1 <chapter xmlns="http://docbook.org/ns/docbook" version="5.0"
2 xml:id="manual.ext.debug_mode" xreflabel="Debug Mode">
3 <?dbhtml filename="debug_mode.html"?>
5 <info><title>Debug Mode</title>
8 <keyword>library</keyword>
9 <keyword>debug</keyword>
15 <section xml:id="manual.ext.debug_mode.intro" xreflabel="Intro"><info><title>Intro</title></info>
18 By default, libstdc++ is built with efficiency in mind, and
19 therefore performs little or no error checking that is not
20 required by the C++ standard. This means that programs that
21 incorrectly use the C++ standard library will exhibit behavior
22 that is not portable and may not even be predictable, because they
23 tread into implementation-specific or undefined behavior. To
24 detect some of these errors before they can become problematic,
25 libstdc++ offers a debug mode that provides additional checking of
26 library facilities, and will report errors in the use of libstdc++
27 as soon as they can be detected by emitting a description of the
28 problem to standard error and aborting the program. This debug
29 mode is available with GCC 3.4.0 and later versions.
33 The libstdc++ debug mode performs checking for many areas of the
34 C++ standard, but the focus is on checking interactions among
35 standard iterators, containers, and algorithms, including:
39 <listitem><para><emphasis>Safe iterators</emphasis>: Iterators keep track of the
40 container whose elements they reference, so errors such as
41 incrementing a past-the-end iterator or dereferencing an iterator
42 that points to a container that has been destructed are diagnosed
43 immediately.</para></listitem>
45 <listitem><para><emphasis>Algorithm preconditions</emphasis>: Algorithms attempt to
46 validate their input parameters to detect errors as early as
47 possible. For instance, the <code>set_intersection</code>
48 algorithm requires that its iterator
49 parameters <code>first1</code> and <code>last1</code> form a valid
50 iterator range, and that the sequence
51 [<code>first1</code>, <code>last1</code>) is sorted according to
52 the same predicate that was passed
53 to <code>set_intersection</code>; the libstdc++ debug mode will
54 detect an error if the sequence is not sorted or was sorted by a
55 different predicate.</para></listitem>
60 <section xml:id="manual.ext.debug_mode.semantics" xreflabel="Semantics"><info><title>Semantics</title></info>
61 <?dbhtml filename="debug_mode_semantics.html"?>
66 <para>A program that uses the C++ standard library correctly
67 will maintain the same semantics under debug mode as it had with
68 the normal (release) library. All functional and exception-handling
69 guarantees made by the normal library also hold for the debug mode
70 library, with one exception: performance guarantees made by the
71 normal library may not hold in the debug mode library. For
72 instance, erasing an element in a <code>std::list</code> is a
73 constant-time operation in normal library, but in debug mode it is
74 linear in the number of iterators that reference that particular
75 list. So while your (correct) program won't change its results, it
76 is likely to execute more slowly.</para>
78 <para>libstdc++ includes many extensions to the C++ standard library. In
79 some cases the extensions are obvious, such as the hashed
80 associative containers, whereas other extensions give predictable
81 results to behavior that would otherwise be undefined, such as
82 throwing an exception when a <code>std::basic_string</code> is
83 constructed from a NULL character pointer. This latter category also
84 includes implementation-defined and unspecified semantics, such as
85 the growth rate of a vector. Use of these extensions is not
86 considered incorrect, so code that relies on them will not be
87 rejected by debug mode. However, use of these extensions may affect
88 the portability of code to other implementations of the C++ standard
89 library, and is therefore somewhat hazardous. For this reason, the
90 libstdc++ debug mode offers a "pedantic" mode (similar to
91 GCC's <code>-pedantic</code> compiler flag) that attempts to emulate
92 the semantics guaranteed by the C++ standard. For
93 instance, constructing a <code>std::basic_string</code> with a NULL
94 character pointer would result in an exception under normal mode or
95 non-pedantic debug mode (this is a libstdc++ extension), whereas
96 under pedantic debug mode libstdc++ would signal an error. To enable
97 the pedantic debug mode, compile your program with
98 both <code>-D_GLIBCXX_DEBUG</code>
99 and <code>-D_GLIBCXX_DEBUG_PEDANTIC</code> .
