Hardening Modes

Using hardening modes

libc++ provides several hardening modes, where each mode enables a set of assertions that prevent undefined behavior caused by violating preconditions of the standard library. Different hardening modes make different trade-offs between the amount of checking and runtime performance. The available hardening modes are:

  • Unchecked mode/none, which disables all hardening checks.

  • Fast mode, which contains a set of security-critical checks that can be done with relatively little overhead in constant time and are intended to be used in production. We recommend most projects adopt this.

  • Extensive mode, which contains all the checks from fast mode and some additional checks for undefined behavior that incur relatively little overhead but aren’t security-critical. Production builds requiring a broader set of checks than fast mode should consider enabling extensive mode. The additional rigour impacts performance more than fast mode: we recommend benchmarking to determine if that is acceptable for your program.

  • Debug mode, which enables all the available checks in the library, including internal assertions, some of which might be very expensive. This mode is intended to be used for testing, not in production.


Enabling hardening has no impact on the ABI.

Notes for users

As a libc++ user, consult with your vendor to determine the level of hardening enabled by default.

Users wishing for a different hardening level to their vendor default are able to control the level by passing one of the following options to the compiler:






The exact numeric values of these macros are unspecified and users should not rely on them (e.g. expect the values to be sorted in any way).


If you would prefer to override the hardening level on a per-translation-unit basis, you must do so before including any headers to avoid ODR issues.


Since the static and shared library components of libc++ are built by the vendor, setting this macro will have no impact on the hardening mode for the pre-built components. Most libc++ code is header-based, so a user-provided value for _LIBCPP_HARDENING_MODE will be mostly respected.

Notes for vendors

Vendors can set the default hardening mode by providing LIBCXX_HARDENING_MODE as a configuration option, with the possible values of none, fast, extensive and debug. The default value is none which doesn’t enable any hardening checks (this mode is sometimes called the unchecked mode).

This option controls both the hardening mode that the precompiled library is built with and the default hardening mode that users will build with. If set to none, the precompiled library will not contain any assertions, and user code will default to building without assertions.

Iterator bounds checking