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<meta name="AUTHOR" content="bkoz@gcc.gnu.org (Benjamin Kosnik)" />
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<h1 class="centered"><a name="top">Debugging schemes and strategies</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.html">
http://gcc.gnu.org/onlinedocs/libstdc++/debug.html</a>.
</em></p>
<p><em>
To the <a href="http://gcc.gnu.org/libstdc++/">libstdc++ homepage</a>.
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<p>There are numerous things that can be done to improve the ease with
which C++ binaries are debugged when using the GNU
tool chain. Here are some of them.
</p>
<h3 class="left"><a name="gplusplus">Compiler flags determine debug info</a></h3>
<p>The default optimizations and debug flags for a libstdc++ build are
<code>-g -O2</code>. However, both debug and optimization flags can
be varied to change debugging characteristics. For instance,
turning off all optimization via the <code>-g -O0</code> flag will
disable inlining, so that stepping through all functions, including
inlined constructors and destructors, is possible. In addition,
<code>-fno-eliminate-unused-debug-types</code> can be used when
additional debug information, such as nested class info, is desired.
</p>
<p>Or, the debug format that the compiler and debugger use to communicate
information about source constructs can be changed via <code>
-gdwarf-2 </code> or <code> -gstabs </code> flags: some debugging
formats permit more expressive type and scope information to be
shown in gdb. The default debug information for a particular
platform can be identified via the value set by the
PREFERRED_DEBUGGING_TYPE macro in the gcc sources.
</p>
<p>Many other options are available: please see
<a href="http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging%20Options">"Options for Debugging Your Program"</a>
in Using the GNU Compiler Collection (GCC) for a complete list.
</p>
<h3 class="left"><a name="lib">Using special flags to make a debug binary</a></h3>
<p>If you would like debug symbols in libstdc++, there are two ways to
build libstdc++ with debug flags. The first is to run make from the
toplevel in a freshly-configured tree with
</p>
<pre>
--enable-libstdcxx-debug
</pre>
<p>and perhaps</p>
<pre>
--enable-libstdcxx-debug-flags='...'
</pre>
<p>to create a separate debug build. Both the normal build and the
debug build will persist, without having to specify
<code>CXXFLAGS</code>, and the debug library will be installed in a
separate directory tree, in <code>(prefix)/lib/debug</code>. For
more information, look at the <a href="configopts.html">configuration
options</a> document.
</p>
<p>A second approach is to use the configuration flags
</p>
<pre>
make CXXFLAGS='-g3 -O0' all
</pre>
<p>This quick and dirty approach is often sufficient for quick
debugging tasks, when you cannot or don't want to recompile your
application to use the <a href="#safe">debug mode</a>.</p>
<h3 class="left"><a name="safe">The libstdc++ debug mode</a></h3>
<p>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. </p>
<p>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:</p>
<ul>
<li><em>Safe iterators</em>: 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.</li>
<li><em>Algorithm preconditions</em>: Algorithms attempt to
validate their input parameters to detect errors as early as
possible. For instance, the <code>set_intersection</code>
algorithm requires that its iterator
parameters <code>first1</code> and <code>last1</code> form a valid
iterator range, and that the sequence
[<code>first1</code>, <code>last1</code>) is sorted according to
the same predicate that was passed
to <code>set_intersection</code>; the libstdc++ debug mode will
detect an error if the sequence is not sorted or was sorted by a
different predicate.</li>
</ul>
<h4 class="left">Using the libstdc++ debug mode</h4>
<p>To use the libstdc++ debug mode, compile your application with the
compiler flag <code>-D_GLIBCXX_DEBUG</code>. Note that this flag
changes the sizes and behavior of standard class templates such
as <code>std::vector</code>, and therefore you can only link code
compiled with debug mode and code compiled without debug mode if no
instantiation of a container is passed between the two translation
units.</p>
<p>By default, error messages are formatted to fit on lines of about
78 characters. The environment variable
<code>GLIBCXX_DEBUG_MESSAGE_LENGTH</code> can be used to request a
different length.</p>
<p>For information about the design of the libstdc++ debug mode,
please see the <a href="debug_mode.html">libstdc++ debug mode design
document</a>.</p>
<h4 class="left">Using the debugging containers without debug
mode</h4>
<p>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:
</p>
<table title="Debugging containers" border="1">
<tr>
<th>Container</th>
<th>Header</th>
<th>Debug container</th>
<th>Debug header</th>
</tr>
<tr>
<td>std::bitset</td>
<td><bitset></td>
<td>__gnu_debug::bitset</td>
<td><debug/bitset></td>
</tr>
<tr>
<td>std::deque</td>
<td><deque></td>
<td>__gnu_debug::deque</td>
<td><debug/deque></td>
</tr>
<tr>
<td>std::list</td>
<td><list></td>
<td>__gnu_debug::list</td>
<td><debug/list></td>
</tr>
<tr>
<td>std::map</td>
<td><map></td>
<td>__gnu_debug::map</td>
<td><debug/map></td>
</tr>
<tr>
<td>std::multimap</td>
<td><map></td>
<td>__gnu_debug::multimap</td>
<td><debug/map></td>
</tr>
<tr>
<td>std::multiset</td>
<td><set></td>
<td>__gnu_debug::multiset</td>
<td><debug/set></td>
</tr>
<tr>
<td>std::set</td>
<td><set></td>
<td>__gnu_debug::set</td>
<td><debug/set></td>
</tr>
<tr>
<td>std::string</td>
<td><string></td>
<td>__gnu_debug::string</td>
<td><debug/string></td>
</tr>
<tr>
<td>std::wstring</td>
<td><string></td>
<td>__gnu_debug::wstring</td>
<td><debug/string></td>
</tr>
<tr>
<td>std::basic_string</td>
<td><string></td>
<td>__gnu_debug::basic_string</td>
<td><debug/string></td>
</tr>
<tr>
<td>std::vector</td>
<td><vector></td>
<td>__gnu_debug::vector</td>
<td><debug/vector></td>
</tr>
</table>
<p>In addition, when compiling in C++0x mode, these additional
containers have additional debug capability.
