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diff --git a/libstdc++-v3/doc/html/ext/debug_mode.html b/libstdc++-v3/doc/html/ext/debug_mode.html new file mode 100644 index 00000000000..7e9d9bafae9 --- /dev/null +++ b/libstdc++-v3/doc/html/ext/debug_mode.html @@ -0,0 +1,578 @@ +<?xml version="1.0" encoding="ISO-8859-1"?> +<!DOCTYPE html + PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" + "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> + +<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> +<head> + <meta name="AUTHOR" content="gregod@cs.rpi.edu (Doug Gregor)" /> + <meta name="KEYWORDS" content="C++, GCC, libstdc++, g++, debug" /> + <meta name="DESCRIPTION" content="Design of the libstdc++ debug mode." /> + <meta name="GENERATOR" content="vi and eight fingers" /> + <title>Design of the libstdc++ debug mode</title> +<link rel="StyleSheet" href="lib3styles.css" /> +</head> +<body> + +<h1 class="centered"><a name="top">Design of the libstdc++ debug mode</a></h1> + +<p class="fineprint"><em> + The latest version of this document is always available at + <a href="http://gcc.gnu.org/onlinedocs/libstdc++/debug_mode.html"> + http://gcc.gnu.org/onlinedocs/libstdc++/debug_mode.html</a>. +</em></p> + +<p><em> + To the <a href="http://gcc.gnu.org/libstdc++/">libstdc++ homepage</a>. +</em></p> + + +<!-- ####################################################### --> + +<hr /> +<h1>Debug mode design</h1> +<p> 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:</p> + + <ul> + + <li><b>Correctness</b>: 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.</li> + + <li><b>Performance</b>: 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).</li> + + <li><b>Usability</b>: 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.</li> + + <li><b>Minimize recompilation</b>: 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. + <ol> + <li><b>Full recompilation</b>: 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.</li> + + <li><b>Full user recompilation</b>: 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.</li> + + <li><b>Partial recompilation</b>: 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.</li> + + <li><b>Per-use recompilation</b>: 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 <em>A</em> that contains a + particular instantiation + (say, <code>std::vector<int></code>) compiled in release + mode can be linked against a translation unit <em>B</em> 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. </li> + + <li><b>Per-unit recompilation</b>: 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).</li> + </ol> + </li> + </ul> + +<h2><a name="other">Other implementations</a></h2> +<p> 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:</p> +<ul> + <li><a + href="http://www.mathcs.sjsu.edu/faculty/horstman/safestl.html">SafeSTL</a>: + 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.</li> + + <li><a href="http://www.stlport.org/">STLport</a>: STLport is a free + implementation of the C++ standard library derived from the <a + href="http://www.sgi.com/tech/stl/">SGI implementation</a>, and + ported to many other platforms. It includes a debug mode that uses a + wrapper model (that in some way 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.</li> + + <li><a href="http://www.metrowerks.com/mw/default.htm">Metrowerks + CodeWarrior</a>: 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.</li> +</ul> + +<h2><a name="design">Debug mode design methodology</a></h2> +<p>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.</p> + +<h3><a name="wrappers">The wrapper model</a></h3> +<p>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 <a + href="#mixing">mixing debug and release code</a> at link time, + although that will not be discussed at this time.</p> + +<p>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.</p> + +<h4><a name="safe_iterator">Safe iterators</a></h4> +<p>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:</p> + +<ul> + <li><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.</li> + + <li><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.</li> +</ul> + +<h4><a name="safe_sequence">Safe sequences (containers)</a></h4> +<p>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).</p> + +<p>The iterators of a container wrapper will be + <a href="#safe_iterator">safe iterators</a> 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.</p> + +<p> The debugging version of <code>std::list</code> will have the + following basic structure:</p> + +<pre> +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 + }; +</pre> + +<h3><a name="precondition">Precondition checking</a></h3> +<p>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.</p> + +<p>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.