mirror of git://gcc.gnu.org/git/gcc.git
886 lines
42 KiB
XML
886 lines
42 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook" version="5.0"
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xml:id="manual.ext.debug_mode" xreflabel="Debug Mode">
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<?dbhtml filename="debug_mode.html"?>
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<info><title>Debug Mode</title>
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<keywordset>
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<keyword>C++</keyword>
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<keyword>library</keyword>
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<keyword>debug</keyword>
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</keywordset>
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</info>
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<section xml:id="manual.ext.debug_mode.intro" xreflabel="Intro"><info><title>Intro</title></info>
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<para>
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By default, libstdc++ is built with efficiency in mind, and
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therefore performs little or no error checking that is not
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required by the C++ standard. This means that programs that
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incorrectly use the C++ standard library will exhibit behavior
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that is not portable and may not even be predictable, because they
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tread into implementation-specific or undefined behavior. To
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detect some of these errors before they can become problematic,
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libstdc++ offers a debug mode that provides additional checking of
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library facilities, and will report errors in the use of libstdc++
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as soon as they can be detected by emitting a description of the
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problem to standard error and aborting the program. This debug
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mode is available with GCC 3.4.0 and later versions.
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</para>
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<para>
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The libstdc++ debug mode performs checking for many areas of the
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C++ standard, but the focus is on checking interactions among
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standard iterators, containers, and algorithms, including:
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</para>
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<itemizedlist>
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<listitem><para><emphasis>Safe iterators</emphasis>: Iterators keep track of the
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container whose elements they reference, so errors such as
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incrementing a past-the-end iterator or dereferencing an iterator
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that points to a container that has been destructed are diagnosed
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immediately.</para></listitem>
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<listitem><para><emphasis>Algorithm preconditions</emphasis>: Algorithms attempt to
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validate their input parameters to detect errors as early as
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possible. For instance, the <code>set_intersection</code>
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algorithm requires that its iterator
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parameters <code>first1</code> and <code>last1</code> form a valid
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iterator range, and that the sequence
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[<code>first1</code>, <code>last1</code>) is sorted according to
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the same predicate that was passed
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to <code>set_intersection</code>; the libstdc++ debug mode will
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detect an error if the sequence is not sorted or was sorted by a
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different predicate.</para></listitem>
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</itemizedlist>
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</section>
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<section xml:id="manual.ext.debug_mode.semantics" xreflabel="Semantics"><info><title>Semantics</title></info>
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<?dbhtml filename="debug_mode_semantics.html"?>
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<para>
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</para>
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<para>A program that uses the C++ standard library correctly
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will maintain the same semantics under debug mode as it had with
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the normal (release) library. All functional and exception-handling
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guarantees made by the normal library also hold for the debug mode
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library, with one exception: performance guarantees made by the
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normal library may not hold in the debug mode library. For
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instance, erasing an element in a <code>std::list</code> is a
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constant-time operation in normal library, but in debug mode it is
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linear in the number of iterators that reference that particular
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list. So while your (correct) program won't change its results, it
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is likely to execute more slowly.</para>
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<para>libstdc++ includes many extensions to the C++ standard library. In
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some cases the extensions are obvious, such as the hashed
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associative containers, whereas other extensions give predictable
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results to behavior that would otherwise be undefined, such as
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throwing an exception when a <code>std::basic_string</code> is
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constructed from a NULL character pointer. This latter category also
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includes implementation-defined and unspecified semantics, such as
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the growth rate of a vector. Use of these extensions is not
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considered incorrect, so code that relies on them will not be
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rejected by debug mode. However, use of these extensions may affect
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the portability of code to other implementations of the C++ standard
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library, and is therefore somewhat hazardous. For this reason, the
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libstdc++ debug mode offers a "pedantic" mode (similar to
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GCC's <code>-pedantic</code> compiler flag) that attempts to emulate
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the semantics guaranteed by the C++ standard. For
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instance, constructing a <code>std::basic_string</code> with a NULL
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character pointer would result in an exception under normal mode or
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non-pedantic debug mode (this is a libstdc++ extension), whereas
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under pedantic debug mode libstdc++ would signal an error. To enable
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the pedantic debug mode, compile your program with
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both <code>-D_GLIBCXX_DEBUG</code>
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and <code>-D_GLIBCXX_DEBUG_PEDANTIC</code> .
