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<title>Values From Inferior - Debugging with GDB</title>
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<a name="Values-From-Inferior"></a>
<p>
Next:&nbsp;<a rel="next" accesskey="n" href="Types-In-Python.html#Types-In-Python">Types In Python</a>,
Previous:&nbsp;<a rel="previous" accesskey="p" href="Exception-Handling.html#Exception-Handling">Exception Handling</a>,
Up:&nbsp;<a rel="up" accesskey="u" href="Python-API.html#Python-API">Python API</a>
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<h5 class="subsubsection">23.2.2.3 Values From Inferior</h5>
<p><a name="index-values-from-inferior_002c-with-Python-1905"></a><a name="index-python_002c-working-with-values-from-inferior-1906"></a>
<a name="index-g_t_0040code_007bgdb_002eValue_007d-1907"></a><span class="sc">gdb</span> provides values it obtains from the inferior program in
an object of type <code>gdb.Value</code>. <span class="sc">gdb</span> uses this object
for its internal bookkeeping of the inferior's values, and for
fetching values when necessary.
<p>Inferior values that are simple scalars can be used directly in
Python expressions that are valid for the value's data type. Here's
an example for an integer or floating-point value <code>some_val</code>:
<pre class="smallexample"> bar = some_val + 2
</pre>
<p class="noindent">As result of this, <code>bar</code> will also be a <code>gdb.Value</code> object
whose values are of the same type as those of <code>some_val</code>. Valid
Python operations can also be performed on <code>gdb.Value</code> objects
representing a <code>struct</code> or <code>class</code> object. For such cases,
the overloaded operator (if present), is used to perform the operation.
For example, if <code>val1</code> and <code>val2</code> are <code>gdb.Value</code> objects
representing instances of a <code>class</code> which overloads the <code>+</code>
operator, then one can use the <code>+</code> operator in their Python script
as follows:
<pre class="smallexample"> val3 = val1 + val2
</pre>
<p class="noindent">The result of the operation <code>val3</code> is also a <code>gdb.Value</code>
object corresponding to the value returned by the overloaded <code>+</code>
operator. In general, overloaded operators are invoked for the
following operations: <code>+</code> (binary addition), <code>-</code> (binary
subtraction), <code>*</code> (multiplication), <code>/</code>, <code>%</code>, <code>&lt;&lt;</code>,
<code>&gt;&gt;</code>, <code>|</code>, <code>&amp;</code>, <code>^</code>.
<p>Inferior values that are structures or instances of some class can
be accessed using the Python <dfn>dictionary syntax</dfn>. For example, if
<code>some_val</code> is a <code>gdb.Value</code> instance holding a structure, you
can access its <code>foo</code> element with:
<pre class="smallexample"> bar = some_val['foo']
</pre>
<p><a name="index-getting-structure-elements-using-gdb_002eField-objects-as-subscripts-1908"></a>Again, <code>bar</code> will also be a <code>gdb.Value</code> object. Structure
elements can also be accessed by using <code>gdb.Field</code> objects as
subscripts (see <a href="Types-In-Python.html#Types-In-Python">Types In Python</a>, for more information on
<code>gdb.Field</code> objects). For example, if <code>foo_field</code> is a
<code>gdb.Field</code> object corresponding to element <code>foo</code> of the above
structure, then <code>bar</code> can also be accessed as follows:
<pre class="smallexample"> bar = some_val[foo_field]
</pre>
<p>A <code>gdb.Value</code> that represents a function can be executed via
inferior function call. Any arguments provided to the call must match
the function's prototype, and must be provided in the order specified
by that prototype.
<p>For example, <code>some_val</code> is a <code>gdb.Value</code> instance
representing a function that takes two integers as arguments. To
execute this function, call it like so:
<pre class="smallexample"> result = some_val (10,20)
</pre>
<p>Any values returned from a function call will be stored as a
<code>gdb.Value</code>.
<p>The following attributes are provided:
<div class="defun">
&mdash; Variable: <b>Value.address</b><var><a name="index-Value_002eaddress-1909"></a></var><br>
<blockquote><p>If this object is addressable, this read-only attribute holds a
<code>gdb.Value</code> object representing the address. Otherwise,
this attribute holds <code>None</code>.