</p> + +<h3><a name="coexistence">Release- and debug-mode coexistence</a></h3> +<p>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 bughunting cycle + and making the debug mode easier to incorporate into development + environments by minimizing dependencies.</p> + +<p>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 (described in the GCC Manual for + C++ Extensions, see <a + href="http://gcc.gnu.org/onlinedocs/gcc/Strong-Using.html">strong + using</a>), 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). +</p> + +<h4><a name="compile_coexistence">Compile-time coexistence of release- and + debug-mode components</a></h4> +<p>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>. </p> + +<p>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++.</p> + +<pre> +namespace std +{ + template<typename _Tp, typename _Alloc = allocator<_Tp> > + class list + { + // ... + }; +} // namespace std +</pre> + +<p>In debug mode we include the release-mode container (which is now +defined in in the namespace <code>__norm</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 GNU namespace association extension. 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:</p> + +<pre> +namespace std +{ + namespace __norm + { + template<typename _Tp, typename _Alloc = allocator<_Tp> > + class list + { + // ... + }; + } // namespace __gnu_norm + + namespace __debug + { + template<typename _Tp, typename _Alloc = allocator<_Tp> > + class list + : public __norm::list<_Tp, _Alloc>, + public __gnu_debug::_Safe_sequence<list<_Tp, _Alloc> > + { + // ... + }; + } // namespace __norm + + using namespace __debug __attribute__ ((strong)); +} +</pre> + +<h4><a name="mixing">Link- and run-time coexistence of release- and + debug-mode components</a></h4> + +<p>Because each component has a distinct and separate release and +debug implementation, there are are no issues with link-time +coexistence: the separate namespaces result in different mangled +names, and thus unique linkage.</p> + +<p>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. +</p> + +<p> Take this translation unit, compiled in debug-mode: </p> +<pre> +// -D_GLIBCXX_DEBUG +#include <string> + +std::string test02(); + +std::string test01() +{ + return test02(); +} + +int main() +{ + test01(); + return 0; +} +</pre> + +<p> ... and linked to this translation unit, compiled in release mode:</p> + +<pre> +#include <string> + +std::string +test02() +{ + return std::string("toast"); +} +</pre> + +<p> 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:</p> + +<ol> + <li>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.</li> + + <li>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.</li> +</ol> + +<p>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.</p> + +<h4><a name="coexistence_alt">Alternatives for Coexistence</a></h4> +<p>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.</p> + +<ul> + <li><em>Completely separate debug/release libraries</em>: 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 <b>usability</b> or <b>minimize recompilation</b> criteria + well.</li> + + <li><em>Add a <code>Debug</code> boolean template parameter</em>: + 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 <em>and</em> release modes. This would + not meet our <b>correctness</b> criteria. </li> + + <li><em>Packaging a debug flag in the allocators</em>: 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 <b>correctness</b> + criteria.</li> + + <li><em>Define debug containers in another namespace, and employ + a <code>using</code> declaration (or directive)</em>: 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 + is C++, because both <code>using</code> directives and using + declarations disallow specialization. This method fails + the <b>correctness</b> criteria.</li> + + <li><em> Use implementation-specific properties of anonymous + namespaces. </em> + See <a + href="http://gcc.gnu.org/ml/libstdc++/2003-08/msg00004.html"> this post + </a> + This method fails the <b>correctness</b> criteria.</li> + + <li><em>Extension: allow reopening on namespaces</em>: 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 <b>minimize + recompilation</b> requirement, because we would only be able to + support option (1) or (2).</li> + + <li><em>Extension: use link name</em>: This option involves + complicated re-naming between debug-mode and release-mode + components at compile time, and then a g++ extension called <em> + link name </em> 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 <a + href="http://gcc.gnu.org/ml/libstdc++/2003-08/msg00177.html">link + name</a> </li> +</ul> + +<p>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.</p> + +<!-- ####################################################### --> + +<hr /> +<p class="fineprint"><em> +See <a href="17_intro/license.html">license.html</a> for copying conditions. +Comments and suggestions are welcome, and may be sent to +<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>. +</em></p> + + +</body> +</html> |
