C++
debug

# 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.

## Using `g++`

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

## Debug Versions of Library Binary Files

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 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 debug mode.

## 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 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

## Using `gdb`

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.

## Debug Mode

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

## Compile Time Checking

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