# Debugging schemes and strategies

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There are numerous things that can be done to improve the ease with which C++ binaries are debugged when using the GNU C++ tool chain. Here are some things to keep in mind when debugging C++ code with GNU tools.

### 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. 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 "Options for Debugging Your Program" in Using the GNU Compiler Collection (GCC) for a complete list.

### Using special flags to make a debug binary

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 specialized debug `CXXFLAGS`, as in

         make CXXFLAGS='-g3 -O0' all

This quick and dirty approach is often sufficient for quick debugging tasks, but the lack of state can be confusing in the long term.

A second approach is to use the configuration flags

         --enable-debug

and perhaps

         --enable-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 options document.

### 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 `purify`. Also highly recommended are `libcwd` and some other one that I forget right now.

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 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 memory is being leaked, when in reality the memory is still being used by the library 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 "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 verbose termination handler gives information about uncaught exceptions which are killing the program. It is described in the linked-to page.

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