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<h1 class="centered"><a name="top">Design of the libstdc++ debug mode</a></h1>
<p class="fineprint"><em>
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<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&lt;int&gt;</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&lt;typename _Tp, typename _Allocator = allocator&lt;_Tp&gt;
class debug-list :
public release-list&lt;_Tp, _Allocator&gt;,
public __gnu_debug::_Safe_sequence&lt;debug-list&lt;_Tp, _Allocator&gt; &gt;
{
typedef release-list&lt;_Tp, _Allocator&gt; _Base;
typedef debug-list&lt;_Tp, _Allocator&gt; _Self;
public:
typedef __gnu_debug::_Safe_iterator&lt;typename _Base::iterator, _Self&gt; iterator;
typedef __gnu_debug::_Safe_iterator&lt;typename _Base::const_iterator, _Self&gt; 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&lt;typename _Tp, typename _Alloc = allocator&lt;_Tp&gt; &gt;
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&lt;typename _Tp, typename _Alloc = allocator&lt;_Tp&gt; &gt;
class list
{
// ...
};
} // namespace __gnu_norm
namespace __debug
{
template&lt;typename _Tp, typename _Alloc = allocator&lt;_Tp&gt; &gt;
class list
: public __norm::list&lt;_Tp, _Alloc&gt;,
public __gnu_debug::_Safe_sequence&lt;list&lt;_Tp, _Alloc&gt; &gt;
{
// ...
};
} // 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 &lt;string&gt;
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 &lt;string&gt;
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&lt;&gt;</code> that
signals the user's intention to use debugging, and pick up
the <code>debug&lt;&gt;</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&lt;T&gt;</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>
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See <a href="17_intro/license.html">license.html</a> for copying conditions.
Comments and suggestions are welcome, and may be sent to
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