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MSYS2 comes with different environments and the first thing you have to decide is which one to use. The differences among the environments are mainly environment variables, default compilers/linkers, architecture, system libraries used etc. If you are unsure, go with UCRT64.

The MSYS environment contains the unix-like/cygwin based tools, lives under /usr and is special in that it is always active. All the other environments inherit from the MSYS environment and add various things on top of it.

For example, in the UCRT64 environment the $PATH variable starts with /ucrt64/bin:/usr/bin so you get all ucrt64 based tools as well as all msys tools.


Name Prefix Toolchain Architecture C Library C++ Library
msys MSYS /usr gcc x86_64 cygwin libstdc++
ucrt64 UCRT64 /ucrt64 gcc x86_64 ucrt libstdc++
clang64 CLANG64 /clang64 llvm x86_64 ucrt libc++
clangarm64 CLANGARM64 /clangarm64 llvm aarch64 ucrt libc++
clang32 CLANG32 /clang32 llvm i686 ucrt libc++
mingw64 MINGW64 /mingw64 gcc x86_64 msvcrt libstdc++
mingw32 MINGW32 /mingw32 gcc i686 msvcrt libstdc++

The active environment is selected via the MSYSTEM environment variable. Setting MSYSTEM to UCRT64 and starting a login shell will put you in that environment.

GCC vs LLVM/Clang

These are the default compilers/toolchains used for building all packages in the respective repositories.

GCC based environments:

  • Widely tested/used at this point
  • Fortran support
  • While there also exists a Clang package in the MINGW environments, that one still uses the GNU linker and the GNU C++ library. In some cases Clang is used to build packages as well there, in case upstream prefers Clang over GCC for example.

LLVM/Clang based environments:

  • Only uses LLVM tools, LLD as a linker, LIBC++ as a C++ standard library
  • Clang provides ASAN support
  • Native support for TLS (Thread-local storage)
  • LLD is faster than LD, but does not support all the features LD supports
  • Some tools lack feature parity with equivalent GNU tools
  • Supports ARM64/AArch64 architecture on Microsoft Windows 10


These are two variants of the C standard library on Microsoft Windows.

MSVCRT (Microsoft Visual C++ Runtime) is available by default on all Microsoft Windows versions, but due to backwards compatibility issues is stuck in the past, not C99 compatible and is missing some features.

  • It isn't C99 compatible, for example the printf() function family, but...
  • mingw-w64 provides replacement functions to make things C99 compatible in many cases
  • It doesn't support the UTF-8 locale
  • Binaries linked with MSVCRT should not be mixed with UCRT ones because the internal structures and data types are different. (More strictly, object files or static libraries built for different targets shouldn't be mixed. DLLs built for different CRTs can be mixed as long as they don't share CRT objects, e.g. FILE*, across DLL boundaries.) Same rule is applied for MSVC compiled binaries because MSVC uses UCRT by default (if not changed).
  • Works out of the box on every version of Microsoft Windows.

UCRT (Universal C Runtime) is a newer version which is also used by Microsoft Visual Studio by default. It should work and behave as if the code was compiled with MSVC.

  • Better compatibility with MSVC, both at build time and at run time.
  • It is included as part of the operating system in Windows 10 or later, and Windows Server 2016 or later. For older Windows versions, you have to provide it explicitly or depend on the user having it installed. More information about that can be found in UCRT deployment.