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      C++
    
    
      library
    
    
      debug
    
  

 
Debug Mode
 

  Intro
  
    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.
  
 
  
    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:
  
 
  
    Safe iterators: 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.
 
    Algorithm preconditions: Algorithms attempt to
    validate their input parameters to detect errors as early as
    possible. For instance, the set_intersection
    algorithm requires that its iterator
    parameters first1 and last1 form a valid
    iterator range, and that the sequence
    [first1, last1) is sorted according to
    the same predicate that was passed
    to set_intersection; the libstdc++ debug mode will
    detect an error if the sequence is not sorted or was sorted by a
    different predicate.
  
 

 

  Semantics
  
  
 
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 std::list 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.
 
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 std::basic_string 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 -pedantic compiler flag) that attempts to emulate
  the semantics guaranteed by the C++ standard. For
  instance, constructing a std::basic_string 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 -D_GLIBCXX_DEBUG
  and -D_GLIBCXX_DEBUG_PEDANTIC .
  (N.B. In GCC 3.4.x and 4.0.0, due to a bug,
  -D_GLIBXX_DEBUG_PEDANTIC was also needed. The problem has
  been fixed in GCC 4.0.1 and later versions.) 
 
The following library components provide extra debugging
  capabilities in debug mode:

  std::basic_string (no safe iterators and see note below)
  std::bitset
  std::deque
  std::list
  std::map
  std::multimap
  std::multiset
  std::set
  std::vector
  std::unordered_map
  std::unordered_multimap
  std::unordered_set
  std::unordered_multiset

 
N.B. although there are precondition checks for some string operations,
e.g.  operator[],
they will not always be run when using the char and
wchar_t specialisations (std::string and
std::wstring).  This is because libstdc++ uses GCC's
extern template extension to provide explicit instantiations
of std::string and std::wstring, 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 -O1 might be enough to enable them.  Alternatively
-D_GLIBCXX_EXTERN_TEMPLATE=0 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
__gnu_debug::basic_string debugging container directly,
which always works correctly.

 

 

  Using
  
  

  Using the Debug Mode
 
To use the libstdc++ debug mode, compile your application with the
  compiler flag -D_GLIBCXX_DEBUG. Note that this flag
  changes the sizes and behavior of standard class templates such
  as std::vector, 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.
 
By default, error messages are formatted to fit on lines of about
  78 characters.  The environment variable
  GLIBCXX_DEBUG_MESSAGE_LENGTH can be used to request a
  different length.
 

 

  Using a Specific Debug Container
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:

 

 
In addition, when compiling in C++0x mode, these additional
containers have additional debug capability.

 



 

  Design
  
  
  
    Goals
    
    
 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:
 
  
 
    Correctness: 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.
 
    Performance: 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).
 
    Usability: 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.
 
    Minimize recompilation: 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.
      
        Full recompilation: 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.
 
        Full user recompilation: 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.
 
        Partial recompilation: 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.
 
        Per-use recompilation: 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 A that contains a
        particular instantiation
        (say, std::vector<int>) compiled in release
        mode can be linked against a translation unit B 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. 
 
        Per-unit recompilation: 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 "-g mode",
        because the -g 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 new (changes the program
        semantics).
      
    
  
  
 
  
    Methods
    
    
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.
 
  
    The Wrapper Model
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 mixing debug and
  release code at link time, although that will not be
  discussed at this time.
 
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.
 
  
    Safe Iterators
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 __gnu_debug::_Safe_iterator,
  which takes two template parameters:
 

  Iterator: The underlying iterator type, which must
    be either the iterator or const_iterator
    typedef from the sequence type this iterator can reference.
 
  Sequence: The type of sequence that this iterator
  references. This sequence must be a safe sequence (discussed below)
  whose iterator or const_iterator typedef
  is the type of the safe iterator.

  
 
  
    Safe Sequences (Containers)
 
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 std::list duplicates the entire
  interface of std::list, adding additional semantic
  checks and then forwarding operations to the
  real std::list (a public base class of the debugging
  version) as appropriate. However, all safe containers inherit from
  the class template __gnu_debug::_Safe_sequence,
  instantiated with the type of the safe container itself (an instance
  of the curiously recurring template pattern).
 
The iterators of a container wrapper will be
  safe
  iterators 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.
 
 The debugging version of std::list will have the
  following basic structure:
 

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
  };

  
  
 
  
    Precondition Checking
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 __check_xxx macros defined
  and documented in the source
  file include/debug/debug.h. Preconditions that may or
  may not be checked, depending on the debug-mode
  macro _GLIBCXX_DEBUG, are checked via
  the __requires_xxx 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.
 