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(N.B. In GCC 3.4.x and 4.0.0, due to a bug,
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<code>-D_GLIBXX_DEBUG_PEDANTIC</code> was also needed. The problem has
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been fixed in GCC 4.0.1 and later versions.) </para>
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<para>The following library components provide extra debugging
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capabilities in debug mode:</para>
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<itemizedlist>
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<listitem><para><code>std::basic_string</code> (no safe iterators and see note below)</para></listitem>
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<listitem><para><code>std::bitset</code></para></listitem>
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<listitem><para><code>std::deque</code></para></listitem>
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<listitem><para><code>std::list</code></para></listitem>
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<listitem><para><code>std::map</code></para></listitem>
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<listitem><para><code>std::multimap</code></para></listitem>
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<listitem><para><code>std::multiset</code></para></listitem>
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<listitem><para><code>std::set</code></para></listitem>
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<listitem><para><code>std::vector</code></para></listitem>
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<listitem><para><code>std::unordered_map</code></para></listitem>
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<listitem><para><code>std::unordered_multimap</code></para></listitem>
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<listitem><para><code>std::unordered_set</code></para></listitem>
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<listitem><para><code>std::unordered_multiset</code></para></listitem>
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</itemizedlist>
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<para>N.B. although there are precondition checks for some string operations,
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e.g. <code>operator[]</code>,
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they will not always be run when using the <code>char</code> and
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<code>wchar_t</code> specialisations (<code>std::string</code> and
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<code>std::wstring</code>). This is because libstdc++ uses GCC's
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<code>extern template</code> extension to provide explicit instantiations
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of <code>std::string</code> and <code>std::wstring</code>, and those
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explicit instantiations don't include the debug-mode checks. If the
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containing functions are inlined then the checks will run, so compiling
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with <code>-O1</code> might be enough to enable them. Alternatively
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<code>-D_GLIBCXX_EXTERN_TEMPLATE=0</code> will suppress the declarations
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of the explicit instantiations and cause the functions to be instantiated
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with the debug-mode checks included, but this is unsupported and not
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guaranteed to work. For full debug-mode support you can use the
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<code>__gnu_debug::basic_string</code> debugging container directly,
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which always works correctly.
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</para>
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</section>
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<section xml:id="manual.ext.debug_mode.using" xreflabel="Using"><info><title>Using</title></info>
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<?dbhtml filename="debug_mode_using.html"?>
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<para>
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</para>
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<section xml:id="debug_mode.using.mode" xreflabel="Using Mode"><info><title>Using the Debug Mode</title></info>
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<para>To use the libstdc++ debug mode, compile your application with the
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compiler flag <code>-D_GLIBCXX_DEBUG</code>. Note that this flag
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changes the sizes and behavior of standard class templates such
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as <code>std::vector</code>, and therefore you can only link code
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compiled with debug mode and code compiled without debug mode if no
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instantiation of a container is passed between the two translation
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units.</para>
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<para>By default, error messages are formatted to fit on lines of about
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78 characters. The environment variable
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<code>GLIBCXX_DEBUG_MESSAGE_LENGTH</code> can be used to request a
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different length.</para>
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</section>
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<section xml:id="debug_mode.using.specific" xreflabel="Using Specific"><info><title>Using a Specific Debug Container</title></info>
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<para>When it is not feasible to recompile your entire application, or
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only specific containers need checking, debugging containers are
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available as GNU extensions. These debugging containers are
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functionally equivalent to the standard drop-in containers used in
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debug mode, but they are available in a separate namespace as GNU
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extensions and may be used in programs compiled with either release
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mode or with debug mode. The
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following table provides the names and headers of the debugging
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containers:
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</para>
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<table frame="all" xml:id="table.debug_mode_containers">
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<title>Debugging Containers</title>
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<tgroup cols="4" align="left" colsep="1" rowsep="1">
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<colspec colname="c1"/>
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<colspec colname="c2"/>
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<colspec colname="c3"/>
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<colspec colname="c4"/>
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<thead>
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<row>
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<entry>Container</entry>
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<entry>Header</entry>
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<entry>Debug container</entry>
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<entry>Debug header</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><classname>std::bitset</classname></entry>
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<entry><filename class="headerfile">bitset</filename></entry>
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<entry><classname>__gnu_debug::bitset</classname></entry>
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<entry><filename class="headerfile"><debug/bitset></filename></entry>
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</row>
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<row>
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<entry><classname>std::deque</classname></entry>
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<entry><filename class="headerfile">deque</filename></entry>
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<entry><classname>__gnu_debug::deque</classname></entry>
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<entry><filename class="headerfile"><debug/deque></filename></entry>
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</row>
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<row>
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<entry><classname>std::list</classname></entry>
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<entry><filename class="headerfile">list</filename></entry>
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<entry><classname>__gnu_debug::list</classname></entry>
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<entry><filename class="headerfile"><debug/list></filename></entry>
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</row>
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<row>
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<entry><classname>std::map</classname></entry>
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<entry><filename class="headerfile">map</filename></entry>
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<entry><classname>__gnu_debug::map</classname></entry>
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<entry><filename class="headerfile"><debug/map></filename></entry>
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</row>
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<row>
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<entry><classname>std::multimap</classname></entry>
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<entry><filename class="headerfile">map</filename></entry>
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<entry><classname>__gnu_debug::multimap</classname></entry>
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<entry><filename class="headerfile"><debug/map></filename></entry>
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</row>
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<row>
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<entry><classname>std::multiset</classname></entry>
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<entry><filename class="headerfile">set</filename></entry>
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<entry><classname>__gnu_debug::multiset</classname></entry>
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<entry><filename class="headerfile"><debug/set></filename></entry>
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</row>
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<row>
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<entry><classname>std::set</classname></entry>
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<entry><filename class="headerfile">set</filename></entry>
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<entry><classname>__gnu_debug::set</classname></entry>
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<entry><filename class="headerfile"><debug/set></filename></entry>
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</row>
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<row>
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<entry><classname>std::string</classname></entry>
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<entry><filename class="headerfile">string</filename></entry>
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<entry><classname>__gnu_debug::string</classname></entry>
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<entry><filename class="headerfile"><debug/string></filename></entry>
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</row>
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<row>
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<entry><classname>std::wstring</classname></entry>
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<entry><filename class="headerfile">string</filename></entry>
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<entry><classname>__gnu_debug::wstring</classname></entry>
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<entry><filename class="headerfile"><debug/string></filename></entry>
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</row>
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<row>
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<entry><classname>std::basic_string</classname></entry>
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<entry><filename class="headerfile">string</filename></entry>
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<entry><classname>__gnu_debug::basic_string</classname></entry>
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<entry><filename class="headerfile"><debug/string></filename></entry>
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</row>
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<row>
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<entry><classname>std::vector</classname></entry>
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<entry><filename class="headerfile">vector</filename></entry>
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<entry><classname>__gnu_debug::vector</classname></entry>
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<entry><filename class="headerfile"><debug/vector></filename></entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>In addition, when compiling in C++11 mode, these additional
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containers have additional debug capability.
