| <chapter xmlns="http://docbook.org/ns/docbook" version="5.0" |
| xml:id="manual.ext.debug_mode" xreflabel="Debug Mode"> |
| <?dbhtml filename="debug_mode.html"?> |
| |
| <info><title>Debug Mode</title> |
| <keywordset> |
| <keyword>C++</keyword> |
| <keyword>library</keyword> |
| <keyword>debug</keyword> |
| </keywordset> |
| </info> |
| |
| |
| |
| <section xml:id="manual.ext.debug_mode.intro" xreflabel="Intro"><info><title>Intro</title></info> |
| |
| <para> |
| By default, libstdc++ is built with efficiency in mind, and |
| therefore performs little or no error checking that is not |
| required by the C++ standard. This means that programs that |
| incorrectly use the C++ standard library will exhibit behavior |
| that is not portable and may not even be predictable, because they |
| tread into implementation-specific or undefined behavior. To |
| detect some of these errors before they can become problematic, |
| libstdc++ offers a debug mode that provides additional checking of |
| library facilities, and will report errors in the use of libstdc++ |
| as soon as they can be detected by emitting a description of the |
| problem to standard error and aborting the program. This debug |
| mode is available with GCC 3.4.0 and later versions. |
| </para> |
| |
| <para> |
| The libstdc++ debug mode performs checking for many areas of the |
| C++ standard, but the focus is on checking interactions among |
| standard iterators, containers, and algorithms, including: |
| </para> |
| |
| <itemizedlist> |
| <listitem><para><emphasis>Safe iterators</emphasis>: Iterators keep track of the |
| container whose elements they reference, so errors such as |
| incrementing a past-the-end iterator or dereferencing an iterator |
| that points to a container that has been destructed are diagnosed |
| immediately.</para></listitem> |
| |
| <listitem><para><emphasis>Algorithm preconditions</emphasis>: Algorithms attempt to |
| validate their input parameters to detect errors as early as |
| possible. For instance, the <code>set_intersection</code> |
| algorithm requires that its iterator |
| parameters <code>first1</code> and <code>last1</code> form a valid |
| iterator range, and that the sequence |
| [<code>first1</code>, <code>last1</code>) is sorted according to |
| the same predicate that was passed |
| to <code>set_intersection</code>; the libstdc++ debug mode will |
| detect an error if the sequence is not sorted or was sorted by a |
| different predicate.</para></listitem> |
| </itemizedlist> |
| |
| </section> |
| |
| <section xml:id="manual.ext.debug_mode.semantics" xreflabel="Semantics"><info><title>Semantics</title></info> |
| <?dbhtml filename="debug_mode_semantics.html"?> |
| |
| <para> |
| </para> |
| |
| <para>A program that uses the C++ standard library correctly |
| will maintain the same semantics under debug mode as it had with |
| the normal (release) library. All functional and exception-handling |
| guarantees made by the normal library also hold for the debug mode |
| library, with one exception: performance guarantees made by the |
| normal library may not hold in the debug mode library. For |
| instance, erasing an element in a <code>std::list</code> is a |
| constant-time operation in normal library, but in debug mode it is |
| linear in the number of iterators that reference that particular |
| list. So while your (correct) program won't change its results, it |
| is likely to execute more slowly.</para> |
| |
| <para>libstdc++ includes many extensions to the C++ standard library. In |
| some cases the extensions are obvious, such as the hashed |
| associative containers, whereas other extensions give predictable |
| results to behavior that would otherwise be undefined, such as |
| throwing an exception when a <code>std::basic_string</code> is |
| constructed from a NULL character pointer. This latter category also |
| includes implementation-defined and unspecified semantics, such as |
| the growth rate of a vector. Use of these extensions is not |
| considered incorrect, so code that relies on them will not be |
| rejected by debug mode. However, use of these extensions may affect |
| the portability of code to other implementations of the C++ standard |
| library, and is therefore somewhat hazardous. For this reason, the |
| libstdc++ debug mode offers a "pedantic" mode (similar to |
| GCC's <code>-pedantic</code> compiler flag) that attempts to emulate |
| the semantics guaranteed by the C++ standard. For |
| instance, constructing a <code>std::basic_string</code> with a NULL |