</p>
<table>
<tr>
<td>std::unordered_map</td>
<td><unordered_map></td>
<td>__gnu_debug::unordered_map</td>
<td><debug/unordered_map></td>
</tr>
<tr>
<td>std::unordered_multimap</td>
<td><unordered_map></td>
<td>__gnu_debug::unordered_multimap</td>
<td><debug/unordered_map></td>
</tr>
<tr>
<td>std::unordered_set</td>
<td><unordered_set></td>
<td>__gnu_debug::unordered_set</td>
<td><debug/unordered_set></td>
</tr>
<tr>
<td>std::unordered_multiset</td>
<td><unordered_set></td>
<td>__gnu_debug::unordered_multiset</td>
<td><debug/unordered_set></td>
</tr>
</table>
<h4 class="left">Debug mode semantics</h4>
<p>A program that uses the C++ standard library correctly
will maintain the same semantics under debug mode as it had with
the normal (release) library. All functional and exception-handling
guarantees made by the normal library also hold for the debug mode
library, with one exception: performance guarantees made by the
normal library may not hold in the debug mode library. For
instance, erasing an element in a <code>std::list</code> is a
constant-time operation in normal library, but in debug mode it is
linear in the number of iterators that reference that particular
list. So while your (correct) program won't change its results, it
is likely to execute more slowly.</p>
<p>libstdc++ includes many extensions to the C++ standard library. In
some cases the extensions are obvious, such as the hashed
associative containers, whereas other extensions give predictable
results to behavior that would otherwise be undefined, such as
throwing an exception when a <code>std::basic_string</code> is
constructed from a NULL character pointer. This latter category also
includes implementation-defined and unspecified semantics, such as
the growth rate of a vector. Use of these extensions is not
considered incorrect, so code that relies on them will not be
rejected by debug mode. However, use of these extensions may affect
the portability of code to other implementations of the C++ standard
library, and is therefore somewhat hazardous. For this reason, the
libstdc++ debug mode offers a "pedantic" mode (similar to
GCC's <code>-pedantic</code> compiler flag) that attempts to emulate
the semantics guaranteed by the C++ standard. For
instance, constructing a <code>std::basic_string</code> with a NULL
character pointer would result in an exception under normal mode or
non-pedantic debug mode (this is a libstdc++ extension), whereas
under pedantic debug mode libstdc++ would signal an error. To enable
the pedantic debug mode, compile your program with
both <code>-D_GLIBCXX_DEBUG</code>
and <code>-D_GLIBCXX_DEBUG_PEDANTIC</code> .