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</para>
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<table frame="all" xml:id="table.debug_mode_containers_cxx11">
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<title>Debugging Containers C++11</title>
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<tgroup cols="4" align="left" colsep="1" rowsep="1">
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<colspec colname="c1"/>
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<colspec colname="c2"/>
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<colspec colname="c3"/>
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<colspec colname="c4"/>
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<thead>
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<row>
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<entry>Container</entry>
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<entry>Header</entry>
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<entry>Debug container</entry>
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<entry>Debug header</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><classname>std::unordered_map</classname></entry>
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<entry><filename class="headerfile">unordered_map</filename></entry>
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<entry><classname>__gnu_debug::unordered_map</classname></entry>
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<entry><filename class="headerfile"><debug/unordered_map></filename></entry>
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</row>
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<row>
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<entry><classname>std::unordered_multimap</classname></entry>
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<entry><filename class="headerfile">unordered_map</filename></entry>
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<entry><classname>__gnu_debug::unordered_multimap</classname></entry>
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<entry><filename class="headerfile"><debug/unordered_map></filename></entry>
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</row>
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<row>
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<entry><classname>std::unordered_set</classname></entry>
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<entry><filename class="headerfile">unordered_set</filename></entry>
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<entry><classname>__gnu_debug::unordered_set</classname></entry>
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<entry><filename class="headerfile"><debug/unordered_set></filename></entry>
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</row>
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<row>
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<entry><classname>std::unordered_multiset</classname></entry>
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<entry><filename class="headerfile">unordered_set</filename></entry>
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<entry><classname>__gnu_debug::unordered_multiset</classname></entry>
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<entry><filename class="headerfile"><debug/unordered_set></filename></entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</section>
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</section>
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<section xml:id="manual.ext.debug_mode.design" xreflabel="Design"><info><title>Design</title></info>
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<?dbhtml filename="debug_mode_design.html"?>
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<para>
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</para>
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<section xml:id="debug_mode.design.goals" xreflabel="Goals"><info><title>Goals</title></info>
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<para>
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</para>
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<para> The libstdc++ debug mode replaces unsafe (but efficient) standard
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containers and iterators with semantically equivalent safe standard
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containers and iterators to aid in debugging user programs. The
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following goals directed the design of the libstdc++ debug mode:</para>
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<itemizedlist>
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<listitem><para><emphasis>Correctness</emphasis>: the libstdc++ debug mode must not change
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the semantics of the standard library for all cases specified in
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the ANSI/ISO C++ standard. The essence of this constraint is that
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any valid C++ program should behave in the same manner regardless
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of whether it is compiled with debug mode or release mode. In
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particular, entities that are defined in namespace std in release
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mode should remain defined in namespace std in debug mode, so that
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legal specializations of namespace std entities will remain
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valid. A program that is not valid C++ (e.g., invokes undefined
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behavior) is not required to behave similarly, although the debug
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mode will abort with a diagnostic when it detects undefined
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behavior.</para></listitem>
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<listitem><para><emphasis>Performance</emphasis>: the additional of the libstdc++ debug mode
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must not affect the performance of the library when it is compiled
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in release mode. Performance of the libstdc++ debug mode is
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secondary (and, in fact, will be worse than the release
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mode).</para></listitem>
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<listitem><para><emphasis>Usability</emphasis>: the libstdc++ debug mode should be easy to
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use. It should be easily incorporated into the user's development
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environment (e.g., by requiring only a single new compiler switch)
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and should produce reasonable diagnostics when it detects a
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problem with the user program. Usability also involves detection
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of errors when using the debug mode incorrectly, e.g., by linking
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a release-compiled object against a debug-compiled object if in
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fact the resulting program will not run correctly.</para></listitem>
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<listitem><para><emphasis>Minimize recompilation</emphasis>: While it is expected that
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users recompile at least part of their program to use debug
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mode, the amount of recompilation affects the
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detect-compile-debug turnaround time. This indirectly affects the
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usefulness of the debug mode, because debugging some applications
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may require rebuilding a large amount of code, which may not be
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feasible when the suspect code may be very localized. There are
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several levels of conformance to this requirement, each with its
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own usability and implementation characteristics. In general, the
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higher-numbered conformance levels are more usable (i.e., require
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less recompilation) but are more complicated to implement than
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the lower-numbered conformance levels.
