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464 lines
22 KiB
HTML
464 lines
22 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Free Software" and "Free Software Needs
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Free Documentation", with the Front-Cover Texts being "A GNU Manual,"
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and with the Back-Cover Texts as in (a) below.
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(a) The FSF's Back-Cover Text is: "You are free to copy and modify
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this GNU Manual. Buying copies from GNU Press supports the FSF in
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developing GNU and promoting software freedom." -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Values From Inferior (Debugging with GDB)</title>
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<meta name="description" content="Values From Inferior (Debugging with GDB)">
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<meta name="keywords" content="Values From Inferior (Debugging with GDB)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Python-API.html#Python-API" rel="up" title="Python API">
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<link href="Types-In-Python.html#Types-In-Python" rel="next" title="Types In Python">
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<link href="Exception-Handling.html#Exception-Handling" rel="prev" title="Exception Handling">
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</style>
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</head>
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<body lang="en">
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<a name="Values-From-Inferior"></a>
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<div class="header">
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<p>
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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>
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</div>
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<hr>
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<a name="Values-From-Inferior-1"></a>
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<h4 class="subsubsection">23.2.2.3 Values From Inferior</h4>
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<a name="index-values-from-inferior_002c-with-Python"></a>
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<a name="index-python_002c-working-with-values-from-inferior"></a>
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<a name="index-gdb_002eValue"></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>Inferior values that are simple scalars can be used directly in
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Python expressions that are valid for the value’s data type. Here’s
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an example for an integer or floating-point value <code>some_val</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">bar = some_val + 2
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</pre></div>
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<p>As result of this, <code>bar</code> will also be a <code>gdb.Value</code> object
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whose values are of the same type as those of <code>some_val</code>. Valid
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Python operations can also be performed on <code>gdb.Value</code> objects
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representing a <code>struct</code> or <code>class</code> object. For such cases,
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the overloaded operator (if present), is used to perform the operation.
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For example, if <code>val1</code> and <code>val2</code> are <code>gdb.Value</code> objects
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representing instances of a <code>class</code> which overloads the <code>+</code>
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operator, then one can use the <code>+</code> operator in their Python script
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as follows:
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</p>
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<div class="smallexample">
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<pre class="smallexample">val3 = val1 + val2
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</pre></div>
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<p>The result of the operation <code>val3</code> is also a <code>gdb.Value</code>
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object corresponding to the value returned by the overloaded <code>+</code>
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operator. In general, overloaded operators are invoked for the
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following operations: <code>+</code> (binary addition), <code>-</code> (binary
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subtraction), <code>*</code> (multiplication), <code>/</code>, <code>%</code>, <code><<</code>,
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<code>>></code>, <code>|</code>, <code>&</code>, <code>^</code>.
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</p>
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<p>Inferior values that are structures or instances of some class can
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be accessed using the Python <em>dictionary syntax</em>. For example, if
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<code>some_val</code> is a <code>gdb.Value</code> instance holding a structure, you
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can access its <code>foo</code> element with:
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</p>
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<div class="smallexample">
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<pre class="smallexample">bar = some_val['foo']
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</pre></div>
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<a name="index-getting-structure-elements-using-gdb_002eField-objects-as-subscripts"></a>
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<p>Again, <code>bar</code> will also be a <code>gdb.Value</code> object. Structure
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elements can also be accessed by using <code>gdb.Field</code> objects as
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subscripts (see <a href="Types-In-Python.html#Types-In-Python">Types In Python</a>, for more information on
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<code>gdb.Field</code> objects). For example, if <code>foo_field</code> is a
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<code>gdb.Field</code> object corresponding to element <code>foo</code> of the above
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structure, then <code>bar</code> can also be accessed as follows:
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</p>
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<div class="smallexample">
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<pre class="smallexample">bar = some_val[foo_field]
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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. Any arguments provided to the call must match
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the function’s prototype, and must be provided in the order specified
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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">result = some_val (10,20)
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</pre></div>
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<p>Any values returned from a function call will be stored as a
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<code>gdb.Value</code>.
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</p>
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<p>The following attributes are provided:
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</p>
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<dl>
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<dt><a name="index-Value_002eaddress"></a>Variable: <strong>Value.address</strong></dt>
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<dd><p>If this object is addressable, this read-only attribute holds a
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<code>gdb.Value</code> object representing the address. Otherwise,
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this attribute holds <code>None</code>.
