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| <title>Debugging with GDB: Values From Inferior</title> |
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| <a name="Values-From-Inferior"></a> |
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| Next: <a href="Types-In-Python.html#Types-In-Python" accesskey="n" rel="next">Types In Python</a>, Previous: <a href="Exception-Handling.html#Exception-Handling" accesskey="p" rel="prev">Exception Handling</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python 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> |
| </div> |
| <hr> |
| <a name="Values-From-Inferior-1"></a> |
| <h4 class="subsubsection">23.2.2.3 Values From Inferior</h4> |
| <a name="index-values-from-inferior_002c-with-Python"></a> |
| <a name="index-python_002c-working-with-values-from-inferior"></a> |
| |
| <a name="index-gdb_002eValue"></a> |
| <p><small>GDB</small> provides values it obtains from the inferior program in |
| an object of type <code>gdb.Value</code>. <small>GDB</small> uses this object |
| for its internal bookkeeping of the inferior’s values, and for |
| fetching values when necessary. |
| </p> |
| <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>: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">bar = some_val + 2 |
| </pre></div> |
| |
| <p>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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">val3 = val1 + val2 |
| </pre></div> |
| |
| <p>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><<</code>, |
| <code>>></code>, <code>|</code>, <code>&</code>, <code>^</code>. |
| </p> |
| <p>Inferior values that are structures or instances of some class can |
| be accessed using the Python <em>dictionary syntax</em>. 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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">bar = some_val['foo'] |
| </pre></div> |
| |
| <a name="index-getting-structure-elements-using-gdb_002eField-objects-as-subscripts"></a> |
| <p>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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">bar = some_val[foo_field] |
| </pre></div> |
| |
| <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> |
| <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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">result = some_val (10,20) |
| </pre></div> |
| |
| <p>Any values returned from a function call will be stored as a |
| <code>gdb.Value</code>. |
| </p> |
| <p>The following attributes are provided: |
| </p> |
| <dl> |
| <dt><a name="index-Value_002eaddress"></a>Variable: <strong>Value.address</strong></dt> |
| <dd><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></dd></dl> |
| |
| <a name="index-optimized-out-value-in-Python"></a> |
| <dl> |
| <dt><a name="index-Value_002eis_005foptimized_005fout"></a>Variable: <strong>Value.is_optimized_out</strong></dt> |
| <dd><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></dd></dl> |
| |
| <dl> |
| <dt><a name="index-Value_002etype"></a>Variable: <strong>Value.type</strong></dt> |
| <dd><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></dd></dl> |
| |
| <dl> |
| <dt><a name="index-Value_002edynamic_005ftype"></a>Variable: <strong>Value.dynamic_type</strong></dt> |
| <dd><p>The dynamic type of this <code>gdb.Value</code>. This uses 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> |
| <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></dd></dl> |
| |
| <dl> |
| <dt><a name="index-Value_002eis_005flazy"></a>Variable: <strong>Value.is_lazy</strong></dt> |
| <dd><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. |
| <small>GDB</small> does not fetch values until necessary, for efficiency. |
| For example: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">myval = gdb.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> |
| method is invoked. |
| </p></dd></dl> |
| |
| <p>The following methods are provided: |
| </p> |
| <dl> |
| <dt><a name="index-Value_002e_005f_005finit_005f_005f"></a>Function: <strong>Value.__init__</strong> <em>(<var>val</var>)</em></dt> |
| <dd><p>Many Python values can be converted directly to a <code>gdb.Value</code> via |
| this object initializer. Specifically: |
| </p> |
| <dl compact="compact"> |
| <dt>Python boolean</dt> |
| <dd><p>A Python boolean is converted to the boolean type from the current |
| language. |
| </p> |
| </dd> |
| <dt>Python integer</dt> |
| <dd><p>A Python integer is converted to the C <code>long</code> type for the |
| current architecture. |
| </p> |
| </dd> |
| <dt>Python long</dt> |
| <dd><p>A Python long is converted to the C <code>long long</code> type for the |
| current architecture. |
| </p> |
| </dd> |
| <dt>Python float</dt> |
| <dd><p>A Python float is converted to the C <code>double</code> type for the |
| current architecture. |
| </p> |
| </dd> |
| <dt>Python string</dt> |
| <dd><p>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. |
| </p> |
| </dd> |
