Debugging Support

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 how debug information is transmitted between compilation and debug or analysis tools.

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 -fno-inline flags will disable inlining and optimizations, and add debugging information, 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. Expressiveness can be enhanced by flags like -g3. 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 "Options for Debugging Your Program" in Using the GNU Compiler Collection (GCC) for a complete list.

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


and perhaps


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 configuration section.

A second approach is to use the configuration flags

     make CXXFLAGS='-g3 -fno-inline -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 debug mode.

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 the mt allocator documentation 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()
     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);
      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

All synchronization primitives used in the library internals need to be understood by race detectors so that they do not produce false reports.

Two annotation macros are used to explain low-level synchronization to race detectors: _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE() and _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(). By default, these macros are defined empty -- anyone who wants to use a race detector needs to redefine them to call an appropriate API. Since these macros are empty by default when the library is built, redefining them will only affect inline functions and template instantiations which are compiled in user code. This allows annotation of templates such as shared_ptr, but not code which is only instantiated in the library. In order to annotate basic_string reference counting it is necessary to disable extern templates (by defining _GLIBCXX_EXTERN_TEMPLATE=-1) or to rebuild the .so file. Annotating the remaining atomic operations (at the time of writing these are in ios_base::Init::~Init, locale::_Impl and locale::facet) requires rebuilding the .so file.

The approach described above is known to work with the following race detection tools: DRD, Helgrind, and ThreadSanitizer.

With DRD, Helgrind and ThreadSanitizer you will need to define the macros like this:


Refer to the documentation of each particular tool for details.

Many options are available for GDB itself: please see "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

Starting with version 7.0, GDB includes support for writing pretty-printers in Python. Pretty printers for STL classes are distributed with GCC from version 4.5.0. The most recent version of these printers are always found in libstdc++ svn repository. To enable these printers, check-out the latest printers to a local directory:

  svn co svn://

Next, add the following section to your ~/.gdbinit The path must match the location where the Python module above was checked-out. So if checked out to: /home/maude/gdb_printers/, the path would be as written in the example below.

  import sys
  sys.path.insert(0, '/home/maude/gdb_printers/python')
  from libstdcxx.v6.printers import register_libstdcxx_printers
  register_libstdcxx_printers (None)

The path should be the only element that needs to be adjusted in the example. Once loaded, STL classes that the printers support should print in a more human-readable format. To print the classes in the old style, use the /r (raw) switch in the print command (i.e., print /r foo). This will print the classes as if the Python pretty-printers were not loaded.

For additional information on STL support and GDB please visit: "GDB Support for STL" in the GDB wiki. Additionally, in-depth documentation and discussion of the pretty printing feature can be found in "Pretty Printing" node in the GDB manual. You can find on-line versions of the GDB user manual in GDB's homepage, at "GDB: The GNU Project Debugger" .

The verbose termination handler gives information about uncaught exceptions which are killing the program. It is described in the linked-to page.

The Debug Mode has compile and run-time checks for many containers.

The Compile-Time Checks Extension has compile-time checks for many algorithms.

The Profile-based Performance Analysis Extension has performance checks for many algorithms.