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

     --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 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()
   {
     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

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

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.