Debugging with 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. This includes uses of memory that has already been freed and other memory-smashing bugs. 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.

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 sort algorithm requires that its iterator parameters first and last form a valid iterator range, and that the predicate is a strict weak ordering; the libstdc++ debug mode will detect an error if either of these conditions can be shown not to hold.

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. This does not mean that you are required to recompile your entire application: for instance, if one source file uses std::vector but std::vector doesn't occur in its public interface, that file can be recompiled in debug mode even if the rest of the program is not compiled in debug mode.

How much should you recompile to use debug mode? Generally, you'll want to recompile your entire application to enable the debug mode's extra run-time checking globally. However, if there is a particular self-contained module that needs checking, recompile only that module. If in fact the module was not self-contained, the result will be a link error.

Using the debugging containers without debug mode

When it is not feasible to recompile your application, or only specific containers need checking, debugging containers are available as GNU extensions. These debugging containers are functionally equivalent to the containers used in debug mode, e.g., __gnu_debug::vector<int> is equivalent to std::vector<int> in debug mode, but the __gnu_debug versions can be used in either release or debug mode without changing semantics. However, unlike the containers in namespace std, these containers may not be specialized, because they are introduced into namespace std with a using declaration. Note that the std::basic_string template (of which std::string and std::wstring are instantiations) differs from the other containers in this area, because using __gnu_debug::basic_string (resp. __gnu_debug::string and __gnu_debug::wstring) is the only way to get a fully-debugging string, and is therefore not equivalent to std::basic_string in debug mode. The following table provides the names and headers of the debugging containers:

Debug mode semantics

A program that does 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. In pedantic mode, 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. Note that _GLIBCXX_DEBUG_PEDANTIC also affects the containers in the __gnu_debug namespace, so specifying _GLIBCXX_DEBUG_PEDANTIC without -D_GLIBCXX_DEBUG will make the __gnu_debug containers more pedantic but will leave the containers in namespace std unchecked.

The following library components provide extra debugging capabilities in debug mode:

• 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 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 GLIBCPP_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 GLIBCPP_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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