100 (N.B. In GCC 3.4.x and 4.0.0, due to a bug,
101 <code>-D_GLIBXX_DEBUG_PEDANTIC</code> was also needed. The problem has
102 been fixed in GCC 4.0.1 and later versions.) </para>
104 <para>The following library components provide extra debugging
105 capabilities in debug mode:</para>
107 <listitem><para><code>std::basic_string</code> (no safe iterators and see note below)</para></listitem>
108 <listitem><para><code>std::bitset</code></para></listitem>
109 <listitem><para><code>std::deque</code></para></listitem>
110 <listitem><para><code>std::list</code></para></listitem>
111 <listitem><para><code>std::map</code></para></listitem>
112 <listitem><para><code>std::multimap</code></para></listitem>
113 <listitem><para><code>std::multiset</code></para></listitem>
114 <listitem><para><code>std::set</code></para></listitem>
115 <listitem><para><code>std::vector</code></para></listitem>
116 <listitem><para><code>std::unordered_map</code></para></listitem>
117 <listitem><para><code>std::unordered_multimap</code></para></listitem>
118 <listitem><para><code>std::unordered_set</code></para></listitem>
119 <listitem><para><code>std::unordered_multiset</code></para></listitem>
122 <para>N.B. although there are precondition checks for some string operations,
123 e.g. <code>operator[]</code>,
124 they will not always be run when using the <code>char</code> and
125 <code>wchar_t</code> specialisations (<code>std::string</code> and
126 <code>std::wstring</code>). This is because libstdc++ uses GCC's
127 <code>extern template</code> extension to provide explicit instantiations
128 of <code>std::string</code> and <code>std::wstring</code>, and those
129 explicit instantiations don't include the debug-mode checks. If the
130 containing functions are inlined then the checks will run, so compiling
131 with <code>-O1</code> might be enough to enable them. Alternatively
132 <code>-D_GLIBCXX_EXTERN_TEMPLATE=0</code> will suppress the declarations
133 of the explicit instantiations and cause the functions to be instantiated
134 with the debug-mode checks included, but this is unsupported and not
135 guaranteed to work. For full debug-mode support you can use the
136 <code>__gnu_debug::basic_string</code> debugging container directly,
137 which always works correctly.
142 <section xml:id="manual.ext.debug_mode.using" xreflabel="Using"><info><title>Using</title></info>
143 <?dbhtml filename="debug_mode_using.html"?>
147 <section xml:id="debug_mode.using.mode" xreflabel="Using Mode"><info><title>Using the Debug Mode</title></info>
150 <para>To use the libstdc++ debug mode, compile your application with the
151 compiler flag <code>-D_GLIBCXX_DEBUG</code>. Note that this flag
152 changes the sizes and behavior of standard class templates such
153 as <code>std::vector</code>, and therefore you can only link code
154 compiled with debug mode and code compiled without debug mode if no
155 instantiation of a container is passed between the two translation
158 <para>By default, error messages are formatted to fit on lines of about
159 78 characters. The environment variable
160 <code>GLIBCXX_DEBUG_MESSAGE_LENGTH</code> can be used to request a
161 different length.</para>
165 <section xml:id="debug_mode.using.specific" xreflabel="Using Specific"><info><title>Using a Specific Debug Container</title></info>
167 <para>When it is not feasible to recompile your entire application, or
168 only specific containers need checking, debugging containers are
169 available as GNU extensions. These debugging containers are
170 functionally equivalent to the standard drop-in containers used in
171 debug mode, but they are available in a separate namespace as GNU
172 extensions and may be used in programs compiled with either release
173 mode or with debug mode. The
174 following table provides the names and headers of the debugging
179 <title>Debugging Containers</title>
181 <tgroup cols="4" align="left" colsep="1" rowsep="1">
182 <colspec colname="c1"/>
183 <colspec colname="c2"/>
184 <colspec colname="c3"/>
185 <colspec colname="c4"/>
189 <entry>Container</entry>
190 <entry>Header</entry>
191 <entry>Debug container</entry>
192 <entry>Debug header</entry>
197 <entry><classname>std::bitset</classname></entry>
198 <entry><filename class="headerfile">bitset</filename></entry>
199 <entry><classname>__gnu_debug::bitset</classname></entry>
200 <entry><filename class="headerfile"><debug/bitset></filename></entry>
203 <entry><classname>std::deque</classname></entry>
204 <entry><filename class="headerfile">deque</filename></entry>
205 <entry><classname>__gnu_debug::deque</classname></entry>
206 <entry><filename class="headerfile"><debug/deque></filename></entry>
209 <entry><classname>std::list</classname></entry>
210 <entry><filename class="headerfile">list</filename></entry>
211 <entry><classname>__gnu_debug::list</classname></entry>
212 <entry><filename class="headerfile"><debug/list></filename></entry>
215 <entry><classname>std::map</classname></entry>
216 <entry><filename class="headerfile">map</filename></entry>
217 <entry><classname>__gnu_debug::map</classname></entry>
218 <entry><filename class="headerfile"><debug/map></filename></entry>
221 <entry><classname>std::multimap</classname></entry>
222 <entry><filename class="headerfile">map</filename></entry>
223 <entry><classname>__gnu_debug::multimap</classname></entry>