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<orderedlist inheritnum="ignore" continuation="restarts">
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<listitem><para><emphasis>Full recompilation</emphasis>: The user must recompile his or
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her entire application and all C++ libraries it depends on,
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including the C++ standard library that ships with the
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compiler. This must be done even if only a small part of the
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program can use debugging features.</para></listitem>
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<listitem><para><emphasis>Full user recompilation</emphasis>: The user must recompile
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his or her entire application and all C++ libraries it depends
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on, but not the C++ standard library itself. This must be done
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even if only a small part of the program can use debugging
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features. This can be achieved given a full recompilation
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system by compiling two versions of the standard library when
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the compiler is installed and linking against the appropriate
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one, e.g., a multilibs approach.</para></listitem>
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<listitem><para><emphasis>Partial recompilation</emphasis>: The user must recompile the
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parts of his or her application and the C++ libraries it
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depends on that will use the debugging facilities
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directly. This means that any code that uses the debuggable
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standard containers would need to be recompiled, but code
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that does not use them (but may, for instance, use IOStreams)
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would not have to be recompiled.</para></listitem>
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<listitem><para><emphasis>Per-use recompilation</emphasis>: The user must recompile the
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parts of his or her application and the C++ libraries it
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depends on where debugging should occur, and any other code
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that interacts with those containers. This means that a set of
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translation units that accesses a particular standard
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container instance may either be compiled in release mode (no
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checking) or debug mode (full checking), but must all be
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compiled in the same way; a translation unit that does not see
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that standard container instance need not be recompiled. This
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also means that a translation unit <emphasis>A</emphasis> that contains a
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particular instantiation
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(say, <code>std::vector<int></code>) compiled in release
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mode can be linked against a translation unit <emphasis>B</emphasis> that
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contains the same instantiation compiled in debug mode (a
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feature not present with partial recompilation). While this
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behavior is technically a violation of the One Definition
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Rule, this ability tends to be very important in
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practice. The libstdc++ debug mode supports this level of
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recompilation. </para></listitem>
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<listitem><para><emphasis>Per-unit recompilation</emphasis>: The user must only
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recompile the translation units where checking should occur,
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regardless of where debuggable standard containers are
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used. This has also been dubbed "<code>-g</code> mode",
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because the <code>-g</code> compiler switch works in this way,
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emitting debugging information at a per--translation-unit
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granularity. We believe that this level of recompilation is in
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fact not possible if we intend to supply safe iterators, leave
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the program semantics unchanged, and not regress in
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performance under release mode because we cannot associate
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|
extra information with an iterator (to form a safe iterator)
|
|
without either reserving that space in release mode
|
|
(performance regression) or allocating extra memory associated
|
|
with each iterator with <code>new</code> (changes the program
|
|
semantics).</para></listitem>
|
|
</orderedlist>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
<section xml:id="debug_mode.design.methods" xreflabel="Methods"><info><title>Methods</title></info>
