475 lines
18 KiB
HTML
475 lines
18 KiB
HTML
<?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="bkoz@gcc.gnu.org (Benjamin Kosnik)" />
|
|
<meta name="KEYWORDS" content="c++, libstdc++, gdb, g++, debug" />
|
|
<meta name="DESCRIPTION" content="Debugging C++ binaries" />
|
|
<meta name="GENERATOR" content="vi and ten fingers" />
|
|
<title>Debugging schemes and strategies</title>
|
|
<link rel="StyleSheet" href="lib3styles.css" type="text/css" />
|
|
<link rel="Copyright" href="17_intro/license.html" type="text/html" />
|
|
</head>
|
|
<body>
|
|
|
|
<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>.
|
|
</em></p>
|
|
|
|
<!-- ####################################################### -->
|
|
<hr />
|
|
<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>
|
|
|
|
|
|
<!-- ####################################################### -->
|
|
|
|
<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>
|