| character pointer would result in an exception under normal mode or |
| non-pedantic debug mode (this is a libstdc++ extension), whereas |
| under pedantic debug mode libstdc++ would signal an error. To enable |
| the pedantic debug mode, compile your program with |
| both <code>-D_GLIBCXX_DEBUG</code> |
| and <code>-D_GLIBCXX_DEBUG_PEDANTIC</code> . |
| (N.B. In GCC 3.4.x and 4.0.0, due to a bug, |
| <code>-D_GLIBXX_DEBUG_PEDANTIC</code> was also needed. The problem has |
| been fixed in GCC 4.0.1 and later versions.) </para> |
| |
| <para>The following library components provide extra debugging |
| capabilities in debug mode:</para> |
| <itemizedlist> |
| <listitem><para><code>std::array</code> (no safe iterators)</para></listitem> |
| <listitem><para><code>std::basic_string</code> (no safe iterators and see note below)</para></listitem> |
| <listitem><para><code>std::bitset</code></para></listitem> |
| <listitem><para><code>std::deque</code></para></listitem> |
| <listitem><para><code>std::list</code></para></listitem> |
| <listitem><para><code>std::map</code></para></listitem> |
| <listitem><para><code>std::multimap</code></para></listitem> |
| <listitem><para><code>std::multiset</code></para></listitem> |
| <listitem><para><code>std::set</code></para></listitem> |
| <listitem><para><code>std::vector</code></para></listitem> |
| <listitem><para><code>std::unordered_map</code></para></listitem> |
| <listitem><para><code>std::unordered_multimap</code></para></listitem> |
| <listitem><para><code>std::unordered_set</code></para></listitem> |
| <listitem><para><code>std::unordered_multiset</code></para></listitem> |
| </itemizedlist> |
| |
| <para>N.B. although there are precondition checks for some string operations, |
| e.g. <code>operator[]</code>, |
| they will not always be run when using the <code>char</code> and |
| <code>wchar_t</code> specializations (<code>std::string</code> and |
| <code>std::wstring</code>). This is because libstdc++ uses GCC's |
| <code>extern template</code> extension to provide explicit instantiations |
| of <code>std::string</code> and <code>std::wstring</code>, and those |
| explicit instantiations don't include the debug-mode checks. If the |
| containing functions are inlined then the checks will run, so compiling |
| with <code>-O1</code> might be enough to enable them. Alternatively |
| <code>-D_GLIBCXX_EXTERN_TEMPLATE=0</code> will suppress the declarations |
| of the explicit instantiations and cause the functions to be instantiated |
| with the debug-mode checks included, but this is unsupported and not |
| guaranteed to work. For full debug-mode support you can use the |
| <code>__gnu_debug::basic_string</code> debugging container directly, |
| which always works correctly. |
| </para> |
| |
| </section> |
| |
| <section xml:id="manual.ext.debug_mode.using" xreflabel="Using"><info><title>Using</title></info> |
| <?dbhtml filename="debug_mode_using.html"?> |
| |
| <para> |
| </para> |
| <section xml:id="debug_mode.using.mode" xreflabel="Using Mode"><info><title>Using the Debug Mode</title></info> |
| |
| |
| <para>To use the libstdc++ debug mode, compile your application with the |
| compiler flag <code>-D_GLIBCXX_DEBUG</code>. Note that this flag |
| changes the sizes and behavior of standard class templates such |
| as <code>std::vector</code>, and therefore you can only link code |
| compiled with debug mode and code compiled without debug mode if no |
| instantiation of a container is passed between the two translation |
| units.</para> |
| |
| <para>By default, error messages are formatted to fit on lines of about |
| 78 characters. The environment variable |
| <code>GLIBCXX_DEBUG_MESSAGE_LENGTH</code> can be used to request a |
| different length.</para> |
| |
| </section> |
| |
| <section xml:id="debug_mode.using.specific" xreflabel="Using Specific"><info><title>Using a Specific Debug Container</title></info> |
| |
| <para>When it is not feasible to recompile your entire application, or |
| only specific containers need checking, debugging containers are |
| available as GNU extensions. These debugging containers are |
| functionally equivalent to the standard drop-in containers used in |
| debug mode, but they are available in a separate namespace as GNU |
| extensions and may be used in programs compiled with either release |
| mode or with debug mode. The |
| following table provides the names and headers of the debugging |
| containers: |
| </para> |
| |
| <table frame="all" xml:id="table.debug_mode_containers"> |
| <title>Debugging Containers</title> |