|
|
|
|
<para>
|
|
</para>
|
|
<para>This section provides an overall view of the design of the
|
|
libstdc++ debug mode and details the relationship between design
|
|
decisions and the stated design goals.</para>
|
|
|
|
<section xml:id="debug_mode.design.methods.wrappers" xreflabel="Method Wrapper"><info><title>The Wrapper Model</title></info>
|
|
|
|
<para>The libstdc++ debug mode uses a wrapper model where the
|
|
debugging versions of library components (e.g., iterators and
|
|
containers) form a layer on top of the release versions of the
|
|
library components. The debugging components first verify that the
|
|
operation is correct (aborting with a diagnostic if an error is
|
|
found) and will then forward to the underlying release-mode
|
|
container that will perform the actual work. This design decision
|
|
ensures that we cannot regress release-mode performance (because the
|
|
release-mode containers are left untouched) and partially
|
|
enables <link linkend="methods.coexistence.link">mixing debug and
|
|
release code</link> at link time, although that will not be
|
|
discussed at this time.</para>
|
|
|
|
<para>Two types of wrappers are used in the implementation of the debug
|
|
mode: container wrappers and iterator wrappers. The two types of
|
|
wrappers interact to maintain relationships between iterators and
|
|
their associated containers, which are necessary to detect certain
|
|
types of standard library usage errors such as dereferencing
|
|
past-the-end iterators or inserting into a container using an
|
|
iterator from a different container.</para>
|
|
|
|
<section xml:id="debug_mode.design.methods.safe_iter" xreflabel="Method Safe Iter"><info><title>Safe Iterators</title></info>
|
|
|
|
<para>Iterator wrappers provide a debugging layer over any iterator that
|
|
is attached to a particular container, and will manage the
|
|
information detailing the iterator's state (singular,
|
|
dereferenceable, etc.) and tracking the container to which the
|
|
iterator is attached. Because iterators have a well-defined, common
|
|
interface the iterator wrapper is implemented with the iterator
|
|
adaptor class template <code>__gnu_debug::_Safe_iterator</code>,
|
|
which takes two template parameters:</para>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><code>Iterator</code>: The underlying iterator type, which must
|
|
be either the <code>iterator</code> or <code>const_iterator</code>
|
|
typedef from the sequence type this iterator can reference.</para></listitem>
|
|
|
|
<listitem><para><code>Sequence</code>: The type of sequence that this iterator
|
|
references. This sequence must be a safe sequence (discussed below)
|
|
whose <code>iterator</code> or <code>const_iterator</code> typedef
|
|
is the type of the safe iterator.</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
<section xml:id="debug_mode.design.methods.safe_seq" xreflabel="Method Safe Seq"><info><title>Safe Sequences (Containers)</title></info>
|
|
|
|
|
|
<para>Container wrappers provide a debugging layer over a particular
|
|
container type. Because containers vary greatly in the member
|
|
functions they support and the semantics of those member functions
|
|
(especially in the area of iterator invalidation), container
|
|
wrappers are tailored to the container they reference, e.g., the
|
|
debugging version of <code>std::list</code> duplicates the entire
|
|
interface of <code>std::list</code>, adding additional semantic
|
|
checks and then forwarding operations to the
|
|
real <code>std::list</code> (a public base class of the debugging
|
|
version) as appropriate. However, all safe containers inherit from
|
|
the class template <code>__gnu_debug::_Safe_sequence</code>,
|
|
instantiated with the type of the safe container itself (an instance
|
|
of the curiously recurring template pattern).</para>
|
|
|
|
<para>The iterators of a container wrapper will be
|
|
<link linkend="debug_mode.design.methods.safe_iter">safe
|
|
iterators</link> that reference sequences of this type and wrap the
|
|
iterators provided by the release-mode base class. The debugging
|
|
container will use only the safe iterators within its own interface
|
|
(therefore requiring the user to use safe iterators, although this
|
|
does not change correct user code) and will communicate with the
|
|
release-mode base class with only the underlying, unsafe,
|
|
release-mode iterators that the base class exports.</para>
|
|
|
|
<para> The debugging version of <code>std::list</code> will have the
|
|
following basic structure:</para>
|
|
|
|
<programlisting>
|
|
template<typename _Tp, typename _Allocator = allocator<_Tp>
|
|
class debug-list :
|
|
public release-list<_Tp, _Allocator>,
|
|
public __gnu_debug::_Safe_sequence<debug-list<_Tp, _Allocator> >
|
|
{
|
|
typedef release-list<_Tp, _Allocator> _Base;
|
|
typedef debug-list<_Tp, _Allocator> _Self;
|
|
|
|
public:
|
|
typedef __gnu_debug::_Safe_iterator<typename _Base::iterator, _Self> iterator;
|
|
typedef __gnu_debug::_Safe_iterator<typename _Base::const_iterator, _Self> const_iterator;
|
|
|
|
// duplicate std::list interface with debugging semantics
|
|
};
|
|
</programlisting>
|
|
</section>
|
|
</section>
|
|
|
|
<section xml:id="debug_mode.design.methods.precond" xreflabel="Precondition check"><info><title>Precondition Checking</title></info>
|
|
|
|
<para>The debug mode operates primarily by checking the preconditions of
|
|
all standard library operations that it supports. Preconditions that
|
|
are always checked (regardless of whether or not we are in debug
|
|
mode) are checked via the <code>__check_xxx</code> macros defined
|
|
and documented in the source
|
|
file <code>include/debug/debug.h</code>. Preconditions that may or
|
|
may not be checked, depending on the debug-mode
|
|
macro <code>_GLIBCXX_DEBUG</code>, are checked via
|
|
the <code>__requires_xxx</code> macros defined and documented in the
|
|
same source file. Preconditions are validated using any additional
|
|
information available at run-time, e.g., the containers that are
|
|
