Debugging schemes and strategies
The latest version of this document is always available at
http://gcc.gnu.org/onlinedocs/libstdc++/debug.html
http://gcc.gnu.org/onlinedocs/libstdc++/debug.html
.
To the
http://gcc.gnu.org/libstdc++/
libstdc++-v3 homepage
.
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.
Compiler flags determine debug info
The default optimizations and debug flags for a libstdc++ build are
-g -O2
. However, both debug and optimization flags can
be varied to change debugging characteristics. For instance,
turning off all optimization via the
-g -O0
flag will
disable inlining, so that stepping through all functions, including
inlined constructors and destructors, is possible. In addition,
-fno-eliminate-unused-debug-types
can be used when
additional debug information, such as nested class info, is desired.
Or, the debug format that the compiler and debugger use to communicate
information about source constructs can be changed via
-gdwarf-2
or
-gstabs
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.
Many other options are available: please see
http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging%20Options
"Options for Debugging Your Program"
in Using the GNU Compiler Collection (GCC) for a complete list.
Using special flags to make a debug binary
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
--enable-libstdcxx-debug
and perhaps
--enable-libstdcxx-debug-flags='...'
to create a separate debug build. Both the normal build and the
debug build will persist, without having to specify
CXXFLAGS
, and the debug library will be installed in a
separate directory tree, in
(prefix)/lib/debug
. For
more information, look at the
configopts.html
configuration
options
document.
A second approach is to use the configuration flags
make CXXFLAGS='-g3 -O0' all
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
#safe
debug mode
.
The libstdc++ debug mode
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.
Using the libstdc++ 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.
For information about the design of the libstdc++ debug mode,
please see the
debug_mode.html
libstdc++ debug mode design
document
.
Using the debugging containers without debug
mode
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:
Container
Header
Debug container
Debug header
std::bitset
<bitset>
__gnu_debug::bitset
<debug/bitset>
std::deque
<deque>
__gnu_debug::deque
<debug/deque>
std::list
<list>
__gnu_debug::list
<debug/list>
std::map
<map>
__gnu_debug::map
<debug/map>
std::multimap
<map>
__gnu_debug::multimap
<debug/map>
std::multiset
<set>
__gnu_debug::multiset
<debug/set>
std::set
<set>
__gnu_debug::set
<debug/set>
std::string
<string>
__gnu_debug::string
<debug/string>
std::wstring
<string>
__gnu_debug::wstring
<debug/string>
std::basic_string
<string>
__gnu_debug::basic_string
<debug/string>
std::vector
<vector>
__gnu_debug::vector
<debug/vector>
__gnu_cxx::hash_map
<ext/hash_map>
__gnu_debug::hash_map
<debug/hash_map>
__gnu_cxx::hash_multimap
<ext/hash_map>
__gnu_debug::hash_multimap
<debug/hash_map>
__gnu_cxx::hash_set
<ext/hash_set>
__gnu_debug::hash_set
<debug/hash_set>
__gnu_cxx::hash_multiset
<ext/hash_set>
__gnu_debug::hash_multiset
<debug/hash_set>
Debug mode 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
.
The following library components provide extra debugging
capabilities in debug mode:
std::basic_string
(no safe iterators)
std::bitset
std::deque
__gnu_cxx::hash_map
__gnu_cxx::hash_multimap
__gnu_cxx::hash_multiset
__gnu_cxx::hash_set
std::list
std::map
std::multimap
std::multiset
std::set
std::vector
Tips for memory leak hunting
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
mtrace
,
valgrind
,
mudflap
, and the non-free commercial product
purify
. In addition,
libcwd
has a
replacement for the global new and delete operators that can track
memory allocation and deallocation and provide useful memory
statistics.
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
new
and
delete
:
there are different kinds of allocation schemes that can be used by
std::allocator
. For implementation details, see this
ext/howto.html#3
document
and look specifically for
GLIBCXX_FORCE_NEW
.
In a nutshell, the default allocator used by
std::allocator
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.
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.
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
__cxa_atexit
or
atexit
functions.
#include <cstdlib>
extern "C" void __libc_freeres(void);
void do_something() { }
int main()
{
atexit(__libc_freeres);
do_something();
return 0;
}
or, using
__cxa_atexit
:
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;
}
Suggested valgrind flags, given the suggestions above about setting
up the runtime environment, library, and test file, might be:
valgrind -v --num-callers=20 --leak-check=yes --leak-resolution=high --show-reachable=yes a.out
Some gdb strategies
Many options are available for gdb itself: please see
http://sources.redhat.com/gdb/current/onlinedocs/gdb_13.html#SEC109
"GDB features for C++"
in the gdb documentation. Also
recommended: the other parts of this manual.
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:
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
Tracking uncaught exceptions
The
19_diagnostics/howto.html#4
verbose termination handler
gives information about uncaught exceptions which are killing the
program.  It is described in the linked-to page.
Return
#top
to the top of the page
or
http://gcc.gnu.org/libstdc++/
to the libstdc++ homepage
.
See
17_intro/license.html
license.html
for copying conditions.
Comments and suggestions are welcome, and may be sent to
mailto:libstdc++@gcc.gnu.org
the libstdc++ mailing list
.
