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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-1609"></a><a name="index-python_002c-working-with-values-from-inferior-1610"></a> |
| <a name="index-g_t_0040code_007bgdb_002eValue_007d-1611"></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>. |
| |
| <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>Again, <code>bar</code> will also be a <code>gdb.Value</code> object. |
| |
| <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: |
| |
| <dl> |
| |
| <div class="defun"> |
| — Instance Variable of Value: <b>address</b><var><a name="index-address-1612"></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-1613"></a> |
| |
| <div class="defun"> |
| — Instance Variable of Value: <b>is_optimized_out</b><var><a name="index-is_005foptimized_005fout-1614"></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"> |
| — Instance Variable of Value: <b>type</b><var><a name="index-type-1615"></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. |
| </p></blockquote></div> |
| |
| <div class="defun"> |
| — Instance Variable of Value: <b>dynamic_type</b><var><a name="index-dynamic_005ftype-1616"></a></var><br> |
| <blockquote> <p>The dynamic type of this <code>gdb.Value</code>. This uses C<tt>++</tt> run-time |
| type information 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></blockquote></div> |
| </dl> |
| |
| <p>The following methods are provided: |
| |
| <dl> |
| |
| <div class="defun"> |
| — Method on Value: <b>cast</b><var> type<a name="index-cast-on-Value-1617"></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"> |
| — Method on Value: <b>dereference</b><var><a name="index-dereference-on-Value-1618"></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></blockquote></div> |
| |
| <div class="defun"> |
| — Method on Value: <b>dynamic_cast</b><var> type<a name="index-dynamic_005fcast-on-Value-1619"></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"> |
| — Method on Value: <b>reinterpret_cast</b><var> type<a name="index-reinterpret_005fcast-on-Value-1620"></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"> |
| — Method on Value: <b>string</b> <span class="roman">[</span><var>encoding</var><span class="roman">]</span> <span class="roman">[</span><var>errors</var><span class="roman">]</span> <span class="roman">[</span><var>length</var><span class="roman">]</span><var><a name="index-string-on-Value-1621"></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>Strings are recognized in a language-specific way; whether a given |
| <code>gdb.Value</code> represents a string is determined by the current |
| language. |
| |
| <p>For C-like languages, a value is a string if it is a pointer to or an |
| array of characters or ints. The string is assumed to be terminated |
| by a zero of the appropriate width. However if the optional length |
| argument is given, the string will be converted to that given length, |
| ignoring any embedded zeros that the string may contain. |
| |
| <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"> |
| — Method on Value: <b>lazy_string</b> <span class="roman">[</span><var>encoding</var><span class="roman">]</span> <span class="roman">[</span><var>length</var><span class="roman">]</span><var><a name="index-lazy_005fstring-on-Value-1622"></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: |
| ‘<samp><span class="samp">ascii</span></samp>’, ‘<samp><span class="samp">iso-8859-6</span></samp>’ or ‘<samp><span class="samp">utf-8</span></samp>’. 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> |
| </dl> |
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