</p></blockquote></div>
<p><a name="index-optimized-out-value-in-Python-1910"></a>
<div class="defun">
&mdash; Variable: <b>Value.is_optimized_out</b><var><a name="index-Value_002eis_005foptimized_005fout-1911"></a></var><br>
<blockquote><p>This read-only boolean attribute is true if the compiler optimized out
this value, thus it is not available for fetching from the inferior.
</p></blockquote></div>
<div class="defun">
&mdash; Variable: <b>Value.type</b><var><a name="index-Value_002etype-1912"></a></var><br>
<blockquote><p>The type of this <code>gdb.Value</code>. The value of this attribute is a
<code>gdb.Type</code> object (see <a href="Types-In-Python.html#Types-In-Python">Types In Python</a>).
</p></blockquote></div>
<div class="defun">
&mdash; Variable: <b>Value.dynamic_type</b><var><a name="index-Value_002edynamic_005ftype-1913"></a></var><br>
<blockquote><p>The dynamic type of this <code>gdb.Value</code>. This uses the object's
virtual table and the C<tt>++</tt> run-time type information
(<acronym>RTTI</acronym>) to determine the dynamic type of the value. If this
value is of class type, it will return the class in which the value is
embedded, if any. If this value is of pointer or reference to a class
type, it will compute the dynamic type of the referenced object, and
return a pointer or reference to that type, respectively. In all
other cases, it will return the value's static type.
<p>Note that this feature will only work when debugging a C<tt>++</tt> program
that includes <acronym>RTTI</acronym> for the object in question. Otherwise,
it will just return the static type of the value as in <kbd>ptype foo</kbd>
(see <a href="Symbols.html#Symbols">ptype</a>).
</p></blockquote></div>
<div class="defun">
&mdash; Variable: <b>Value.is_lazy</b><var><a name="index-Value_002eis_005flazy-1914"></a></var><br>
<blockquote><p>The value of this read-only boolean attribute is <code>True</code> if this
<code>gdb.Value</code> has not yet been fetched from the inferior.
<span class="sc">gdb</span> does not fetch values until necessary, for efficiency.
For example:
<pre class="smallexample"> myval = gdb.parse_and_eval ('somevar')
</pre>
<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>
method is invoked.
</p></blockquote></div>
<p>The following methods are provided:
<div class="defun">
&mdash; Function: <b>Value.__init__</b> (<var>val</var>)<var><a name="index-Value_002e_005f_005finit_005f_005f-1915"></a></var><br>
<blockquote><p>Many Python values can be converted directly to a <code>gdb.Value</code> via
this object initializer. Specifically:
<dl>
<dt>Python boolean<dd>A Python boolean is converted to the boolean type from the current
language.
<br><dt>Python integer<dd>A Python integer is converted to the C <code>long</code> type for the
current architecture.
<br><dt>Python long<dd>A Python long is converted to the C <code>long long</code> type for the
current architecture.
<br><dt>Python float<dd>A Python float is converted to the C <code>double</code> type for the
current architecture.
<br><dt>Python string<dd>A Python string is converted to a target string in the current target
language using the current target encoding.
If a character cannot be represented in the current target encoding,
then an exception is thrown.
<br><dt><code>gdb.Value</code><dd>If <code>val</code> is a <code>gdb.Value</code>, then a copy of the value is made.
<br><dt><code>gdb.LazyString</code><dd>If <code>val</code> is a <code>gdb.LazyString</code> (see <a href="Lazy-Strings-In-Python.html#Lazy-Strings-In-Python">Lazy Strings In Python</a>), then the lazy string's <code>value</code> method is called, and
its result is used.