224 <entry><filename class="headerfile"><debug/map></filename></entry>
227 <entry><classname>std::multiset</classname></entry>
228 <entry><filename class="headerfile">set</filename></entry>
229 <entry><classname>__gnu_debug::multiset</classname></entry>
230 <entry><filename class="headerfile"><debug/set></filename></entry>
233 <entry><classname>std::set</classname></entry>
234 <entry><filename class="headerfile">set</filename></entry>
235 <entry><classname>__gnu_debug::set</classname></entry>
236 <entry><filename class="headerfile"><debug/set></filename></entry>
239 <entry><classname>std::string</classname></entry>
240 <entry><filename class="headerfile">string</filename></entry>
241 <entry><classname>__gnu_debug::string</classname></entry>
242 <entry><filename class="headerfile"><debug/string></filename></entry>
245 <entry><classname>std::wstring</classname></entry>
246 <entry><filename class="headerfile">string</filename></entry>
247 <entry><classname>__gnu_debug::wstring</classname></entry>
248 <entry><filename class="headerfile"><debug/string></filename></entry>
251 <entry><classname>std::basic_string</classname></entry>
252 <entry><filename class="headerfile">string</filename></entry>
253 <entry><classname>__gnu_debug::basic_string</classname></entry>
254 <entry><filename class="headerfile"><debug/string></filename></entry>
257 <entry><classname>std::vector</classname></entry>
258 <entry><filename class="headerfile">vector</filename></entry>
259 <entry><classname>__gnu_debug::vector</classname></entry>
260 <entry><filename class="headerfile"><debug/vector></filename></entry>
266 <para>In addition, when compiling in C++11 mode, these additional
267 containers have additional debug capability.
271 <title>Debugging Containers C++11</title>
273 <tgroup cols="4" align="left" colsep="1" rowsep="1">
274 <colspec colname="c1"/>
275 <colspec colname="c2"/>
276 <colspec colname="c3"/>
277 <colspec colname="c4"/>
281 <entry>Container</entry>
282 <entry>Header</entry>
283 <entry>Debug container</entry>
284 <entry>Debug header</entry>
289 <entry><classname>std::unordered_map</classname></entry>
290 <entry><filename class="headerfile">unordered_map</filename></entry>
291 <entry><classname>__gnu_debug::unordered_map</classname></entry>
292 <entry><filename class="headerfile"><debug/unordered_map></filename></entry>
295 <entry><classname>std::unordered_multimap</classname></entry>
296 <entry><filename class="headerfile">unordered_map</filename></entry>
297 <entry><classname>__gnu_debug::unordered_multimap</classname></entry>
298 <entry><filename class="headerfile"><debug/unordered_map></filename></entry>
301 <entry><classname>std::unordered_set</classname></entry>
302 <entry><filename class="headerfile">unordered_set</filename></entry>
303 <entry><classname>__gnu_debug::unordered_set</classname></entry>
304 <entry><filename class="headerfile"><debug/unordered_set></filename></entry>
307 <entry><classname>std::unordered_multiset</classname></entry>
308 <entry><filename class="headerfile">unordered_set</filename></entry>
309 <entry><classname>__gnu_debug::unordered_multiset</classname></entry>
310 <entry><filename class="headerfile"><debug/unordered_set></filename></entry>
318 <section xml:id="manual.ext.debug_mode.design" xreflabel="Design"><info><title>Design</title></info>
319 <?dbhtml filename="debug_mode_design.html"?>
323 <section xml:id="debug_mode.design.goals" xreflabel="Goals"><info><title>Goals</title></info>
327 <para> The libstdc++ debug mode replaces unsafe (but efficient) standard
328 containers and iterators with semantically equivalent safe standard
329 containers and iterators to aid in debugging user programs. The
330 following goals directed the design of the libstdc++ debug mode:</para>
334 <listitem><para><emphasis>Correctness</emphasis>: the libstdc++ debug mode must not change
335 the semantics of the standard library for all cases specified in
336 the ANSI/ISO C++ standard. The essence of this constraint is that
337 any valid C++ program should behave in the same manner regardless
338 of whether it is compiled with debug mode or release mode. In
339 particular, entities that are defined in namespace std in release
340 mode should remain defined in namespace std in debug mode, so that
341 legal specializations of namespace std entities will remain
342 valid. A program that is not valid C++ (e.g., invokes undefined
343 behavior) is not required to behave similarly, although the debug
344 mode will abort with a diagnostic when it detects undefined
345 behavior.</para></listitem>
347 <listitem><para><emphasis>Performance</emphasis>: the additional of the libstdc++ debug mode
348 must not affect the performance of the library when it is compiled
349 in release mode. Performance of the libstdc++ debug mode is
350 secondary (and, in fact, will be worse than the release
351 mode).</para></listitem>
353 <listitem><para><emphasis>Usability</emphasis>: the libstdc++ debug mode should be easy to
354 use. It should be easily incorporated into the user's development
355 environment (e.g., by requiring only a single new compiler switch)
356 and should produce reasonable diagnostics when it detects a
357 problem with the user program. Usability also involves detection
358 of errors when using the debug mode incorrectly, e.g., by linking