associated with a particular iterator, the position of the iterator
|
|
within those containers, the distance between two iterators that may
|
|
form a valid range, etc. In the absence of suitable information,
|
|
e.g., an input iterator that is not a safe iterator, these
|
|
precondition checks will silently succeed.</para>
|
|
|
|
<para>The majority of precondition checks use the aforementioned macros,
|
|
which have the secondary benefit of having prewritten debug
|
|
messages that use information about the current status of the
|
|
objects involved (e.g., whether an iterator is singular or what
|
|
sequence it is attached to) along with some static information
|
|
(e.g., the names of the function parameters corresponding to the
|
|
objects involved). When not using these macros, the debug mode uses
|
|
either the debug-mode assertion
|
|
macro <code>_GLIBCXX_DEBUG_ASSERT</code> , its pedantic
|
|
cousin <code>_GLIBCXX_DEBUG_PEDASSERT</code>, or the assertion
|
|
check macro that supports more advance formulation of error
|
|
messages, <code>_GLIBCXX_DEBUG_VERIFY</code>. These macros are
|
|
documented more thoroughly in the debug mode source code.</para>
|
|
</section>
|
|
|
|
<section xml:id="debug_mode.design.methods.coexistence" xreflabel="Coexistence"><info><title>Release- and debug-mode coexistence</title></info>
|
|
|
|
<para>The libstdc++ debug mode is the first debug mode we know of that
|
|
is able to provide the "Per-use recompilation" (4) guarantee, that
|
|
allows release-compiled and debug-compiled code to be linked and
|
|
executed together without causing unpredictable behavior. This
|
|
guarantee minimizes the recompilation that users are required to
|
|
perform, shortening the detect-compile-debug bug hunting cycle
|
|
and making the debug mode easier to incorporate into development
|
|
environments by minimizing dependencies.</para>
|
|
|
|
<para>Achieving link- and run-time coexistence is not a trivial
|
|
implementation task. To achieve this goal we required a small
|
|
extension to the GNU C++ compiler (since incorporated into the C++11 language specification, described in the GCC Manual for the C++ language as
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/onlinedocs/gcc/Namespace-Association.html#Namespace-Association">namespace
|
|
association</link>), and a complex organization of debug- and
|
|
release-modes. The end result is that we have achieved per-use
|
|
recompilation but have had to give up some checking of the
|
|
<code>std::basic_string</code> class template (namely, safe
|
|
iterators).
|
|
</para>
|
|
|
|
<section xml:id="methods.coexistence.compile" xreflabel="Compile"><info><title>Compile-time coexistence of release- and debug-mode components</title></info>
|
|
|
|
|
|
<para>Both the release-mode components and the debug-mode
|
|
components need to exist within a single translation unit so that
|
|
the debug versions can wrap the release versions. However, only one
|
|
of these components should be user-visible at any particular
|
|
time with the standard name, e.g., <code>std::list</code>. </para>
|
|
|
|
<para>In release mode, we define only the release-mode version of the
|
|
component with its standard name and do not include the debugging
|
|
component at all. The release mode version is defined within the
|
|
namespace <code>std</code>. Minus the namespace associations, this
|
|
method leaves the behavior of release mode completely unchanged from
|
|
its behavior prior to the introduction of the libstdc++ debug
|
|
mode. Here's an example of what this ends up looking like, in
|
|
C++.</para>
|
|
|
|
<programlisting>
|
|
namespace std
|
|
{
|
|
template<typename _Tp, typename _Alloc = allocator<_Tp> >
|
|
class list
|
|
{
|
|
// ...
|
|
};
|
|
} // namespace std
|
|
</programlisting>
|
|
|
|
<para>In debug mode we include the release-mode container (which is now
|
|
defined in the namespace <code>__cxx1998</code>) and also the
|
|
debug-mode container. The debug-mode container is defined within the
|
|
namespace <code>__debug</code>, which is associated with namespace
|
|
<code>std</code> via the C++11 namespace association language feature. This
|
|
method allows the debug and release versions of the same component to
|
|
coexist at compile-time and link-time without causing an unreasonable
|
|
maintenance burden, while minimizing confusion. Again, this boils down
|
|
to C++ code as follows:</para>
|
|
|
|
<programlisting>
|
|
namespace std
|
|
{
|
|
namespace __cxx1998
|
|
{
|
|
template<typename _Tp, typename _Alloc = allocator<_Tp> >
|
|
class list
|
|
{
|
|
// ...
|
|
};
|
|
} // namespace __gnu_norm
|
|
|
|
namespace __debug
|
|
{
|
|
template<typename _Tp, typename _Alloc = allocator<_Tp> >
|
|
class list
|
|
: public __cxx1998::list<_Tp, _Alloc>,
|
|
public __gnu_debug::_Safe_sequence<list<_Tp, _Alloc> >
|
|
{
|
|
// ...
|
|
};
|
|
} // namespace __cxx1998
|
|
|
|
// namespace __debug __attribute__ ((strong));
|
|
inline namespace __debug { }
|
|
}
|
|
</programlisting>
|
|
</section>
|
|
|
|
<section xml:id="methods.coexistence.link" xreflabel="Link"><info><title>Link- and run-time coexistence of release- and
|
|
debug-mode components</title></info>
|
|
|
|
|
|
<para>Because each component has a distinct and separate release and
|
|
debug implementation, there is no issue with link-time
|
|
coexistence: the separate namespaces result in different mangled
|
|
names, and thus unique linkage.</para>
|
|
|
|
<para>However, components that are defined and used within the C++
|
|
standard library itself face additional constraints. For instance,
|
|
some of the member functions of <code> std::moneypunct</code> return
|
|
<code>std::basic_string</code>. Normally, this is not a problem, but
|
|
with a mixed mode standard library that could be using either
|
|
debug-mode or release-mode <code> basic_string</code> objects, things
|
|
get more complicated. As the return value of a function is not
|
|
encoded into the mangled name, there is no way to specify a
|
|
release-mode or a debug-mode string. In practice, this results in
|
|
runtime errors. A simplified example of this problem is as follows.
