configure: Use ucrt for msvcrt by default.

Signed-off-by: Jacek Caban <jacek@codeweavers.com>

mingw-w64-crt/configure

@@ -1513,7 +1513,7 @@ Optional Packages:
   --with-genlib=PATH      Whether to build import libraries via the genlib
                           tool (default: no)
   --with-default-msvcrt=LIB
-                          Lib to provide as libmsvcrt.a (default: msvcrt-os)
+                          Lib to provide as libmsvcrt.a (default: ucrt)
 
 Some influential environment variables:
   CC          C compiler command
@@ -2068,9 +2068,7 @@ struct stat;
 /* Most of the following tests are stolen from RCS 5.7 src/conf.sh.  */
 struct buf { int x; };
 struct buf * (*rcsopen) (struct buf *, struct stat *, int);
-static char *e (p, i)
-     char **p;
-     int i;
+static char *e (char **p, int i)
 {
   return p[i];
 }
@@ -2121,6 +2119,7 @@ extern int puts (const char *);
 extern int printf (const char *, ...);
 extern int dprintf (int, const char *, ...);
 extern void *malloc (size_t);
+extern void free (void *);
 
 // Check varargs macros.  These examples are taken from C99 6.10.3.5.
 // dprintf is used instead of fprintf to avoid needing to declare
@@ -6787,7 +6786,7 @@ if test ${with_default_msvcrt+y}
 then :
   withval=$with_default_msvcrt;
 else $as_nop
-  with_default_msvcrt=msvcrt-os
+  with_default_msvcrt=ucrt
 fi
 
 if test "$with_default_msvcrt" = "msvcrt"; then

mingw-w64-crt/configure.ac

@@ -267,9 +267,9 @@ AC_MSG_RESULT([$enable_delay_import_libs])
 AC_MSG_CHECKING([what to provide as libmsvcrt.a])
 AC_ARG_WITH([default-msvcrt],
   [AS_HELP_STRING([--with-default-msvcrt=LIB],
-    [Lib to provide as libmsvcrt.a (default: msvcrt-os)])],
+    [Lib to provide as libmsvcrt.a (default: ucrt)])],
   [],
-  [with_default_msvcrt=msvcrt-os])
+  [with_default_msvcrt=ucrt])
 if test "$with_default_msvcrt" = "msvcrt"; then
   with_default_msvcrt=msvcrt-os
 fi

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

@@ -0,0 +1,58 @@
+== 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/

mingw-w64-headers/configure

@@ -1387,7 +1387,7 @@ Optional Packages:
   --with-default-win32-winnt=VER
                           Default value of _WIN32_WINNT (default: 0xa00)
   --with-default-msvcrt=LIB
-                          Default msvcrt to target (default: msvcrt)
+                          Default msvcrt to target (default: ucrt)
 
 Report bugs to <mingw-w64-public@lists.sourceforge.net>.
 _ACEOF
@@ -3002,7 +3002,7 @@ if test ${with_default_msvcrt+y}
 then :
   withval=$with_default_msvcrt;
 else $as_nop
-  with_default_msvcrt=msvcrt
+  with_default_msvcrt=ucrt
 fi
 
 case $with_default_msvcrt in
@@ -3036,12 +3036,12 @@ msvcr110*)
 msvcr120*)
   default_msvcrt_version=0xC00
   ;;
-ucrt*)
-  default_msvcrt_version=0xE00
-  ;;
-msvcrt|*)
+msvcrt*)
   default_msvcrt_version=0x700
   ;;
+ucrt*|*)
+  default_msvcrt_version=0xE00
+  ;;
 esac
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $with_default_msvcrt ($default_msvcrt_version)" >&5
 printf "%s\n" "$with_default_msvcrt ($default_msvcrt_version)" >&6; }

mingw-w64-headers/configure.ac

@@ -144,9 +144,9 @@ AC_SUBST([DEFAULT_WIN32_WINNT])
 AC_MSG_CHECKING([default msvcrt])
 AC_ARG_WITH([default-msvcrt],
   [AS_HELP_STRING([--with-default-msvcrt=LIB],
-    [Default msvcrt to target (default: msvcrt)])],
+    [Default msvcrt to target (default: ucrt)])],
   [],
-  [with_default_msvcrt=msvcrt])
+  [with_default_msvcrt=ucrt])
 case $with_default_msvcrt in
 msvcrt10*)
   default_msvcrt_version=0x100
@@ -178,12 +178,12 @@ msvcr110*)
 msvcr120*)
   default_msvcrt_version=0xC00
   ;;
-ucrt*)
-  default_msvcrt_version=0xE00
-  ;;
-msvcrt|*)
+msvcrt*)
   default_msvcrt_version=0x700
   ;;
+ucrt*|*)
+  default_msvcrt_version=0xE00
+  ;;
 esac
 AC_MSG_RESULT([$with_default_msvcrt ($default_msvcrt_version)])
 AS_VAR_SET([DEFAULT_MSVCRT_VERSION],[$default_msvcrt_version])