359 a release-compiled object against a debug-compiled object if in
360 fact the resulting program will not run correctly.</para></listitem>
362 <listitem><para><emphasis>Minimize recompilation</emphasis>: While it is expected that
363 users recompile at least part of their program to use debug
364 mode, the amount of recompilation affects the
365 detect-compile-debug turnaround time. This indirectly affects the
366 usefulness of the debug mode, because debugging some applications
367 may require rebuilding a large amount of code, which may not be
368 feasible when the suspect code may be very localized. There are
369 several levels of conformance to this requirement, each with its
370 own usability and implementation characteristics. In general, the
371 higher-numbered conformance levels are more usable (i.e., require
372 less recompilation) but are more complicated to implement than
373 the lower-numbered conformance levels.
374 <orderedlist inheritnum="ignore" continuation="restarts">
375 <listitem><para><emphasis>Full recompilation</emphasis>: The user must recompile his or
376 her entire application and all C++ libraries it depends on,
377 including the C++ standard library that ships with the
378 compiler. This must be done even if only a small part of the
379 program can use debugging features.</para></listitem>
381 <listitem><para><emphasis>Full user recompilation</emphasis>: The user must recompile
382 his or her entire application and all C++ libraries it depends
383 on, but not the C++ standard library itself. This must be done
384 even if only a small part of the program can use debugging
385 features. This can be achieved given a full recompilation
386 system by compiling two versions of the standard library when
387 the compiler is installed and linking against the appropriate
388 one, e.g., a multilibs approach.</para></listitem>
390 <listitem><para><emphasis>Partial recompilation</emphasis>: The user must recompile the
391 parts of his or her application and the C++ libraries it
392 depends on that will use the debugging facilities
393 directly. This means that any code that uses the debuggable
394 standard containers would need to be recompiled, but code
395 that does not use them (but may, for instance, use IOStreams)
396 would not have to be recompiled.</para></listitem>
398 <listitem><para><emphasis>Per-use recompilation</emphasis>: The user must recompile the
399 parts of his or her application and the C++ libraries it
400 depends on where debugging should occur, and any other code
401 that interacts with those containers. This means that a set of
402 translation units that accesses a particular standard
403 container instance may either be compiled in release mode (no
404 checking) or debug mode (full checking), but must all be
405 compiled in the same way; a translation unit that does not see
406 that standard container instance need not be recompiled. This
407 also means that a translation unit <emphasis>A</emphasis> that contains a
408 particular instantiation
409 (say, <code>std::vector<int></code>) compiled in release
410 mode can be linked against a translation unit <emphasis>B</emphasis> that
411 contains the same instantiation compiled in debug mode (a
412 feature not present with partial recompilation). While this
413 behavior is technically a violation of the One Definition
414 Rule, this ability tends to be very important in
415 practice. The libstdc++ debug mode supports this level of
416 recompilation. </para></listitem>
418 <listitem><para><emphasis>Per-unit recompilation</emphasis>: The user must only
419 recompile the translation units where checking should occur,
420 regardless of where debuggable standard containers are
421 used. This has also been dubbed "<code>-g</code> mode",
422 because the <code>-g</code> compiler switch works in this way,
423 emitting debugging information at a per--translation-unit
424 granularity. We believe that this level of recompilation is in
425 fact not possible if we intend to supply safe iterators, leave
426 the program semantics unchanged, and not regress in
427 performance under release mode because we cannot associate
428 extra information with an iterator (to form a safe iterator)
429 without either reserving that space in release mode
430 (performance regression) or allocating extra memory associated
431 with each iterator with <code>new</code> (changes the program
432 semantics).</para></listitem>
438 <section xml:id="debug_mode.design.methods" xreflabel="Methods"><info><title>Methods</title></info>
442 <para>This section provides an overall view of the design of the
443 libstdc++ debug mode and details the relationship between design
444 decisions and the stated design goals.</para>
446 <section xml:id="debug_mode.design.methods.wrappers" xreflabel="Method Wrapper"><info><title>The Wrapper Model</title></info>
448 <para>The libstdc++ debug mode uses a wrapper model where the
449 debugging versions of library components (e.g., iterators and
450 containers) form a layer on top of the release versions of the
451 library components. The debugging components first verify that the
452 operation is correct (aborting with a diagnostic if an error is
453 found) and will then forward to the underlying release-mode
454 container that will perform the actual work. This design decision
