from-inferior_002c-in-guile"></a>
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<a name="index-guile_002c-working-with-values-from-inferior"></a>
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<a name="index-_003cgdb_003avalue_003e"></a>
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<p><small>GDB</small> provides values it obtains from the inferior program in
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an object of type <code><gdb:value></code>. <small>GDB</small> uses this object
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for its internal bookkeeping of the inferior’s values, and for
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fetching values when necessary.
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</p>
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<p><small>GDB</small> does not memoize <code><gdb:value></code> objects.
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<code>make-value</code> always returns a fresh object.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) guile (eq? (make-value 1) (make-value 1))
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$1 = #f
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(gdb) guile (equal? (make-value 1) (make-value 1))
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$1 = #t
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</pre></div>
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<p>A <code><gdb:value></code> that represents a function can be executed via
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inferior function call with <code>value-call</code>.
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Any arguments provided to the call must match the function’s prototype,
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and must be provided in the order specified by that prototype.
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</p>
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<p>For example, <code>some-val</code> is a <code><gdb:value></code> instance
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representing a function that takes two integers as arguments. To
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execute this function, call it like so:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define result (value-call some-val 10 20))
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</pre></div>
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<p>Any values returned from a function call are <code><gdb:value></code> objects.
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</p>
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<p>Note: Unlike Python scripting in <small>GDB</small>,
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inferior values that are simple scalars cannot be used directly in
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Scheme expressions that are valid for the value’s data type.
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For example, <code>(+ (parse-and-eval "int_variable") 2)</code> does not work.
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And inferior values that are structures or instances of some class cannot
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be accessed using any special syntax, instead <code>value-field</code> must be used.
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</p>
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<p>The following value-related procedures are provided by the
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<code>(gdb)</code> module.
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</p>
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<dl>
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<dt><a name="index-value_003f"></a>Scheme Procedure: <strong>value?</strong> <em>object</em></dt>
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<dd><p>Return <code>#t</code> if <var>object</var> is a <code><gdb:value></code> object.
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Otherwise return <code>#f</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-make_002dvalue"></a>Scheme Procedure: <strong>make-value</strong> <em>value <span class="roman">[</span>#:type type<span class="roman">]</span></em></dt>
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<dd><p>Many Scheme values can be converted directly to a <code><gdb:value></code>
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with this procedure. If <var>type</var> is specified, the result is a value
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of this type, and if <var>value</var> can’t be represented with this type
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an exception is thrown. Otherwise the type of the result is determined from
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<var>value</var> as described below.
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</p>
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<p>See <a href="Architectures-In-Guile.html#Architectures-In-Guile">Architectures In Guile</a>, for a list of the builtin
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types for an architecture.
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</p>
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<p>Here’s how Scheme values are converted when <var>type</var> argument to
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<code>make-value</code> is not specified:
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</p>
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<dl compact="compact">
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<dt>Scheme boolean</dt>
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<dd><p>A Scheme boolean is converted the boolean type for the current language.
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</p>
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</dd>
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<dt>Scheme integer</dt>
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<dd><p>A Scheme integer is converted to the first of a C <code>int</code>,
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<code>unsigned int</code>, <code>long</code>, <code>unsigned long</code>,
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<code>long long</code> or <code>unsigned long long</code> type
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for the current architecture that can represent the value.
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</p>
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<p>If the Scheme integer cannot be represented as a target integer
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an <code>out-of-range</code> exception is thrown.
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</p>
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</dd>
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<dt>Scheme real</dt>
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<dd><p>A Scheme real is converted to the C <code>double</code> type for the
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current architecture.
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</p>
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</dd>
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<dt>Scheme string</dt>
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<dd><p>A Scheme string is converted to a string in the current target
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language using the current target encoding.
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Characters that cannot be represented in the current target encoding
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are replaced with the corresponding escape sequence. This is Guile’s
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<code>SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE</code> conversion strategy
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(see <a href="http://www.gnu.org/software/guile/manual/html_node/Strings.html#Strings">Strings</a> in <cite>GNU Guile Reference Manual</cite>).
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</p>
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<p>Passing <var>type</var> is not supported in this case,
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if it is provided a <code>wrong-type-arg</code> exception is thrown.
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</p>
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</dd>
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<dt><code><gdb:lazy-string></code></dt>
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<dd><p>If <var>value</var> is a <code><gdb:lazy-string></code> object (see <a href="Lazy-Strings-In-Guile.html#Lazy-Strings-In-Guile">Lazy Strings In Guile</a>), then the <code>lazy-string->value</code> procedure is called, and
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its result is used.