</dl>
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>Value.__init__</b> (<var>val, </var><span class="roman">[</span><var>, type </var><span class="roman">]</span>)<var><a name="index-Value_002e_005f_005finit_005f_005f-1916"></a></var><br>
<blockquote><p>This second form of the <code>gdb.Value</code> constructor returns a
<code>gdb.Value</code> of type <var>type</var> where the value contents are taken
from the Python buffer object specified by <var>val</var>. The number of
bytes in the Python buffer object must be greater than or equal to the
size of <var>type</var>.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>Value.cast</b> (<var>type</var>)<var><a name="index-Value_002ecast-1917"></a></var><br>
<blockquote><p>Return a new instance of <code>gdb.Value</code> that is the result of
casting this instance to the type described by <var>type</var>, which must
be a <code>gdb.Type</code> object. If the cast cannot be performed for some
reason, this method throws an exception.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>Value.dereference</b> ()<var><a name="index-Value_002edereference-1918"></a></var><br>
<blockquote><p>For pointer data types, this method returns a new <code>gdb.Value</code> object
whose contents is the object pointed to by the pointer. For example, if
<code>foo</code> is a C pointer to an <code>int</code>, declared in your C program as
<pre class="smallexample"> int *foo;
</pre>
<p class="noindent">then you can use the corresponding <code>gdb.Value</code> to access what
<code>foo</code> points to like this:
<pre class="smallexample"> bar = foo.dereference ()
</pre>
<p>The result <code>bar</code> will be a <code>gdb.Value</code> object holding the
value pointed to by <code>foo</code>.
<p>A similar function <code>Value.referenced_value</code> exists which also
returns <code>gdb.Value</code> objects corresonding to the values pointed to
by pointer values (and additionally, values referenced by reference
values). However, the behavior of <code>Value.dereference</code>
differs from <code>Value.referenced_value</code> by the fact that the
behavior of <code>Value.dereference</code> is identical to applying the C
unary operator <code>*</code> on a given value. For example, consider a
reference to a pointer <code>ptrref</code>, declared in your C<tt>++</tt> program
as
<pre class="smallexample"> typedef int *intptr;
...
int val = 10;
intptr ptr = &amp;val;
intptr &amp;ptrref = ptr;
</pre>
<p>Though <code>ptrref</code> is a reference value, one can apply the method
<code>Value.dereference</code> to the <code>gdb.Value</code> object corresponding
to it and obtain a <code>gdb.Value</code> which is identical to that
corresponding to <code>val</code>. However, if you apply the method
<code>Value.referenced_value</code>, the result would be a <code>gdb.Value</code>
object identical to that corresponding to <code>ptr</code>.
<pre class="smallexample"> py_ptrref = gdb.parse_and_eval ("ptrref")
py_val = py_ptrref.dereference ()
py_ptr = py_ptrref.referenced_value ()
</pre>
<p>The <code>gdb.Value</code> object <code>py_val</code> is identical to that
corresponding to <code>val</code>, and <code>py_ptr</code> is identical to that
corresponding to <code>ptr</code>. In general, <code>Value.dereference</code> can
be applied whenever the C unary operator <code>*</code> can be applied
to the corresponding C value. For those cases where applying both
<code>Value.dereference</code> and <code>Value.referenced_value</code> is allowed,
the results obtained need not be identical (as we have seen in the above
example). The results are however identical when applied on
<code>gdb.Value</code> objects corresponding to pointers (<code>gdb.Value</code>
objects with type code <code>TYPE_CODE_PTR</code>) in a C/C<tt>++</tt> program.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>Value.referenced_value</b> ()<var><a name="index-Value_002ereferenced_005fvalue-1919"></a></var><br>
<blockquote><p>For pointer or reference data types, this method returns a new
<code>gdb.Value</code> object corresponding to the value referenced by the
pointer/reference value. For pointer data types,
<code>Value.dereference</code> and <code>Value.referenced_value</code> produce
identical results. The difference between these methods is that
<code>Value.dereference</code> cannot get the values referenced by reference
values. For example, consider a reference to an <code>int</code>, declared
in your C<tt>++</tt> program as
<pre class="smallexample"> int val = 10;
int &amp;ref = val;
</pre>
<p class="noindent">then applying <code>Value.dereference</code> to the <code>gdb.Value</code> object
corresponding to <code>ref</code> will result in an error, while applying
<code>Value.referenced_value</code> will result in a <code>gdb.Value</code> object
identical to that corresponding to <code>val</code>.