455 ensures that we cannot regress release-mode performance (because the
456 release-mode containers are left untouched) and partially
457 enables <link linkend="methods.coexistence.link">mixing debug and
458 release code</link> at link time, although that will not be
459 discussed at this time.</para>
461 <para>Two types of wrappers are used in the implementation of the debug
462 mode: container wrappers and iterator wrappers. The two types of
463 wrappers interact to maintain relationships between iterators and
464 their associated containers, which are necessary to detect certain
465 types of standard library usage errors such as dereferencing
466 past-the-end iterators or inserting into a container using an
467 iterator from a different container.</para>
469 <section xml:id="debug_mode.design.methods.safe_iter" xreflabel="Method Safe Iter"><info><title>Safe Iterators</title></info>
471 <para>Iterator wrappers provide a debugging layer over any iterator that
472 is attached to a particular container, and will manage the
473 information detailing the iterator's state (singular,
474 dereferenceable, etc.) and tracking the container to which the
475 iterator is attached. Because iterators have a well-defined, common
476 interface the iterator wrapper is implemented with the iterator
477 adaptor class template <code>__gnu_debug::_Safe_iterator</code>,
478 which takes two template parameters:</para>
481 <listitem><para><code>Iterator</code>: The underlying iterator type, which must
482 be either the <code>iterator</code> or <code>const_iterator</code>
483 typedef from the sequence type this iterator can reference.</para></listitem>
485 <listitem><para><code>Sequence</code>: The type of sequence that this iterator
486 references. This sequence must be a safe sequence (discussed below)
487 whose <code>iterator</code> or <code>const_iterator</code> typedef
488 is the type of the safe iterator.</para></listitem>
492 <section xml:id="debug_mode.design.methods.safe_seq" xreflabel="Method Safe Seq"><info><title>Safe Sequences (Containers)</title></info>
495 <para>Container wrappers provide a debugging layer over a particular
496 container type. Because containers vary greatly in the member
497 functions they support and the semantics of those member functions
498 (especially in the area of iterator invalidation), container
499 wrappers are tailored to the container they reference, e.g., the
500 debugging version of <code>std::list</code> duplicates the entire
501 interface of <code>std::list</code>, adding additional semantic
502 checks and then forwarding operations to the
503 real <code>std::list</code> (a public base class of the debugging
504 version) as appropriate. However, all safe containers inherit from
505 the class template <code>__gnu_debug::_Safe_sequence</code>,
506 instantiated with the type of the safe container itself (an instance
507 of the curiously recurring template pattern).</para>
509 <para>The iterators of a container wrapper will be
510 <link linkend="debug_mode.design.methods.safe_iter">safe
511 iterators</link> that reference sequences of this type and wrap the
512 iterators provided by the release-mode base class. The debugging
513 container will use only the safe iterators within its own interface
514 (therefore requiring the user to use safe iterators, although this
515 does not change correct user code) and will communicate with the
516 release-mode base class with only the underlying, unsafe,
517 release-mode iterators that the base class exports.</para>
519 <para> The debugging version of <code>std::list</code> will have the
520 following basic structure:</para>
523 template<typename _Tp, typename _Allocator = allocator<_Tp>
525 public release-list<_Tp, _Allocator>,
526 public __gnu_debug::_Safe_sequence<debug-list<_Tp, _Allocator> >
528 typedef release-list<_Tp, _Allocator> _Base;
529 typedef debug-list<_Tp, _Allocator> _Self;
532 typedef __gnu_debug::_Safe_iterator<typename _Base::iterator, _Self> iterator;
533 typedef __gnu_debug::_Safe_iterator<typename _Base::const_iterator, _Self> const_iterator;
535 // duplicate std::list interface with debugging semantics
541 <section xml:id="debug_mode.design.methods.precond" xreflabel="Precondition check"><info><title>Precondition Checking</title></info>
543 <para>The debug mode operates primarily by checking the preconditions of
544 all standard library operations that it supports. Preconditions that
545 are always checked (regardless of whether or not we are in debug
546 mode) are checked via the <code>__check_xxx</code> macros defined
547 and documented in the source
548 file <code>include/debug/debug.h</code>. Preconditions that may or
549 may not be checked, depending on the debug-mode
550 macro <code>_GLIBCXX_DEBUG</code>, are checked via
551 the <code>__requires_xxx</code> macros defined and documented in the
552 same source file. Preconditions are validated using any additional
553 information available at run-time, e.g., the containers that are
554 associated with a particular iterator, the position of the iterator
555 within those containers, the distance between two iterators that may
556 form a valid range, etc. In the absence of suitable information,
557 e.g., an input iterator that is not a safe iterator, these
558 precondition checks will silently succeed.</para>