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</p>
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<p>Passing <var>type</var> is not supported in this case,
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if it is provided a <code>wrong-type-arg</code> exception is thrown.
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</p>
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</dd>
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<dt>Scheme bytevector</dt>
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<dd><p>If <var>value</var> is a Scheme bytevector and <var>type</var> is provided,
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<var>value</var> must be the same size, in bytes, of values of type <var>type</var>,
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and the result is essentially created by using <code>memcpy</code>.
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</p>
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<p>If <var>value</var> is a Scheme bytevector and <var>type</var> is not provided,
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the result is an array of type <code>uint8</code> of the same length.
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</p></dd>
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</dl>
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</dd></dl>
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<a name="index-optimized-out-value-in-guile"></a>
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<dl>
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<dt><a name="index-value_002doptimized_002dout_003f"></a>Scheme Procedure: <strong>value-optimized-out?</strong> <em>value</em></dt>
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<dd><p>Return <code>#t</code> if the compiler optimized out <var>value</var>,
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thus it is not available for fetching from the inferior.
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Otherwise return <code>#f</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-value_002daddress"></a>Scheme Procedure: <strong>value-address</strong> <em>value</em></dt>
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<dd><p>If <var>value</var> is addressable, returns a
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<code><gdb:value></code> object representing the address.
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Otherwise, <code>#f</code> is returned.
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</p></dd></dl>
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<dl>
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<dt><a name="index-value_002dtype"></a>Scheme Procedure: <strong>value-type</strong> <em>value</em></dt>
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<dd><p>Return the type of <var>value</var> as a <code><gdb:type></code> object
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(see <a href="Types-In-Guile.html#Types-In-Guile">Types In Guile</a>).
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</p></dd></dl>
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<dl>
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<dt><a name="index-value_002ddynamic_002dtype"></a>Scheme Procedure: <strong>value-dynamic-type</strong> <em>value</em></dt>
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<dd><p>Return the dynamic type of <var>value</var>. This uses C<tt>++</tt> run-time
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type information (<acronym>RTTI</acronym>) to determine the dynamic type of the
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value. If the value is of class type, it will return the class in
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which the value is embedded, if any. If the value is of pointer or
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reference to a class type, it will compute the dynamic type of the
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referenced object, and return a pointer or reference to that type,
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respectively. In all other cases, it will return the value’s static
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type.
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</p>
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<p>Note that this feature will only work when debugging a C<tt>++</tt> program
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that includes <acronym>RTTI</acronym> for the object in question. Otherwise,
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it will just return the static type of the value as in <kbd>ptype foo</kbd>.
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See <a href="Symbols.html#Symbols">ptype</a>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-value_002dcast"></a>Scheme Procedure: <strong>value-cast</strong> <em>value type</em></dt>
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<dd><p>Return a new instance of <code><gdb:value></code> that is the result of
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casting <var>value</var> to the type described by <var>type</var>, which must
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be a <code><gdb:type></code> object. If the cast cannot be performed for some
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reason, this method throws an exception.
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</p></dd></dl>
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<dl>
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<dt><a name="index-value_002ddynamic_002dcast"></a>Scheme Procedure: <strong>value-dynamic-cast</strong> <em>value type</em></dt>
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<dd><p>Like <code>value-cast</code>, but works as if the C<tt>++</tt> <code>dynamic_cast</code>
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operator were used. Consult a C<tt>++</tt> reference for details.
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</p></dd></dl>
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<dl>
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<dt><a name="index-value_002dreinterpret_002dcast"></a>Scheme Procedure: <strong>value-reinterpret-cast</strong> <em>value type</em></dt>
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<dd><p>Like <code>value-cast</code>, but works as if the C<tt>++</tt> <code>reinterpret_cast</code>
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operator were used. Consult a C<tt>++</tt> reference for details.
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</p></dd></dl>
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<dl>
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<dt><a name="index-value_002ddereference"></a>Scheme Procedure: <strong>value-dereference</strong> <em>value</em></dt>
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<dd><p>For pointer data types, this method returns a new <code><gdb:value></code> object
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whose contents is the object pointed to by <var>value</var>. For example, if
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<code>foo</code> is a C pointer to an <code>int</code>, declared in your C program as
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</p>
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<div class="smallexample">
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<pre class="smallexample">int *foo;
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</pre></div>
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<p>then you can use the corresponding <code><gdb:value></code> to access what
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<code>foo</code> points to like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define bar (value-dereference foo))
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</pre></div>
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<p>The result <code>bar</code> will be a <code><gdb:value></code> object holding the
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value pointed to by <code>foo</code>.