| |
| <tgroup cols="4" align="left" colsep="1" rowsep="1"> |
| <colspec colname="c1"/> |
| <colspec colname="c2"/> |
| <colspec colname="c3"/> |
| <colspec colname="c4"/> |
| |
| <thead> |
| <row> |
| <entry>Container</entry> |
| <entry>Header</entry> |
| <entry>Debug container</entry> |
| <entry>Debug header</entry> |
| </row> |
| </thead> |
| <tbody> |
| <row> |
| <entry><classname>std::bitset</classname></entry> |
| <entry><filename class="headerfile">bitset</filename></entry> |
| <entry><classname>__gnu_debug::bitset</classname></entry> |
| <entry><filename class="headerfile"><debug/bitset></filename></entry> |
| </row> |
| <row> |
| <entry><classname>std::deque</classname></entry> |
| <entry><filename class="headerfile">deque</filename></entry> |
| <entry><classname>__gnu_debug::deque</classname></entry> |
| <entry><filename class="headerfile"><debug/deque></filename></entry> |
| </row> |
| <row> |
| <entry><classname>std::list</classname></entry> |
| <entry><filename class="headerfile">list</filename></entry> |
| <entry><classname>__gnu_debug::list</classname></entry> |
| <entry><filename class="headerfile"><debug/list></filename></entry> |
| </row> |
| <row> |
| <entry><classname>std::map</classname></entry> |
| <entry><filename class="headerfile">map</filename></entry> |
| <entry><classname>__gnu_debug::map</classname></entry> |
| <entry><filename class="headerfile"><debug/map></filename></entry> |
| </row> |
| <row> |
| <entry><classname>std::multimap</classname></entry> |
| <entry><filename class="headerfile">map</filename></entry> |
| <entry><classname>__gnu_debug::multimap</classname></entry> |
| <entry><filename class="headerfile"><debug/map></filename></entry> |
| </row> |
| <row> |
| <entry><classname>std::multiset</classname></entry> |
| <entry><filename class="headerfile">set</filename></entry> |
| <entry><classname>__gnu_debug::multiset</classname></entry> |
| <entry><filename class="headerfile"><debug/set></filename></entry> |
| </row> |
| <row> |
| <entry><classname>std::set</classname></entry> |
| <entry><filename class="headerfile">set</filename></entry> |
| <entry><classname>__gnu_debug::set</classname></entry> |
| <entry><filename class="headerfile"><debug/set></filename></entry> |
| </row> |
| <row> |
| <entry><classname>std::string</classname></entry> |
| <entry><filename class="headerfile">string</filename></entry> |
| <entry><classname>__gnu_debug::string</classname></entry> |
| <entry><filename class="headerfile"><debug/string></filename></entry> |
| </row> |
| <row> |
| <entry><classname>std::wstring</classname></entry> |
| <entry><filename class="headerfile">string</filename></entry> |
| <entry><classname>__gnu_debug::wstring</classname></entry> |
| <entry><filename class="headerfile"><debug/string></filename></entry> |
| </row> |
| <row> |
| <entry><classname>std::basic_string</classname></entry> |
| <entry><filename class="headerfile">string</filename></entry> |
| <entry><classname>__gnu_debug::basic_string</classname></entry> |
| <entry><filename class="headerfile"><debug/string></filename></entry> |
| </row> |
| <row> |
| <entry><classname>std::vector</classname></entry> |
| <entry><filename class="headerfile">vector</filename></entry> |
| <entry><classname>__gnu_debug::vector</classname></entry> |
| <entry><filename class="headerfile"><debug/vector></filename></entry> |
| </row> |
| </tbody> |
| </tgroup> |
| </table> |
| |
| <para>When compiling in C++11 mode (or newer), these |
| containers have additional debug capability. |
| </para> |
| |
| <table frame="all" xml:id="table.debug_mode_containers_cxx11"> |
| <title>Debugging Containers C++11</title> |
| |
| <tgroup cols="4" align="left" colsep="1" rowsep="1"> |
| <colspec colname="c1"/> |
| <colspec colname="c2"/> |
| <colspec colname="c3"/> |
| <colspec colname="c4"/> |
| |
| <thead> |
| <row> |
| <entry>Container</entry> |
| <entry>Header</entry> |
| <entry>Debug container</entry> |
| <entry>Debug header</entry> |
| </row> |
| </thead> |
| <tbody> |
| <row> |
| <entry><classname>std::forward_list</classname></entry> |
| <entry><filename class="headerfile">forward_list</filename></entry> |
| <entry><classname>__gnu_debug::forward_list</classname></entry> |
| <entry><filename class="headerfile"><debug/forward_list></filename></entry> |
| </row> |
| <row> |
| <entry><classname>std::unordered_map</classname></entry> |
| <entry><filename class="headerfile">unordered_map</filename></entry> |
| <entry><classname>__gnu_debug::unordered_map</classname></entry> |