560 <para>The majority of precondition checks use the aforementioned macros,
561 which have the secondary benefit of having prewritten debug
562 messages that use information about the current status of the
563 objects involved (e.g., whether an iterator is singular or what
564 sequence it is attached to) along with some static information
565 (e.g., the names of the function parameters corresponding to the
566 objects involved). When not using these macros, the debug mode uses
567 either the debug-mode assertion
568 macro <code>_GLIBCXX_DEBUG_ASSERT</code> , its pedantic
569 cousin <code>_GLIBCXX_DEBUG_PEDASSERT</code>, or the assertion
570 check macro that supports more advance formulation of error
571 messages, <code>_GLIBCXX_DEBUG_VERIFY</code>. These macros are
572 documented more thoroughly in the debug mode source code.</para>
575 <section xml:id="debug_mode.design.methods.coexistence" xreflabel="Coexistence"><info><title>Release- and debug-mode coexistence</title></info>
577 <para>The libstdc++ debug mode is the first debug mode we know of that
578 is able to provide the "Per-use recompilation" (4) guarantee, that
579 allows release-compiled and debug-compiled code to be linked and
580 executed together without causing unpredictable behavior. This
581 guarantee minimizes the recompilation that users are required to
582 perform, shortening the detect-compile-debug bug hunting cycle
583 and making the debug mode easier to incorporate into development
584 environments by minimizing dependencies.</para>
586 <para>Achieving link- and run-time coexistence is not a trivial
587 implementation task. To achieve this goal we required a small
588 extension to the GNU C++ compiler (since incorporated into the C++11 language specification, described in the GCC Manual for the C++ language as
589 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/onlinedocs/gcc/Namespace-Association.html#Namespace-Association">namespace
590 association</link>), and a complex organization of debug- and
591 release-modes. The end result is that we have achieved per-use
592 recompilation but have had to give up some checking of the
593 <code>std::basic_string</code> class template (namely, safe
597 <section xml:id="methods.coexistence.compile" xreflabel="Compile"><info><title>Compile-time coexistence of release- and debug-mode components</title></info>
600 <para>Both the release-mode components and the debug-mode
601 components need to exist within a single translation unit so that
602 the debug versions can wrap the release versions. However, only one
603 of these components should be user-visible at any particular
604 time with the standard name, e.g., <code>std::list</code>. </para>
606 <para>In release mode, we define only the release-mode version of the
607 component with its standard name and do not include the debugging
608 component at all. The release mode version is defined within the
609 namespace <code>std</code>. Minus the namespace associations, this
610 method leaves the behavior of release mode completely unchanged from
611 its behavior prior to the introduction of the libstdc++ debug
612 mode. Here's an example of what this ends up looking like, in
618 template<typename _Tp, typename _Alloc = allocator<_Tp> >
626 <para>In debug mode we include the release-mode container (which is now
627 defined in the namespace <code>__cxx1998</code>) and also the
628 debug-mode container. The debug-mode container is defined within the
629 namespace <code>__debug</code>, which is associated with namespace
630 <code>std</code> via the C++11 namespace association language feature. This
631 method allows the debug and release versions of the same component to
632 coexist at compile-time and link-time without causing an unreasonable
633 maintenance burden, while minimizing confusion. Again, this boils down
634 to C++ code as follows:</para>
641 template<typename _Tp, typename _Alloc = allocator<_Tp> >
646 } // namespace __gnu_norm
650 template<typename _Tp, typename _Alloc = allocator<_Tp> >
652 : public __cxx1998::list<_Tp, _Alloc>,
653 public __gnu_debug::_Safe_sequence<list<_Tp, _Alloc> >
657 } // namespace __cxx1998
659 // namespace __debug __attribute__ ((strong));
660 inline namespace __debug { }
665 <section xml:id="methods.coexistence.link" xreflabel="Link"><info><title>Link- and run-time coexistence of release- and
666 debug-mode components</title></info>
669 <para>Because each component has a distinct and separate release and
670 debug implementation, there is no issue with link-time
671 coexistence: the separate namespaces result in different mangled
672 names, and thus unique linkage.</para>
674 <para>However, components that are defined and used within the C++
675 standard library itself face additional constraints. For instance,
676 some of the member functions of <code> std::moneypunct</code> return
677 <code>std::basic_string</code>. Normally, this is not a problem, but
678 with a mixed mode standard library that could be using either
679 debug-mode or release-mode <code> basic_string</code> objects, things
680 get more complicated. As the return value of a function is not
681 encoded into the mangled name, there is no way to specify a
682 release-mode or a debug-mode string. In practice, this results in
683 runtime errors. A simplified example of this problem is as follows.