| <dt><code>gdb.Value</code></dt> |
| <dd><p>If <code>val</code> is a <code>gdb.Value</code>, then a copy of the value is made. |
| </p> |
| </dd> |
| <dt><code>gdb.LazyString</code></dt> |
| <dd><p>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. |
| </p></dd> |
| </dl> |
| </dd></dl> |
| |
| <dl> |
| <dt><a name="index-Value_002ecast"></a>Function: <strong>Value.cast</strong> <em>(type)</em></dt> |
| <dd><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></dd></dl> |
| |
| <dl> |
| <dt><a name="index-Value_002edereference"></a>Function: <strong>Value.dereference</strong> <em>()</em></dt> |
| <dd><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 |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">int *foo; |
| </pre></div> |
| |
| <p>then you can use the corresponding <code>gdb.Value</code> to access what |
| <code>foo</code> points to like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">bar = foo.dereference () |
| </pre></div> |
| |
| <p>The result <code>bar</code> will be a <code>gdb.Value</code> object holding the |
| value pointed to by <code>foo</code>. |
| </p> |
| <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 |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">typedef int *intptr; |
| ... |
| int val = 10; |
| intptr ptr = &val; |
| intptr &ptrref = ptr; |
| </pre></div> |
| |
| <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>. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">py_ptrref = gdb.parse_and_eval ("ptrref") |
| py_val = py_ptrref.dereference () |
| py_ptr = py_ptrref.referenced_value () |
| </pre></div> |
| |
| <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></dd></dl> |
| |
| <dl> |
| <dt><a name="index-Value_002ereferenced_005fvalue"></a>Function: <strong>Value.referenced_value</strong> <em>()</em></dt> |
| <dd><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 |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">int val = 10; |
| int &ref = val; |
| </pre></div> |
| |
| <p>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>. |
| </p> |
| <div class="smallexample"> |
| <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></div> |
| |
| <p>The <code>gdb.Value</code> object <code>py_val</code> is identical to that |
| corresponding to <code>val</code>. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-Value_002ereference_005fvalue"></a>Function: <strong>Value.reference_value</strong> <em>()</em></dt> |
| <dd><p>Return a <code>gdb.Value</code> object which is a reference to the value |
| encapsulated by this instance. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-Value_002econst_005fvalue"></a>Function: <strong>Value.const_value</strong> <em>()</em></dt> |
| <dd><p>Return a <code>gdb.Value</code> object which is a <code>const</code> version of the |
| value encapsulated by this instance. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-Value_002edynamic_005fcast"></a>Function: <strong>Value.dynamic_cast</strong> <em>(type)</em></dt> |
| <dd><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></dd></dl> |
| |
| <dl> |
| <dt><a name="index-Value_002ereinterpret_005fcast"></a>Function: <strong>Value.reinterpret_cast</strong> <em>(type)</em></dt> |
| <dd><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></dd></dl> |
| |
| <dl> |
| <dt><a name="index-Value_002estring"></a>Function: <strong>Value.string</strong> <em>(<span class="roman">[</span>encoding<span class="roman">[</span>, errors<span class="roman">[</span>, length<span class="roman">]]]</span>)</em></dt> |
| <dd><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> |
| <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 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> |
| <p>The optional <var>errors</var> argument is the same as the corresponding |
| argument to Python’s <code>string.decode</code> method. |
| </p> |
| <p>If the optional <var>length</var> argument is given, the string will be |
| fetched and converted to the given length. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-Value_002elazy_005fstring"></a>Function: <strong>Value.lazy_string</strong> <em>(<span class="roman">[</span>encoding <span class="roman">[</span>, length<span class="roman">]]</span>)</em></dt> |
| <dd><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> |
| <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>ascii</samp>’, ‘<samp>iso-8859-6</samp>’ or ‘<samp>utf-8</samp>’. If the |
| <var>encoding</var> argument is an encoding that <small>GDB</small> does |
| 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. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-Value_002efetch_005flazy"></a>Function: <strong>Value.fetch_lazy</strong> <em>()</em></dt> |
| <dd><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> |
| <p>If the <code>gdb.Value</code> object is not a lazy value, this method |
| has no effect. |
| </p> |
| <p>This method does not return a value. |
| </p></dd></dl> |
| |
| |
| <hr> |
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