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</p>
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<p>A similar function <code>value-referenced-value</code> exists which also
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returns <code><gdb:value></code> objects corresonding to the values pointed to
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by pointer values (and additionally, values referenced by reference
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values). However, the behavior of <code>value-dereference</code>
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differs from <code>value-referenced-value</code> by the fact that the
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behavior of <code>value-dereference</code> is identical to applying the C
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unary operator <code>*</code> on a given value. For example, consider a
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reference to a pointer <code>ptrref</code>, declared in your C<tt>++</tt> program
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as
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</p>
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<div class="smallexample">
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<pre class="smallexample">typedef int *intptr;
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...
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int val = 10;
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intptr ptr = &val;
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intptr &ptrref = ptr;
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</pre></div>
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<p>Though <code>ptrref</code> is a reference value, one can apply the method
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<code>value-dereference</code> to the <code><gdb:value></code> object corresponding
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to it and obtain a <code><gdb:value></code> which is identical to that
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corresponding to <code>val</code>. However, if you apply the method
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<code>value-referenced-value</code>, the result would be a <code><gdb:value></code>
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object identical to that corresponding to <code>ptr</code>.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define scm-ptrref (parse-and-eval "ptrref"))
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(define scm-val (value-dereference scm-ptrref))
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(define scm-ptr (value-referenced-value scm-ptrref))
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</pre></div>
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<p>The <code><gdb:value></code> object <code>scm-val</code> is identical to that
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corresponding to <code>val</code>, and <code>scm-ptr</code> is identical to that
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corresponding to <code>ptr</code>. In general, <code>value-dereference</code> can
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be applied whenever the C unary operator <code>*</code> can be applied
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to the corresponding C value. For those cases where applying both
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<code>value-dereference</code> and <code>value-referenced-value</code> is allowed,
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the results obtained need not be identical (as we have seen in the above
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example). The results are however identical when applied on
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<code><gdb:value></code> objects corresponding to pointers (<code><gdb:value></code>
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objects with type code <code>TYPE_CODE_PTR</code>) in a C/C<tt>++</tt> program.
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</p></dd></dl>
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<dl>
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<dt><a name="index-value_002dreferenced_002dvalue"></a>Scheme Procedure: <strong>value-referenced-value</strong> <em>value</em></dt>
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<dd><p>For pointer or reference data types, this method returns a new
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<code><gdb:value></code> object corresponding to the value referenced by the
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pointer/reference value. For pointer data types,
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<code>value-dereference</code> and <code>value-referenced-value</code> produce
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identical results. The difference between these methods is that
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<code>value-dereference</code> cannot get the values referenced by reference
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values. For example, consider a reference to an <code>int</code>, declared
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in your C<tt>++</tt> program as
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</p>
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<div class="smallexample">
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<pre class="smallexample">int val = 10;
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int &ref = val;
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</pre></div>
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|
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<p>then applying <code>value-dereference</code> to the <code><gdb:value></code> object
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corresponding to <code>ref</code> will result in an error, while applying
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<code>value-referenced-value</code> will result in a <code><gdb:value></code> object
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identical to that corresponding to <code>val</code>.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define scm-ref (parse-and-eval "ref"))
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(define err-ref (value-dereference scm-ref)) ;; error
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(define scm-val (value-referenced-value scm-ref)) ;; ok
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</pre></div>
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<p>The <code><gdb:value></code> object <code>scm-val</code> is identical to that
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corresponding to <code>val</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-value_002dfield"></a>Scheme Procedure: <strong>value-field</strong> <em>value field-name</em></dt>
|
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<dd><p>Return field <var>field-name</var> from <code><gdb:value></code> object <var>value</var>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-value_002dsubscript"></a>Scheme Procedure: <strong>value-subscript</strong> <em>value index</em></dt>
|
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<dd><p>Return the value of array <var>value</var> at index <var>index</var>.
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The <var>value</var> argument must be a subscriptable <code><gdb:value></code> object.
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</p></dd></dl>
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<dl>
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<dt><a name="index-value_002dcall"></a>Scheme Procedure: <strong>value-call</strong> <em>value arg-list</em></dt>
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<dd><p>Perform an inferior function call, taking <var>value</var> as a pointer
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to the function to call.
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Each element of list <var>arg-list</var> must be a <gdb:value> object or an object
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that can be converted to a value.
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The result is the value returned by the function.