686 <para> Take this translation unit, compiled in debug-mode: </para>
689 #include <string>
691 std::string test02();
705 <para> ... and linked to this translation unit, compiled in release mode:</para>
708 #include <string>
713 return std::string("toast");
717 <para> For this reason we cannot easily provide safe iterators for
718 the <code>std::basic_string</code> class template, as it is present
719 throughout the C++ standard library. For instance, locale facets
720 define typedefs that include <code>basic_string</code>: in a mixed
721 debug/release program, should that typedef be based on the
722 debug-mode <code>basic_string</code> or the
723 release-mode <code>basic_string</code>? While the answer could be
724 "both", and the difference hidden via renaming a la the
725 debug/release containers, we must note two things about locale
728 <orderedlist inheritnum="ignore" continuation="restarts">
729 <listitem><para>They exist as shared state: one can create a facet in one
730 translation unit and access the facet via the same type name in a
731 different translation unit. This means that we cannot have two
732 different versions of locale facets, because the types would not be
733 the same across debug/release-mode translation unit barriers.</para></listitem>
735 <listitem><para>They have virtual functions returning strings: these functions
736 mangle in the same way regardless of the mangling of their return
737 types (see above), and their precise signatures can be relied upon
738 by users because they may be overridden in derived classes.</para></listitem>
741 <para>With the design of libstdc++ debug mode, we cannot effectively hide
742 the differences between debug and release-mode strings from the
743 user. Failure to hide the differences may result in unpredictable
744 behavior, and for this reason we have opted to only
745 perform <code>basic_string</code> changes that do not require ABI
746 changes. The effect on users is expected to be minimal, as there are
747 simple alternatives (e.g., <code>__gnu_debug::basic_string</code>),
748 and the usability benefit we gain from the ability to mix debug- and
749 release-compiled translation units is enormous.</para>
752 <section xml:id="methods.coexistence.alt" xreflabel="Alternatives"><info><title>Alternatives for Coexistence</title></info>
755 <para>The coexistence scheme above was chosen over many alternatives,
756 including language-only solutions and solutions that also required
757 extensions to the C++ front end. The following is a partial list of
758 solutions, with justifications for our rejection of each.</para>
761 <listitem><para><emphasis>Completely separate debug/release libraries</emphasis>: This is by
762 far the simplest implementation option, where we do not allow any
763 coexistence of debug- and release-compiled translation units in a
764 program. This solution has an extreme negative affect on usability,
765 because it is quite likely that some libraries an application
766 depends on cannot be recompiled easily. This would not meet
767 our <emphasis>usability</emphasis> or <emphasis>minimize recompilation</emphasis> criteria
768 well.</para></listitem>
770 <listitem><para><emphasis>Add a <code>Debug</code> boolean template parameter</emphasis>:
771 Partial specialization could be used to select the debug
772 implementation when <code>Debug == true</code>, and the state
773 of <code>_GLIBCXX_DEBUG</code> could decide whether the
774 default <code>Debug</code> argument is <code>true</code>
775 or <code>false</code>. This option would break conformance with the
776 C++ standard in both debug <emphasis>and</emphasis> release modes. This would
777 not meet our <emphasis>correctness</emphasis> criteria. </para></listitem>
779 <listitem><para><emphasis>Packaging a debug flag in the allocators</emphasis>: We could
780 reuse the <code>Allocator</code> template parameter of containers
781 by adding a sentinel wrapper <code>debug<></code> that
782 signals the user's intention to use debugging, and pick up
783 the <code>debug<></code> allocator wrapper in a partial
784 specialization. However, this has two drawbacks: first, there is a
785 conformance issue because the default allocator would not be the
786 standard-specified <code>std::allocator<T></code>. Secondly
787 (and more importantly), users that specify allocators instead of
788 implicitly using the default allocator would not get debugging
789 containers. Thus this solution fails the <emphasis>correctness</emphasis>
790 criteria.</para></listitem>
792 <listitem><para><emphasis>Define debug containers in another namespace, and employ
793 a <code>using</code> declaration (or directive)</emphasis>: This is an
794 enticing option, because it would eliminate the need for