| <entry><filename class="headerfile"><debug/unordered_map></filename></entry> |
| </row> |
| <row> |
| <entry><classname>std::unordered_multimap</classname></entry> |
| <entry><filename class="headerfile">unordered_map</filename></entry> |
| <entry><classname>__gnu_debug::unordered_multimap</classname></entry> |
| <entry><filename class="headerfile"><debug/unordered_map></filename></entry> |
| </row> |
| <row> |
| <entry><classname>std::unordered_set</classname></entry> |
| <entry><filename class="headerfile">unordered_set</filename></entry> |
| <entry><classname>__gnu_debug::unordered_set</classname></entry> |
| <entry><filename class="headerfile"><debug/unordered_set></filename></entry> |
| </row> |
| <row> |
| <entry><classname>std::unordered_multiset</classname></entry> |
| <entry><filename class="headerfile">unordered_set</filename></entry> |
| <entry><classname>__gnu_debug::unordered_multiset</classname></entry> |
| <entry><filename class="headerfile"><debug/unordered_set></filename></entry> |
| </row> |
| </tbody> |
| </tgroup> |
| </table> |
| |
| <para>Prior to GCC 11 a debug version of <classname>std::array</classname> |
| was available as <classname>__gnu_debug::array</classname> in the |
| header <filename class="headerfile"><debug/array></filename>. |
| Because <classname>array::iterator</classname> is just a pointer, |
| the debug <classname>array</classname> can't check iterator operations, |
| it can only check direct accesses to the container. |
| Starting with GCC 11 all the debug capabilities are available in |
| <classname>std::array</classname>, without needing a separate type, |
| so <classname>__gnu_debug::array</classname> is just an alias for |
| <classname>std::array</classname>. |
| That alias is deprecated and may be removed in a future release. |
| </para> |
| |
| </section> |
| </section> |
| |
| <section xml:id="manual.ext.debug_mode.design" xreflabel="Design"><info><title>Design</title></info> |
| <?dbhtml filename="debug_mode_design.html"?> |
| |
| <para> |
| </para> |
| <section xml:id="debug_mode.design.goals" xreflabel="Goals"><info><title>Goals</title></info> |
| |
| <para> |
| </para> |
| <para> The libstdc++ debug mode replaces unsafe (but efficient) standard |
| containers and iterators with semantically equivalent safe standard |
| containers and iterators to aid in debugging user programs. The |
| following goals directed the design of the libstdc++ debug mode:</para> |
| |
| <itemizedlist> |
| |
| <listitem><para><emphasis>Correctness</emphasis>: the libstdc++ debug mode must not change |
| the semantics of the standard library for all cases specified in |
| the ANSI/ISO C++ standard. The essence of this constraint is that |
| any valid C++ program should behave in the same manner regardless |
| of whether it is compiled with debug mode or release mode. In |
| particular, entities that are defined in namespace std in release |
| mode should remain defined in namespace std in debug mode, so that |
| legal specializations of namespace std entities will remain |
| valid. A program that is not valid C++ (e.g., invokes undefined |
| behavior) is not required to behave similarly, although the debug |
| mode will abort with a diagnostic when it detects undefined |
| behavior.</para></listitem> |
| |
| <listitem><para><emphasis>Performance</emphasis>: the additional of the libstdc++ debug mode |
| must not affect the performance of the library when it is compiled |
| in release mode. Performance of the libstdc++ debug mode is |
| secondary (and, in fact, will be worse than the release |
| mode).</para></listitem> |
| |
| <listitem><para><emphasis>Usability</emphasis>: the libstdc++ debug mode should be easy to |
| use. It should be easily incorporated into the user's development |
| environment (e.g., by requiring only a single new compiler switch) |
| and should produce reasonable diagnostics when it detects a |
| problem with the user program. Usability also involves detection |
| of errors when using the debug mode incorrectly, e.g., by linking |
| a release-compiled object against a debug-compiled object if in |
| fact the resulting program will not run correctly.</para></listitem> |
| |
| <listitem><para><emphasis>Minimize recompilation</emphasis>: While it is expected that |
| users recompile at least part of their program to use debug |
| mode, the amount of recompilation affects the |
| detect-compile-debug turnaround time. This indirectly affects the |
| usefulness of the debug mode, because debugging some applications |
| may require rebuilding a large amount of code, which may not be |