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</p></dd></dl>
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<a name="index-optimized-out-value-in-Python"></a>
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<dl>
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<dt><a name="index-Value_002eis_005foptimized_005fout"></a>Variable: <strong>Value.is_optimized_out</strong></dt>
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<dd><p>This read-only boolean attribute is true if the compiler optimized out
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this value, thus it is not available for fetching from the inferior.
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</p></dd></dl>
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<dl>
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<dt><a name="index-Value_002etype"></a>Variable: <strong>Value.type</strong></dt>
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<dd><p>The type of this <code>gdb.Value</code>. The value of this attribute is a
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<code>gdb.Type</code> object (see <a href="Types-In-Python.html#Types-In-Python">Types In Python</a>).
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</p></dd></dl>
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<dl>
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<dt><a name="index-Value_002edynamic_005ftype"></a>Variable: <strong>Value.dynamic_type</strong></dt>
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<dd><p>The dynamic type of this <code>gdb.Value</code>. This uses the object’s
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virtual table and the C<tt>++</tt> run-time type information
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(<acronym>RTTI</acronym>) to determine the dynamic type of the value. If this
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value is of class type, it will return the class in which the value is
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embedded, if any. If this value is of pointer or reference to a class
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type, it will compute the dynamic type of the referenced object, and
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return a pointer or reference to that type, respectively. In all
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other cases, it will return the value’s static 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_002eis_005flazy"></a>Variable: <strong>Value.is_lazy</strong></dt>
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<dd><p>The value of this read-only boolean attribute is <code>True</code> if this
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<code>gdb.Value</code> has not yet been fetched from the inferior.
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<small>GDB</small> does not fetch values until necessary, for efficiency.
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For example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">myval = gdb.parse_and_eval ('somevar')
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</pre></div>
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<p>The value of <code>somevar</code> is not fetched at this time. It will be
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fetched when the value is needed, or when the <code>fetch_lazy</code>
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method is invoked.
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</p></dd></dl>
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<p>The following methods are provided:
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</p>
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<dl>
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<dt><a name="index-Value_002e_005f_005finit_005f_005f"></a>Function: <strong>Value.__init__</strong> <em>(<var>val</var>)</em></dt>
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<dd><p>Many Python values can be converted directly to a <code>gdb.Value</code> via
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this object initializer. Specifically:
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</p>
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<dl compact="compact">
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<dt>Python boolean</dt>
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<dd><p>A Python boolean is converted to the boolean type from the current
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language.
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</p>
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</dd>
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<dt>Python integer</dt>
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<dd><p>A Python integer is converted to the C <code>long</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>Python long</dt>
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<dd><p>A Python long is converted to the C <code>long long</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>Python float</dt>
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<dd><p>A Python float 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>Python string</dt>
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<dd><p>A Python string is converted to a target string in the current target
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language using the current target encoding.
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If a character cannot be represented in the current target encoding,
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then an exception is thrown.
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</p>
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</dd>
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<dt><code>gdb.Value</code></dt>
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<dd><p>If <code>val</code> is a <code>gdb.Value</code>, then a copy of the value is made.
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</p>
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</dd>
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<dt><code>gdb.LazyString</code></dt>
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<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
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its result is used.
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</p></dd>
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</dl>
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</dd></dl>
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<dl>
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<dt><a name="index-Value_002e_005f_005finit_005f_005f-1"></a>Function: <strong>Value.__init__</strong> <em>(<var>val</var>, <span class="roman">[</span>, type <span class="roman">]</span>)</em></dt>
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<dd><p>This second form of the <code>gdb.Value</code> constructor returns a
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<code>gdb.Value</code> of type <var>type</var> where the value contents are taken
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from the Python buffer object specified by <var>val</var>. The number of
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bytes in the Python buffer object must be greater than or equal to the
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size of <var>type</var>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-Value_002ecast"></a>Function: <strong>Value.cast</strong> <em>(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 this instance 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_002edereference"></a>Function: <strong>Value.dereference</strong> <em>()</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 the pointer. 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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||
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<div class="smallexample">
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||
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<pre class="smallexample">int *foo;
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</pre></div>
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||
|
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|
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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">bar = foo.dereference ()
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</pre></div>
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|
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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>
|
||
|
|
<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>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
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>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|