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Freestanding and hosted implementations

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C++
 

There are two kinds of implementations defined by the C++ standard: hosted and freestanding implementations. For hosted implementations, the set of standard library headers required by the C++ standard is much larger than for freestanding ones. In a freestanding implementation, execution may happen without an operating system.

The kind of the implementation is implementation-defined. The macro __STDC_HOSTED__ is predefined to 1 for hosted implementations and 0 for freestanding implementations.(since C++11)

Contents

Requirements on multi-threaded executions and data races

freestanding hosted
Under a freestanding implementation, it is implementation-defined whether a program can have more than one thread of execution. Under a hosted implementation, a C++ program can have more than one thread running concurrently.
(since C++11)

Requirements on the main function

freestanding hosted
In a freestanding implementation, it is implementation-defined whether a program is required to define a main function. Start-up and termination is implementation-defined; start-up contains the execution of constructors for objects of namespace scope with static storage duration; termination contains the execution of destructors for objects with static storage duration. In a hosted implementation, a program must contain a global function called main. Executing a program starts a main thread of execution in which the main function is invoked, and in which variables of static storage duration might be initialized and destroyed.

Requirements on standard library headers

A freestanding implementation has an implementation-defined set of headers. This set includes at least the headers in the following table.

For partially freestanding headers, freestanding implementations only needs to provide part of the entities in the corresponding synopsis:

  • If an entity is commented // freestanding, it is guaranteed to be provided.
  • If an entity (function or function template) is commented // freestanding-deleted, it is guaranteed to be either provided or deleted.
(since C++26)
Headers required for a freestanding implementation
Library Component Headers  Freestanding 
Language support Common definitions <cstddef> All
C standard library <cstdlib> Partial
Implementation properties <cfloat>
<climits> (since C++11)
<limits>
<version> (since C++20) 		All
Integer types <cstdint> (since C++11) All
Dynamic memory management <new> All
Type identification <typeinfo> All
Source location <source_location> (since C++20) All
Exception handling <exception> All
Initializer lists <initializer_list> (since C++11) All
Comparisons <compare> (since C++20) All
Coroutines support <coroutine> (since C++20) All
Other runtime support <cstdarg> All
Debugging support <debugging> (since C++26) All
Concepts <concepts> (since C++20) All
Diagnostics Error numbers <cerrno> (since C++26) Partial
System error support <system_error> (since C++26) Partial
Memory management  Memory <memory> (since C++23) Partial
Metaprogramming Type traits <type_traits> (since C++11) All
Compile-time rational arithmetic  <ratio> (since C++23) All
General utilities Utility components <utility> (since C++23) All
Tuples <tuple> (since C++23) All
Function objects <functional> (since C++20) Partial
Primitive numeric conversions <charconv> (since C++26) Partial
Bit manipulation <bit> (since C++20) All
Strings String classes <string> (since C++26) Partial
Null-terminated
sequence utilities 		<cstring> (since C++26)
<cwchar> (since C++26) 		Partial
Iterators <iterator> (since C++23) Partial
Ranges <ranges> (since C++23) Partial
Numerics Mathematical functions
for floating-point types 		<cmath> (since C++26) Partial
Concurrency support Atomics <atomic> (since C++11)     All[1]
Deprecated headers <ciso646> (until C++20)
<cstdalign> (since C++11)(until C++20) 
<cstdbool> (since C++11)(until C++20) 		All
  1. ↑ Support for always lock-free integral atomic types and presence of type aliases std::atomic_signed_lock_free and std::atomic_unsigned_lock_free are implementation-defined in a freestanding implementation.(since C++20)

Notes

It is important to note that some compiler vendors may not fully support freestanding implementation. For example, GCC libstdc++ has had implementation and build issues before version 13, while LLVM libcxx and MSVC STL do not support freestanding.

In C++23, many features will be made freestanding with partial headers. However, it is still up for discussion in WG21 whether headers like <array> will be made freestanding in the future standard (C++26, maybe). Regardless, containers like vector, list, deque, and map will never be freestanding due to their dependencies on exceptions and heap.

