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diff --git a/libstdc++-v3/doc/xml/manual/debug_mode.xml b/libstdc++-v3/doc/xml/manual/debug_mode.xml new file mode 100644 index 000000000..c58bde340 --- /dev/null +++ b/libstdc++-v3/doc/xml/manual/debug_mode.xml @@ -0,0 +1,889 @@ +<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> + + <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::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> specialisations (<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> + + <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"> +<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>In addition, when compiling in C++0x mode, these additional +containers have additional debug capability. +</para> + +<table frame="all"> +<title>Debugging Containers C++0x</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::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> +</section> +</section> + +<section xml:id="manual.ext.debug_mode.design" xreflabel="Design"><info><title>Design</title></info> + + <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 required a small + extension to the GNU C++ compiler (since incorporated into the C++0x 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++0x 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">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.mathcs.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 + 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> |