mingw-w64-doc/howto-build/ucrt-vs-msvcrt.txt - mingw-w64 - Git at Google

 == Summary ==

 Starting from mingw-w64 version 12 and onwards, UCRT (Universal C Runtime) will
 be the default CRT runtime, unless specified otherwise during the minwgw-64
 build process. We believe that this configuration is the best choice for the
 majority of users. While we encourage users to consider switching to UCRT, we
 understand that it may not always be desirable, especially for compatibility
 reasons. Therefore, all previously supported configurations will continue to
 be supported. Users who wish to continue using msvcrt.dll can easily do so by
 adding the --with-default-msvcrt=msvcrt argument to both the
 mingw-w64-headers and mingw-w64-crt configure scripts.


 == Background ==

 mingw-w64 provides C runtime libraries that directly or indirectly utilize
 Microsoft DLLs at runtime. Over the years, Microsoft has released various
 versions of these DLLs, some of which were designed for specific MSVC versions
 while others aimed for backward compatibility. When building the mingw-w64
 toolchain, users can specify a version of the runtime DLL using the
 --with-default-msvcrt= configure option. The most commonly used versions are
 ucrt (utilizing ucrtbase.dll) and msvcrt (utilizing msvcrt.dll).

 The implementation of msvcrt.dll dates back to the Windows 9x era. While it has
 been extended for a period, it eventually became frozen in favor of providing
 separate DLLs for future MSVC versions. Although msvcrt.dll continues to be
 shipped with modern Windows versions, it remains compatible with the version
 from the 1990s. msvcrt.dll is known to deviate from standard behavior in many
 aspects. mingw-w64 includes compatibility wrappers and extensions to improve
 standard compatibility, but there are limitations to what can be practically
 achieved.

 On the other hand, ucrtbase.dll was introduced with MSVC 14 and is currently
 included with all Windows versions supported by Microsoft. It is also available
 as a redistributable package for older versions of Windows. ucrtbase.dll aims to
 replace MSVC version-specific DLLs and has been used by all MSVC versions since
 its introduction. It offers a more modern approach compared to msvcrt.dll and
 provides better standard compatibility out of the box, reducing overhead on the
 mingw-w64 side.


 == Compatibility ==

 When switching between toolchains that use different runtime libraries, it is
 generally advised not to mix static libraries, unless certain exceptions apply.
 If in doubt, it is recommended to rebuild all static libraries when switching
 the toolchain's runtime DLL.

 Dynamic libraries are less likely to be affected. If the CRT is not part of the
 ABI of a library you intend to use, meaning it does not involve passing FILE
 structs or similar types between modules or depending on the CRT allocator of
 other modules, it should remain unaffected. Otherwise, a compatible build of
 that library will be required.


 == See Also ==

 "MSVCRT vs UCRT": https://www.msys2.org/docs/environments/
	== Summary ==

	Starting from mingw-w64 version 12 and onwards, UCRT (Universal C Runtime) will
	be the default CRT runtime, unless specified otherwise during the minwgw-64
	build process. We believe that this configuration is the best choice for the
	majority of users. While we encourage users to consider switching to UCRT, we
	understand that it may not always be desirable, especially for compatibility
	reasons. Therefore, all previously supported configurations will continue to
	be supported. Users who wish to continue using msvcrt.dll can easily do so by
	adding the --with-default-msvcrt=msvcrt argument to both the
	mingw-w64-headers and mingw-w64-crt configure scripts.


	== Background ==

	mingw-w64 provides C runtime libraries that directly or indirectly utilize
	Microsoft DLLs at runtime. Over the years, Microsoft has released various
	versions of these DLLs, some of which were designed for specific MSVC versions
	while others aimed for backward compatibility. When building the mingw-w64
	toolchain, users can specify a version of the runtime DLL using the
	--with-default-msvcrt= configure option. The most commonly used versions are
	ucrt (utilizing ucrtbase.dll) and msvcrt (utilizing msvcrt.dll).

	The implementation of msvcrt.dll dates back to the Windows 9x era. While it has
	been extended for a period, it eventually became frozen in favor of providing
	separate DLLs for future MSVC versions. Although msvcrt.dll continues to be
	shipped with modern Windows versions, it remains compatible with the version
	from the 1990s. msvcrt.dll is known to deviate from standard behavior in many
	aspects. mingw-w64 includes compatibility wrappers and extensions to improve
	standard compatibility, but there are limitations to what can be practically
	achieved.

	On the other hand, ucrtbase.dll was introduced with MSVC 14 and is currently
	included with all Windows versions supported by Microsoft. It is also available
	as a redistributable package for older versions of Windows. ucrtbase.dll aims to
	replace MSVC version-specific DLLs and has been used by all MSVC versions since
	its introduction. It offers a more modern approach compared to msvcrt.dll and
	provides better standard compatibility out of the box, reducing overhead on the
	mingw-w64 side.


	== Compatibility ==

	When switching between toolchains that use different runtime libraries, it is
	generally advised not to mix static libraries, unless certain exceptions apply.
	If in doubt, it is recommended to rebuild all static libraries when switching
	the toolchain's runtime DLL.

	Dynamic libraries are less likely to be affected. If the CRT is not part of the
	ABI of a library you intend to use, meaning it does not involve passing FILE
	structs or similar types between modules or depending on the CRT allocator of
	other modules, it should remain unaffected. Otherwise, a compatible build of
	that library will be required.


	== See Also ==

	"MSVCRT vs UCRT": https://www.msys2.org/docs/environments/