| feasible when the suspect code may be very localized. There are |
| several levels of conformance to this requirement, each with its |
| own usability and implementation characteristics. In general, the |
| higher-numbered conformance levels are more usable (i.e., require |
| less recompilation) but are more complicated to implement than |
| the lower-numbered conformance levels. |
| <orderedlist inheritnum="ignore" continuation="restarts"> |
| <listitem><para><emphasis>Full recompilation</emphasis>: The user must recompile his or |
| her entire application and all C++ libraries it depends on, |
| including the C++ standard library that ships with the |
| compiler. This must be done even if only a small part of the |
| program can use debugging features.</para></listitem> |
| |
| <listitem><para><emphasis>Full user recompilation</emphasis>: The user must recompile |
| his or her entire application and all C++ libraries it depends |
| on, but not the C++ standard library itself. This must be done |
| even if only a small part of the program can use debugging |
| features. This can be achieved given a full recompilation |
| system by compiling two versions of the standard library when |
| the compiler is installed and linking against the appropriate |
| one, e.g., a multilibs approach.</para></listitem> |
| |
| <listitem><para><emphasis>Partial recompilation</emphasis>: The user must recompile the |
| parts of his or her application and the C++ libraries it |
| depends on that will use the debugging facilities |
| directly. This means that any code that uses the debuggable |
| standard containers would need to be recompiled, but code |
| that does not use them (but may, for instance, use IOStreams) |
| would not have to be recompiled.</para></listitem> |
| |
| <listitem><para><emphasis>Per-use recompilation</emphasis>: The user must recompile the |
| parts of his or her application and the C++ libraries it |
| depends on where debugging should occur, and any other code |
| that interacts with those containers. This means that a set of |
| translation units that accesses a particular standard |
| container instance may either be compiled in release mode (no |
| checking) or debug mode (full checking), but must all be |
| compiled in the same way; a translation unit that does not see |
| that standard container instance need not be recompiled. This |
| also means that a translation unit <emphasis>A</emphasis> that contains a |
| particular instantiation |
| (say, <code>std::vector<int></code>) compiled in release |
| mode can be linked against a translation unit <emphasis>B</emphasis> that |
| contains the same instantiation compiled in debug mode (a |
| feature not present with partial recompilation). While this |
| behavior is technically a violation of the One Definition |
| Rule, this ability tends to be very important in |
| practice. The libstdc++ debug mode supports this level of |
| recompilation. </para></listitem> |
| |
| <listitem><para><emphasis>Per-unit recompilation</emphasis>: The user must only |
| recompile the translation units where checking should occur, |
| regardless of where debuggable standard containers are |
| used. This has also been dubbed "<code>-g</code> mode", |
| because the <code>-g</code> compiler switch works in this way, |
| emitting debugging information at a per--translation-unit |
| granularity. We believe that this level of recompilation is in |
| fact not possible if we intend to supply safe iterators, leave |
| the program semantics unchanged, and not regress in |
| performance under release mode because we cannot associate |
| 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 use inline namespaces 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 |
| |
| 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="https://web.archive.org/web/20171225062613/http://www.sgi.com/tech/stl/">SGI implementation</link>, and |
| ported to many other platforms. It includes a debug mode that uses a |
| wrapper model (that in some ways inspired the libstdc++ debug mode |
| design), although at the time of this writing the debug mode is |
| somewhat incomplete and meets only the "Full user recompilation" (2) |
| recompilation guarantee by requiring the user to link against a |
| different library in debug mode vs. release mode.</para></listitem> |
| |
| <listitem><para>Metrowerks CodeWarrior: The C++ standard library |
| 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> |
| </itemizedlist> |
| |
| </section> |
| </section> |
| |
| </chapter> |