GCC 13 provides more headers, such as optional, span, array, and bitset, for freestanding. However, it is important to note that these headers may not be portable or provide the same experience as a hosted implementation. It is better to avoid using them in a freestanding environment, even if the toolchain provides them.

Feature-test macro Value Std Feature
__cpp_lib_freestanding_feature_test_macros 202306L (C++26) freestanding feature test macros
__cpp_lib_freestanding_char_traits 202306L (C++26) freestanding std::char_traits
__cpp_lib_freestanding_charconv 202306L (C++26) freestanding <charconv>
__cpp_lib_freestanding_cstdlib 202306L (C++26) freestanding <cstdlib>
__cpp_lib_freestanding_cstring 202306L (C++26) freestanding <cstring>
__cpp_lib_freestanding_cwchar 202306L (C++26) freestanding <cwchar>
__cpp_lib_freestanding_errc 202306L (C++26) freestanding std::errc
__cpp_lib_freestanding_functional 202306L (C++26) freestanding <functional>
__cpp_lib_freestanding_iterator 202306L (C++26) freestanding <iterator>
__cpp_lib_freestanding_memory 202306L (C++26) freestanding <memory>
__cpp_lib_freestanding_ranges 202306L (C++26) freestanding <ranges>
__cpp_lib_freestanding_ratio 202306L (C++26) freestanding <ratio>
__cpp_lib_freestanding_tuple 202306L (C++26) freestanding <tuple>
__cpp_lib_freestanding_utility 202306L (C++26) freestanding <utility>

References

  • C++23 standard (ISO/IEC 14882:2024):
  • 4.1 Implementation compliance [intro.compliance] (p: 10)
  • 6.9.2 Multi-threaded executions and data races [intro.multithread] (p: 84)
  • 6.9.3.1 main function [basic.start.main] (p: 89)
  • 16.4.2.5 Freestanding implementations [compliance] (p: 483)
  • C++20 standard (ISO/IEC 14882:2020):
  • 4.1 Implementation compliance [intro.compliance] (p: 7)
  • 6.9.2 Multi-threaded executions and data races [intro.multithread] (p: 77)
  • 6.9.3.1 main function [basic.start.main] (p: 82)
  • 16.5.1.3 Freestanding implementations [compliance] (p: 470)
  • C++17 standard (ISO/IEC 14882:2017):
  • 4.1 Implementation compliance [intro.compliance] (p: 5)
  • 4.7 Multi-threaded executions and data races [intro.multithread] (p: 15)
  • 6.6.1 main function [basic.start.main] (p: 66)
  • 20.5.1.3 Freestanding implementations [compliance] (p: 458)
  • C++14 standard (ISO/IEC 14882:2014):
  • 1.4 Implementation compliance [intro.compliance] (p: 5)
  • 1.10 Multi-threaded executions and data races [intro.multithread] (p: 11)
  • 3.6.1 Main function [basic.start.main] (p: 62)
  • 17.6.1.3 Freestanding implementations [compliance] (p: 441)
  • C++11 standard (ISO/IEC 14882:2011):
  • 1.4 Implementation compliance [intro.compliance] (p: 5)
  • 1.10 Multi-threaded executions and data races [intro.multithread] (p: 11)
  • 3.6.1 Main function [basic.start.main] (p: 58)
  • 17.6.1.3 Freestanding implementations [compliance] (p: 408)
  • C++03 standard (ISO/IEC 14882:2003):
  • 1.4 Implementation compliance [intro.compliance] (p: 3)
  • 3.6.1 Main function [basic.start.main] (p: 43)
  • 17.4.1.3 Freestanding implementations [lib.compliance] (p: 326)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 1938 C++98 an implementation did not need
to document whether it is hosted 		made the implementation kind implementation-
defined (thus requires a documentation)
LWG 3653
(P1642R11) 		C++20 <coroutine> is freestanding, but
uses std::hash which was not 		made <functional> being
partially freestanding

See also

C documentation for Conformance
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