795 the <code>link_name</code> extension by aliasing the
796 templates. However, there is no true template aliasing mechanism
797 in C++, because both <code>using</code> directives and using
798 declarations disallow specialization. This method fails
799 the <emphasis>correctness</emphasis> criteria.</para></listitem>
801 <listitem><para><emphasis> Use implementation-specific properties of anonymous
802 namespaces. </emphasis>
803 See <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2003-08/msg00004.html"> this post
805 This method fails the <emphasis>correctness</emphasis> criteria.</para></listitem>
807 <listitem><para><emphasis>Extension: allow reopening on namespaces</emphasis>: This would
808 allow the debug mode to effectively alias the
809 namespace <code>std</code> to an internal namespace, such
810 as <code>__gnu_std_debug</code>, so that it is completely
811 separate from the release-mode <code>std</code> namespace. While
812 this will solve some renaming problems and ensure that
813 debug- and release-compiled code cannot be mixed unsafely, it ensures that
814 debug- and release-compiled code cannot be mixed at all. For
815 instance, the program would have two <code>std::cout</code>
816 objects! This solution would fails the <emphasis>minimize
817 recompilation</emphasis> requirement, because we would only be able to
818 support option (1) or (2).</para></listitem>
820 <listitem><para><emphasis>Extension: use link name</emphasis>: This option involves
821 complicated re-naming between debug-mode and release-mode
822 components at compile time, and then a g++ extension called <emphasis>
823 link name </emphasis> to recover the original names at link time. There
824 are two drawbacks to this approach. One, it's very verbose,
825 relying on macro renaming at compile time and several levels of
826 include ordering. Two, ODR issues remained with container member
827 functions taking no arguments in mixed-mode settings resulting in
828 equivalent link names, <code> vector::push_back() </code> being
830 See <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2003-08/msg00177.html">link
831 name</link> </para></listitem>
834 <para>Other options may exist for implementing the debug mode, many of
835 which have probably been considered and others that may still be
836 lurking. This list may be expanded over time to include other
837 options that we could have implemented, but in all cases the full
838 ramifications of the approach (as measured against the design goals
839 for a libstdc++ debug mode) should be considered first. The DejaGNU
840 testsuite includes some testcases that check for known problems with
841 some solutions (e.g., the <code>using</code> declaration solution
842 that breaks user specialization), and additional testcases will be
843 added as we are able to identify other typical problem cases. These
844 test cases will serve as a benchmark by which we can compare debug
845 mode implementations.</para>
850 <section xml:id="debug_mode.design.other" xreflabel="Other"><info><title>Other Implementations</title></info>
854 <para> There are several existing implementations of debug modes for C++
855 standard library implementations, although none of them directly
856 supports debugging for programs using libstdc++. The existing
857 implementations include:</para>
859 <listitem><para><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.cs.sjsu.edu/faculty/horstman/safestl.html">SafeSTL</link>:
860 SafeSTL was the original debugging version of the Standard Template
861 Library (STL), implemented by Cay S. Horstmann on top of the
862 Hewlett-Packard STL. Though it inspired much work in this area, it
863 has not been kept up-to-date for use with modern compilers or C++
864 standard library implementations.</para></listitem>
866 <listitem><para><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.stlport.org/">STLport</link>: STLport is a free
867 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
868 ported to many other platforms. It includes a debug mode that uses a
869 wrapper model (that in some ways inspired the libstdc++ debug mode
870 design), although at the time of this writing the debug mode is
871 somewhat incomplete and meets only the "Full user recompilation" (2)
872 recompilation guarantee by requiring the user to link against a
873 different library in debug mode vs. release mode.</para></listitem>
875 <listitem><para>Metrowerks CodeWarrior: The C++ standard library
876 that ships with Metrowerks CodeWarrior includes a debug mode. It is
877 a full debug-mode implementation (including debugging for
878 CodeWarrior extensions) and is easy to use, although it meets only
879 the "Full recompilation" (1) recompilation
880 guarantee.</para></listitem>