(N.B. In GCC 3.4.x and 4.0.0, due to a bug,
<code>-D_GLIBXX_DEBUG_PEDANTIC</code> was also needed. The problem has
been fixed in GCC 4.0.1 and later versions.) </p>
<p>The following library components provide extra debugging
capabilities in debug mode:</p>
<ul>
<li><code>std::basic_string</code> (no safe iterators and see note below)</li>
<li><code>std::bitset</code></li>
<li><code>std::deque</code></li>
<li><code>std::list</code></li>
<li><code>std::map</code></li>
<li><code>std::multimap</code></li>
<li><code>std::multiset</code></li>
<li><code>std::set</code></li>
<li><code>std::vector</code></li>
<li><code>std::unordered_map</code></li>
<li><code>std::unordered_multimap</code></li>
<li><code>std::unordered_set</code></li>
<li><code>std::unordered_multiset</code></li>
</ul>
<p>N.B. although there are precondition checks for some string operations,
e.g. <code>operator[]</code>,
they will not always be run when using the <code>char</code> and
<code>wchar_t</code> specialisations (<code>std::string</code> and
<code>std::wstring</code>). This is because libstdc++ uses GCC's
<code>extern template</code> extension to provide explicit instantiations
of <code>std::string</code> and <code>std::wstring</code>, and those
explicit instantiations don't include the debug-mode checks. If the
containing functions are inlined then the checks will run, so compiling
with <code>-O1</code> might be enough to enable them. Alternatively
<code>-D_GLIBCXX_EXTERN_TEMPLATE=0</code> will suppress the declarations
of the explicit instantiations and cause the functions to be instantiated
with the debug-mode checks included, but this is unsupported and not
guaranteed to work. For full debug-mode support you can use the
<code>__gnu_debug::basic_string</code> debugging container directly,
which always works correctly.
</p>
<h3 class="left"><a name="mem">Tips for memory leak hunting</a></h3>
<p>There are various third party memory tracing and debug utilities
that can be used to provide detailed memory allocation information
about C++ code. An exhaustive list of tools is not going to be
attempted, but includes <code>mtrace</code>, <code>valgrind</code>,
<code>mudflap</code>, and the non-free commercial product
<code>purify</code>. In addition, <code>libcwd</code> has a
replacement for the global new and delete operators that can track
memory allocation and deallocation and provide useful memory
statistics.
</p>
<p>Regardless of the memory debugging tool being used, there is one
thing of great importance to keep in mind when debugging C++ code
that uses <code>new</code> and <code>delete</code>:
there are different kinds of allocation schemes that can be used by
<code> std::allocator </code>. For implementation details, see the
<a href="ext/mt_allocator.html">mt allocator</a> documentation and
look specifically for <code>GLIBCXX_FORCE_NEW</code>.
</p>
<p>In a nutshell, the default allocator used by <code>
std::allocator</code> is a high-performance pool allocator, and can
give the mistaken impression that in a suspect executable, memory
is being leaked, when in reality the memory "leak" is a pool being
used by the library's allocator and is reclaimed after program
termination.
</p>
<p>For valgrind, there are some specific items to keep in mind. First
of all, use a version of valgrind that will work with current GNU
C++ tools: the first that can do this is valgrind 1.0.4, but later
versions should work at least as well. Second of all, use a
completely unoptimized build to avoid confusing valgrind. Third,
use GLIBCXX_FORCE_NEW to keep extraneous pool allocation noise from
cluttering debug information.
</p>
<p>Fourth, it may be necessary to force deallocation in other
libraries as well, namely the "C" library. On linux, this can be
accomplished with the appropriate use of the
<code>__cxa_atexit</code> or <code>atexit</code> functions.
</p>
<pre>
#include <cstdlib>
extern "C" void __libc_freeres(void);
void do_something() { }
int main()
{
atexit(__libc_freeres);
do_something();
return 0;
}
</pre>
<p>or, using <code>__cxa_atexit</code>:</p>
<pre>
extern "C" void __libc_freeres(void);
extern "C" int __cxa_atexit(void (*func) (void *), void *arg, void *d);
void do_something() { }
int main()
{
extern void* __dso_handle __attribute__ ((__weak__));
__cxa_atexit((void (*) (void *)) __libc_freeres, NULL,
&__dso_handle ? __dso_handle : NULL);
do_test();
return 0;
}
</pre>
<p>Suggested valgrind flags, given the suggestions above about setting
up the runtime environment, library, and test file, might be:
</p>
<pre>
valgrind -v --num-callers=20 --leak-check=yes --leak-resolution=high --show-reachable=yes a.out
</pre>
<h3 class="left"><a name="gdb">Some gdb strategies</a></h3>
<p>Many options are available for gdb itself: please see <a
href="http://sources.redhat.com/gdb/current/onlinedocs/gdb_13.html#SEC109">
"GDB features for C++" </a> in the gdb documentation. Also
recommended: the other parts of this manual.
</p>
<p>These settings can either be switched on in at the gdb command
line, or put into a .gdbint file to establish default debugging
characteristics, like so:
</p>
<pre>
set print pretty on
set print object on
set print static-members on
set print vtbl on
set print demangle on
set demangle-style gnu-v3
</pre>
<h3 class="left"><a name="verbterm">Tracking uncaught exceptions</a></h3>
<p>The <a href="18_support/howto.html#4">verbose termination handler</a>
gives information about uncaught exceptions which are killing the
program. It is described in the linked-to page.
</p>
<p>Return <a href="#top">to the top of the page</a> or
<a href="http://gcc.gnu.org/libstdc++/">to the libstdc++ homepage</a>.
</p>
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<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>.
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