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.

Comparator consistency checks (_LIBCPP_DEBUG == 1)

Libc++ provides some checks for the consistency of comparators passed to algorithms. Specifically, 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 the debug mode, and causes the library to violate some of the Standard’s complexity clauses.

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