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</p></dd></dl>
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<dl>
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<dt><a name="index-value_002d_003ebool"></a>Scheme Procedure: <strong>value->bool</strong> <em>value</em></dt>
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<dd><p>Return the Scheme boolean representing <code><gdb:value></code> <var>value</var>.
|
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The value must be “integer like”. Pointers are ok.
|
|
</p></dd></dl>
|
|
|
|
<dl>
|
|
<dt><a name="index-value_002d_003einteger"></a>Scheme Procedure: <strong>value->integer</strong></dt>
|
|
<dd><p>Return the Scheme integer representing <code><gdb:value></code> <var>value</var>.
|
|
The value must be “integer like”. Pointers are ok.
|
|
</p></dd></dl>
|
|
|
|
<dl>
|
|
<dt><a name="index-value_002d_003ereal"></a>Scheme Procedure: <strong>value->real</strong></dt>
|
|
<dd><p>Return the Scheme real number representing <code><gdb:value></code> <var>value</var>.
|
|
The value must be a number.
|
|
</p></dd></dl>
|
|
|
|
<dl>
|
|
<dt><a name="index-value_002d_003ebytevector"></a>Scheme Procedure: <strong>value->bytevector</strong></dt>
|
|
<dd><p>Return a Scheme bytevector with the raw contents of <code><gdb:value></code>
|
|
<var>value</var>. No transformation, endian or otherwise, is performed.
|
|
</p></dd></dl>
|
|
|
|
<dl>
|
|
<dt><a name="index-value_002d_003estring"></a>Scheme Procedure: <strong>value->string</strong> <em>value <span class="roman">[</span>#:encoding encoding<span class="roman">]</span> <span class="roman">[</span>#:errors errors<span class="roman">]</span> <span class="roman">[</span>#:length length<span class="roman">]</span></em></dt>
|
|
<dd><p>If <var>value></var> represents a string, then this method
|
|
converts the contents to a Guile string. Otherwise, this method will
|
|
throw an exception.
|
|
</p>
|
|
<p>Values are interpreted as strings according to the rules of the
|
|
current language. If the optional length argument is given, the
|
|
string will be converted to that length, and will include any embedded
|
|
zeroes that the string may contain. Otherwise, for languages
|
|
where the string is zero-terminated, the entire string will be
|
|
converted.
|
|
</p>
|
|
<p>For example, in C-like languages, a value is a string if it is a pointer
|
|
to or an array of characters or ints of type <code>wchar_t</code>, <code>char16_t</code>,
|
|
or <code>char32_t</code>.
|
|
</p>
|
|
<p>If the optional <var>encoding</var> argument is given, it must be a string
|
|
naming the encoding of the string in the <code><gdb:value></code>, such as
|
|
<code>"ascii"</code>, <code>"iso-8859-6"</code> or <code>"utf-8"</code>. It accepts
|
|
the same encodings as the corresponding argument to Guile’s
|
|
<code>scm_from_stringn</code> function, and the Guile codec machinery will be used
|
|
to convert the string. If <var>encoding</var> is not given, or if
|
|
<var>encoding</var> is the empty string, then either the <code>target-charset</code>
|
|
(see <a href="Character-Sets.html#Character-Sets">Character Sets</a>) will be used, or a language-specific encoding
|
|
will be used, if the current language is able to supply one.
|
|
</p>
|
|
<p>The optional <var>errors</var> argument is one of <code>#f</code>, <code>error</code> or
|
|
<code>substitute</code>. <code>error</code> and <code>substitute</code> must be symbols.
|
|
If <var>errors</var> is not specified, or if its value is <code>#f</code>, then the
|
|
default conversion strategy is used, which is set with the Scheme function
|
|
<code>set-port-conversion-strategy!</code>.
|
|
If the value is <code>'error</code> then an exception is thrown if there is any
|
|
conversion error. If the value is <code>'substitute</code> then any conversion
|
|
error is replaced with question marks.
|
|
See <a href="http://www.gnu.org/software/guile/manual/html_node/Strings.html#Strings">Strings</a> in <cite>GNU Guile Reference Manual</cite>.
|
|
</p>
|
|
<p>If the optional <var>length</var> argument is given, the string will be
|
|
fetched and converted to the given length.
|
|
The length must be a Scheme integer and not a <code><gdb:value></code> integer.