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 _GLIBCXX_DEBUG_ASSERT , its pedantic
  cousin _GLIBCXX_DEBUG_PEDASSERT, or the assertion
  check macro that supports more advance formulation of error
  messages, _GLIBCXX_DEBUG_VERIFY. These macros are
  documented more thoroughly in the debug mode source code.
  
 
  
    Release- and debug-mode coexistence
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.
 
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
  namespace
  association), 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
  std::basic_string class template (namely, safe
  iterators).

 
 
   Compile-time coexistence of release- and debug-mode components
 
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., std::list. 
 
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 std. 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++.
 

namespace std
{
  template<typename _Tp, typename _Alloc = allocator<_Tp> >
    class list
    {
      // ...
     };
} // namespace std

 
In debug mode we include the release-mode container (which is now
defined in the namespace __norm) and also the
debug-mode container. The debug-mode container is defined within the
namespace __debug, which is associated with namespace
std 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:
 

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
 
  // namespace __debug __attribute__ ((strong));
  inline namespace __debug { }
}

 
 
 
   Link- and run-time coexistence of release- and</code></pre></td>
      </tr>
      <tr valign="middle">
         <td>672</td>
         <td></td>
         <td></td>
         <td class="code"><pre><code>    debug-mode components
 
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.
 
However, components that are defined and used within the C++
standard library itself face additional constraints. For instance,
some of the member functions of  std::moneypunct return
std::basic_string. Normally, this is not a problem, but
with a mixed mode standard library that could be using either
debug-mode or release-mode  basic_string 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.

 
 Take this translation unit, compiled in debug-mode: 

// -D_GLIBCXX_DEBUG
#include <string>
 
std::string test02();
 
std::string test01()
{
  return test02();
}
 
int main()
{
  test01();
  return 0;
}

 
 ... and linked to this translation unit, compiled in release mode:
 

#include <string>
 
std::string
test02()
{
  return std::string("toast");
}

 
 For this reason we cannot easily provide safe iterators for
  the std::basic_string class template, as it is present
  throughout the C++ standard library. For instance, locale facets
  define typedefs that include basic_string: in a mixed
  debug/release program, should that typedef be based on the
  debug-mode basic_string or the
  release-mode basic_string? 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:
 

  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.
 
  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.

 
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 basic_string changes that do not require ABI
  changes. The effect on users is expected to be minimal, as there are
  simple alternatives (e.g., __gnu_debug::basic_string),
  and the usability benefit we gain from the ability to mix debug- and
  release-compiled translation units is enormous.
 
 
 
Alternatives for Coexistence
 
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.
 

  Completely separate debug/release libraries: 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 usability or minimize recompilation criteria
  well.
 
  Add a Debug boolean template parameter:
  Partial specialization could be used to select the debug
  implementation when Debug == true, and the state
  of _GLIBCXX_DEBUG could decide whether the
  default Debug argument is true
  or false. This option would break conformance with the
  C++ standard in both debug and release modes. This would
  not meet our correctness criteria. 
 
  Packaging a debug flag in the allocators: We could
    reuse the Allocator template parameter of containers
    by adding a sentinel wrapper debug<> that
    signals the user's intention to use debugging, and pick up
    the debug<> 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 std::allocator<T>. 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 correctness
    criteria.
 
  Define debug containers in another namespace, and employ
      a using declaration (or directive): This is an
      enticing option, because it would eliminate the need for
      the link_name extension by aliasing the
      templates. However, there is no true template aliasing mechanism
      in C++, because both using directives and using
      declarations disallow specialization. This method fails
      the correctness criteria.
 
   Use implementation-specific properties of anonymous
    namespaces. 
    See  this post
    
    This method fails the correctness criteria.
 
  Extension: allow reopening on namespaces: This would
    allow the debug mode to effectively alias the
    namespace std to an internal namespace, such
    as __gnu_std_debug, so that it is completely
    separate from the release-mode std 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 std::cout
    objects! This solution would fails the minimize
    recompilation requirement, because we would only be able to
    support option (1) or (2).
 
  Extension: use link name: This option involves
    complicated re-naming between debug-mode and release-mode
    components at compile time, and then a g++ extension called 
    link name  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,  vector::push_back()  being
    one example.
    See link
    name 

 
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 using 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.
 
  
  
 
  
    Other Implementations
    
    
 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:

  SafeSTL:
  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.
 
  STLport: STLport is a free
  implementation of the C++ standard library derived from the SGI implementation, 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.
 
  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.

 
  

 

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