|
|
</para>
|
|
|
|
<para> Take this translation unit, compiled in debug-mode: </para>
|
|
<programlisting>
|
|
// -D_GLIBCXX_DEBUG
|
|
#include <string>
|
|
|
|
std::string test02();
|
|
|
|
std::string test01()
|
|
{
|
|
return test02();
|
|
}
|
|
|
|
int main()
|
|
{
|
|
test01();
|
|
return 0;
|
|
}
|
|
</programlisting>
|
|
|
|
<para> ... and linked to this translation unit, compiled in release mode:</para>
|
|
|
|
<programlisting>
|
|
#include <string>
|
|
|
|
std::string
|
|
test02()
|
|
{
|
|
return std::string("toast");
|
|
}
|
|
</programlisting>
|
|
|
|
<para> For this reason we cannot easily provide safe iterators for
|
|
the <code>std::basic_string</code> class template, as it is present
|
|
throughout the C++ standard library. For instance, locale facets
|
|
define typedefs that include <code>basic_string</code>: in a mixed
|
|
debug/release program, should that typedef be based on the
|
|
debug-mode <code>basic_string</code> or the
|
|
release-mode <code>basic_string</code>? While the answer could be
|
|
"both", and the difference hidden via renaming a la the
|
|
debug/release containers, we must note two things about locale
|
|
facets:</para>
|
|
|
|
<orderedlist inheritnum="ignore" continuation="restarts">
|
|
<listitem><para>They exist as shared state: one can create a facet in one
|
|
translation unit and access the facet via the same type name in a
|
|
different translation unit. This means that we cannot have two
|
|
different versions of locale facets, because the types would not be
|
|
the same across debug/release-mode translation unit barriers.</para></listitem>
|
|
|
|
<listitem><para>They have virtual functions returning strings: these functions
|
|
mangle in the same way regardless of the mangling of their return
|
|
types (see above), and their precise signatures can be relied upon
|
|
by users because they may be overridden in derived classes.</para></listitem>
|
|
</orderedlist>
|
|
|
|
<para>With the design of libstdc++ debug mode, we cannot effectively hide
|
|
the differences between debug and release-mode strings from the
|
|
user. Failure to hide the differences may result in unpredictable
|
|
behavior, and for this reason we have opted to only
|
|
perform <code>basic_string</code> changes that do not require ABI
|
|
changes. The effect on users is expected to be minimal, as there are
|
|
simple alternatives (e.g., <code>__gnu_debug::basic_string</code>),
|
|
and the usability benefit we gain from the ability to mix debug- and
|
|
release-compiled translation units is enormous.</para>
|
|
</section>
|
|
|
|
<section xml:id="methods.coexistence.alt" xreflabel="Alternatives"><info><title>Alternatives for Coexistence</title></info>
|
|
|
|
|
|
<para>The coexistence scheme above was chosen over many alternatives,
|
|
including language-only solutions and solutions that also required
|
|
extensions to the C++ front end. The following is a partial list of
|
|
solutions, with justifications for our rejection of each.</para>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Completely separate debug/release libraries</emphasis>: This is by
|
|
far the simplest implementation option, where we do not allow any
|
|
coexistence of debug- and release-compiled translation units in a
|
|
program. This solution has an extreme negative affect on usability,
|
|
because it is quite likely that some libraries an application
|
|
depends on cannot be recompiled easily. This would not meet
|
|
our <emphasis>usability</emphasis> or <emphasis>minimize recompilation</emphasis> criteria
|
|
well.</para></listitem>
|
|
|
|
<listitem><para><emphasis>Add a <code>Debug</code> boolean template parameter</emphasis>:
|
|
Partial specialization could be used to select the debug
|
|
implementation when <code>Debug == true</code>, and the state
|
|
of <code>_GLIBCXX_DEBUG</code> could decide whether the
|
|
default <code>Debug</code> argument is <code>true</code>
|
|
or <code>false</code>. This option would break conformance with the
|
|
C++ standard in both debug <emphasis>and</emphasis> release modes. This would
|
|
not meet our <emphasis>correctness</emphasis> criteria. </para></listitem>
|
|
|
|
<listitem><para><emphasis>Packaging a debug flag in the allocators</emphasis>: We could
|
|
reuse the <code>Allocator</code> template parameter of containers
|
|
by adding a sentinel wrapper <code>debug<></code> that
|
|
signals the user's intention to use debugging, and pick up
|
|
the <code>debug<></code> allocator wrapper in a partial
|
|
specialization. However, this has two drawbacks: first, there is a
|
|
conformance issue because the default allocator would not be the
|
|
standard-specified <code>std::allocator<T></code>. Secondly
|
|
(and more importantly), users that specify allocators instead of
|
|
implicitly using the default allocator would not get debugging
|
|
containers. Thus this solution fails the <emphasis>correctness</emphasis>
|
|
criteria.</para></listitem>
|
|
|
|
<listitem><para><emphasis>Define debug containers in another namespace, and employ
|
|
a <code>using</code> declaration (or directive)</emphasis>: This is an
|
|
enticing option, because it would eliminate the need for
|
|
the <code>link_name</code> extension by aliasing the
|
|
templates. However, there is no true template aliasing mechanism
|
|
in C++, because both <code>using</code> directives and using
|
|
declarations disallow specialization. This method fails
|
|
the <emphasis>correctness</emphasis> criteria.</para></listitem>
|
|
|
|
<listitem><para><emphasis> Use implementation-specific properties of anonymous
|
|
namespaces. </emphasis>
|
|
See <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2003-08/msg00004.html">this post</link>.