|
|
</p></dd></dl>
|
|
|
|
<dl>
|
|
<dt><a name="index-value_002d_003elazy_002dstring"></a>Scheme Procedure: <strong>value->lazy-string</strong> <em>value <span class="roman">[</span>#:encoding encoding<span class="roman">]</span> <span class="roman">[</span>#:length length<span class="roman">]</span></em></dt>
|
|
<dd><p>If this <code><gdb:value></code> represents a string, then this method
|
|
converts <var>value</var> to a <code><gdb:lazy-string</code> (see <a href="Lazy-Strings-In-Guile.html#Lazy-Strings-In-Guile">Lazy Strings In Guile</a>). Otherwise, this method will throw an exception.
|
|
</p>
|
|
<p>If the optional <var>encoding</var> argument is given, it must be a string
|
|
naming the encoding of the <code><gdb:lazy-string</code>. Some examples are:
|
|
<code>"ascii"</code>, <code>"iso-8859-6"</code> or <code>"utf-8"</code>. If the
|
|
<var>encoding</var> argument is an encoding that <small>GDB</small> does not
|
|
recognize, <small>GDB</small> will raise an error.
|
|
</p>
|
|
<p>When a lazy string is printed, the <small>GDB</small> encoding machinery is
|
|
used to convert the string during printing. If the optional
|
|
<var>encoding</var> argument is not provided, or is an empty string,
|
|
<small>GDB</small> will automatically select the encoding most suitable for
|
|
the string type. For further information on encoding in <small>GDB</small>
|
|
please see <a href="Character-Sets.html#Character-Sets">Character Sets</a>.
|
|
</p>
|
|
<p>If the optional <var>length</var> argument is given, the string will be
|
|
fetched and encoded to the length of characters specified. If
|
|
the <var>length</var> argument is not provided, the string will be fetched
|
|
and encoded until a null of appropriate width is found.
|
|
The length must be a Scheme integer and not a <code><gdb:value></code> integer.
|
|
</p></dd></dl>
|
|
|
|
<dl>
|
|
<dt><a name="index-value_002dlazy_003f"></a>Scheme Procedure: <strong>value-lazy?</strong> <em>value</em></dt>
|
|
<dd><p>Return <code>#t</code> if <var>value</var> has not yet been fetched
|
|
from the inferior.
|
|
Otherwise return <code>#f</code>.
|
|
<small>GDB</small> does not fetch values until necessary, for efficiency.
|
|
For example:
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">(define myval (parse-and-eval "somevar"))
|
|
</pre></div>
|
|
|
|
<p>The value of <code>somevar</code> is not fetched at this time. It will be
|
|
fetched when the value is needed, or when the <code>fetch-lazy</code>
|
|
procedure is invoked.
|
|
</p></dd></dl>
|
|
|
|
<dl>
|
|
<dt><a name="index-make_002dlazy_002dvalue"></a>Scheme Procedure: <strong>make-lazy-value</strong> <em>type address</em></dt>
|
|
<dd><p>Return a <code><gdb:value></code> that will be lazily fetched from the
|
|
target. The object of type <code><gdb:type></code> whose value to fetch is
|
|
specified by its <var>type</var> and its target memory <var>address</var>, which
|
|
is a Scheme integer.
|
|
</p></dd></dl>
|
|
|
|
<dl>
|
|
<dt><a name="index-value_002dfetch_002dlazy_0021"></a>Scheme Procedure: <strong>value-fetch-lazy!</strong> <em>value</em></dt>
|
|
<dd><p>If <var>value</var> is a lazy value (<code>(value-lazy? value)</code> is <code>#t</code>),
|
|
then the value is fetched from the inferior.
|
|
Any errors that occur in the process will produce a Guile exception.
|
|
</p>
|
|
<p>If <var>value</var> is not a lazy value, this method has no effect.
|
|
</p>
|
|
<p>The result of this function is unspecified.
|
|
</p></dd></dl>
|
|
|
|
<dl>
|
|
<dt><a name="index-value_002dprint"></a>Scheme Procedure: <strong>value-print</strong> <em>value</em></dt>
|
|
<dd><p>Return the string representation (print form) of <code><gdb:value></code>
|
|
<var>value</var>.
|
|
</p></dd></dl>
|
|
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Arithmetic-In-Guile.html#Arithmetic-In-Guile" accesskey="n" rel="next">Arithmetic In Guile</a>, Previous: <a href="Guile-Exception-Handling.html#Guile-Exception-Handling" accesskey="p" rel="prev">Guile Exception Handling</a>, Up: <a href="Guile-API.html#Guile-API" accesskey="u" rel="up">Guile API</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
|
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</div>
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</body>
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</html>
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