<pre class="smallexample"> py_ref = gdb.parse_and_eval ("ref")
er_ref = py_ref.dereference () # Results in error
py_val = py_ref.referenced_value () # Returns the referenced value
</pre>
<p>The <code>gdb.Value</code> object <code>py_val</code> is identical to that
corresponding to <code>val</code>.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>Value.reference_value</b> ()<var><a name="index-Value_002ereference_005fvalue-1920"></a></var><br>
<blockquote><p>Return a <code>gdb.Value</code> object which is a reference to the value
encapsulated by this instance.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>Value.const_value</b> ()<var><a name="index-Value_002econst_005fvalue-1921"></a></var><br>
<blockquote><p>Return a <code>gdb.Value</code> object which is a <code>const</code> version of the
value encapsulated by this instance.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>Value.dynamic_cast</b> (<var>type</var>)<var><a name="index-Value_002edynamic_005fcast-1922"></a></var><br>
<blockquote><p>Like <code>Value.cast</code>, but works as if the C<tt>++</tt> <code>dynamic_cast</code>
operator were used. Consult a C<tt>++</tt> reference for details.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>Value.reinterpret_cast</b> (<var>type</var>)<var><a name="index-Value_002ereinterpret_005fcast-1923"></a></var><br>
<blockquote><p>Like <code>Value.cast</code>, but works as if the C<tt>++</tt> <code>reinterpret_cast</code>
operator were used. Consult a C<tt>++</tt> reference for details.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>Value.string</b> (<span class="roman">[</span><var>encoding</var><span class="roman">[</span><var>, errors</var><span class="roman">[</span><var>, length</var><span class="roman">]]]</span>)<var><a name="index-Value_002estring-1924"></a></var><br>
<blockquote><p>If this <code>gdb.Value</code> represents a string, then this method
converts the contents to a Python string. Otherwise, this method will
throw an exception.
<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>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>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 Python's
<code>string.decode</code> method, and the Python 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>The optional <var>errors</var> argument is the same as the corresponding
argument to Python's <code>string.decode</code> method.
<p>If the optional <var>length</var> argument is given, the string will be
fetched and converted to the given length.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>Value.lazy_string</b> (<span class="roman">[</span><var>encoding </var><span class="roman">[</span><var>, length</var><span class="roman">]]</span>)<var><a name="index-Value_002elazy_005fstring-1925"></a></var><br>
<blockquote><p>If this <code>gdb.Value</code> represents a string, then this method
converts the contents to a <code>gdb.LazyString</code> (see <a href="Lazy-Strings-In-Python.html#Lazy-Strings-In-Python">Lazy Strings In Python</a>). Otherwise, this method will throw an exception.
<p>If the optional <var>encoding</var> argument is given, it must be a string
naming the encoding of the <code>gdb.LazyString</code>. Some examples are:
&lsquo;<samp><span class="samp">ascii</span></samp>&rsquo;, &lsquo;<samp><span class="samp">iso-8859-6</span></samp>&rsquo; or &lsquo;<samp><span class="samp">utf-8</span></samp>&rsquo;. If the
<var>encoding</var> argument is an encoding that <span class="sc">gdb</span> does
recognize, <span class="sc">gdb</span> will raise an error.
<p>When a lazy string is printed, the <span class="sc">gdb</span> 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,
<span class="sc">gdb</span> will automatically select the encoding most suitable for
the string type. For further information on encoding in <span class="sc">gdb</span>
please see <a href="Character-Sets.html#Character-Sets">Character Sets</a>.
<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.
</p></blockquote></div>
<div class="defun">
&mdash; Function: <b>Value.fetch_lazy</b> ()<var><a name="index-Value_002efetch_005flazy-1926"></a></var><br>
<blockquote><p>If the <code>gdb.Value</code> object is currently a lazy value
(<code>gdb.Value.is_lazy</code> is <code>True</code>), then the value is
fetched from the inferior. Any errors that occur in the process
will produce a Python exception.
<p>If the <code>gdb.Value</code> object is not a lazy value, this method
has no effect.
<p>This method does not return a value.
</p></blockquote></div>
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