|
|
This method fails the <emphasis>correctness</emphasis> criteria.</para></listitem>
|
|
|
|
<listitem><para><emphasis>Extension: allow reopening on namespaces</emphasis>: This would
|
|
allow the debug mode to effectively alias the
|
|
namespace <code>std</code> to an internal namespace, such
|
|
as <code>__gnu_std_debug</code>, so that it is completely
|
|
separate from the release-mode <code>std</code> namespace. While
|
|
this will solve some renaming problems and ensure that
|
|
debug- and release-compiled code cannot be mixed unsafely, it ensures that
|
|
debug- and release-compiled code cannot be mixed at all. For
|
|
instance, the program would have two <code>std::cout</code>
|
|
objects! This solution would fails the <emphasis>minimize
|
|
recompilation</emphasis> requirement, because we would only be able to
|
|
support option (1) or (2).</para></listitem>
|
|
|
|
<listitem><para><emphasis>Extension: use link name</emphasis>: This option
|
|
involves complicated re-naming between debug-mode and release-mode
|
|
components at compile time, and then a g++ extension called <emphasis>
|
|
link name </emphasis> to recover the original names at link time. There
|
|
are two drawbacks to this approach. One, it's very verbose,
|
|
relying on macro renaming at compile time and several levels of
|
|
include ordering. Two, ODR issues remained with container member
|
|
functions taking no arguments in mixed-mode settings resulting in
|
|
equivalent link names, <code> vector::push_back() </code> being
|
|
one example.
|
|
See <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2003-08/msg00177.html">proof-of-concept using link
|
|
name</link>. </para></listitem>
|
|
</itemizedlist>
|
|
|
|
<para>Other options may exist for implementing the debug mode, many of
|
|
which have probably been considered and others that may still be
|
|
lurking. This list may be expanded over time to include other
|
|
options that we could have implemented, but in all cases the full
|
|
ramifications of the approach (as measured against the design goals
|
|
for a libstdc++ debug mode) should be considered first. The DejaGNU
|
|
testsuite includes some testcases that check for known problems with
|
|
some solutions (e.g., the <code>using</code> declaration solution
|
|
that breaks user specialization), and additional testcases will be
|
|
added as we are able to identify other typical problem cases. These
|
|
test cases will serve as a benchmark by which we can compare debug
|
|
mode implementations.</para>
|
|
</section>
|
|
</section>
|
|
</section>
|
|
|
|
<section xml:id="debug_mode.design.other" xreflabel="Other"><info><title>Other Implementations</title></info>
|
|
|
|
<para>
|
|
</para>
|
|
<para> There are several existing implementations of debug modes for C++
|
|
standard library implementations, although none of them directly
|
|
supports debugging for programs using libstdc++. The existing
|
|
implementations include:</para>
|
|
<itemizedlist>
|
|
<listitem><para><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.cs.sjsu.edu/faculty/horstman/safestl.html">SafeSTL</link>:
|
|
SafeSTL was the original debugging version of the Standard Template
|
|
Library (STL), implemented by Cay S. Horstmann on top of the
|
|
Hewlett-Packard STL. Though it inspired much work in this area, it
|
|
has not been kept up-to-date for use with modern compilers or C++
|
|
standard library implementations.</para></listitem>
|
|
|
|
<listitem><para><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.stlport.org/">STLport</link>: STLport is a free
|
|
implementation of the C++ standard library derived from the <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.sgi.com/tech/stl/">SGI implementation</link>, and
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|
ported to many other platforms. It includes a debug mode that uses a
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|
wrapper model (that in some ways inspired the libstdc++ debug mode
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|
design), although at the time of this writing the debug mode is
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|
somewhat incomplete and meets only the "Full user recompilation" (2)
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|
recompilation guarantee by requiring the user to link against a
|
|
different library in debug mode vs. release mode.</para></listitem>
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|
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|
<listitem><para>Metrowerks CodeWarrior: The C++ standard library
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|
that ships with Metrowerks CodeWarrior includes a debug mode. It is
|
|
a full debug-mode implementation (including debugging for
|
|
CodeWarrior extensions) and is easy to use, although it meets only
|
|
the "Full recompilation" (1) recompilation
|
|
guarantee.</para></listitem>
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|
</itemizedlist>
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|
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|
</section>
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|
</section>
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|
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</chapter>
|