Debug Mode

Using the debug mode

Libc++ provides a debug mode that enables special debugging checks meant to detect incorrect usage of the standard library. These checks are disabled by default, but they can be enabled using the _LIBCPP_DEBUG macro.

Note that using the debug mode discussed in this document requires that the library has been compiled with support for the debug mode (see LIBCXX_ENABLE_DEBUG_MODE_SUPPORT).

Also note that while the debug mode has no effect on libc++’s ABI, it does have broad ODR implications. Users should compile their whole program at the same debugging level.

The various levels of checking provided by the debug mode follow.

No debugging checks (_LIBCPP_DEBUG not defined)

When _LIBCPP_DEBUG is not defined, there are no debugging checks performed by the library. This is the default.

Basic checks (_LIBCPP_DEBUG == 0)

When _LIBCPP_DEBUG is defined to 0 (to be understood as level 0), some debugging checks are enabled. The non-exhaustive list of things is:

  • Many algorithms, such as binary_search, merge, next_permutation, and sort, wrap the user-provided comparator to assert that !comp(y, x) whenever comp(x, y). This can cause the user-provided comparator to be evaluated up to twice as many times as it would be without _LIBCPP_DEBUG, and causes the library to violate some of the Standard’s complexity clauses.

  • FIXME: Update this list

Iterator debugging checks (_LIBCPP_DEBUG == 1)

Defining _LIBCPP_DEBUG to 1 enables “iterator debugging”, which provides additional assertions about the validity of iterators used by the program.

The following containers and classes support iterator debugging:

  • std::string

  • std::vector<T> (T != bool)

  • std::list

  • std::unordered_map

  • std::unordered_multimap

  • std::unordered_set

  • std::unordered_multiset

The remaining containers do not currently support iterator debugging. Patches welcome.

Randomizing Unspecified Behavior (_LIBCPP_DEBUG == 1)

This also enables the randomization of unspecified behavior, for example, for equal elements in std::sort or randomizing both parts of the partition after std::nth_element call. This effort helps you to migrate to potential future faster versions of these algorithms and deflake your tests which depend on such behavior. To fix the seed, use _LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY_SEED=seed definition.

Handling Assertion Failures

When a debug assertion fails the assertion handler is called via the std::__libcpp_debug_function function pointer. It is possible to override this function pointer using a different handler function. Libc++ provides a the default handler, std::__libcpp_abort_debug_handler, which aborts the program. The handler may not return. Libc++ can be changed to use a custom assertion handler as follows.

#define _LIBCPP_DEBUG 1
#include <string>
void my_handler(std::__libcpp_debug_info const&);
int main(int, char**) {
  std::__libcpp_debug_function = &my_handler;

  std::string::iterator bad_it;
  std::string str("hello world");
  str.insert(bad_it, '!'); // causes debug assertion
  // control flow doesn't return