Porting

Wolfgang Stoeggl edited this page Jan 10, 2018 · 5 revisions

While in our humble opinions, MSYS2 makes collaborative, organised development of open-source software on Windows a workable proposition, there are a few things to be aware of that we commonly run into, mostly due to the design decisions made by Microsoft, our preference for using native tools and compilers rather than cross msys2-to-native ones and our wish to be as flexible as we can be.

mingw32-make

MSYS2 provides two versions of make, one in the make package and one in the mingw-w64-{i686,x86_64}-make packages. The latter one is called mingw32-make on command line, is fully native and doesn't depend on msys2 shells. The downside is that it doesn't work with many Makefiles. Unless you know what you're doing, use the regular make.

Detecting version of MSYS from GNU make

You can use the following Makefile snippet to detect whether you are running GNU make from an MSYS or an MSYS2 shell. If you run it through mingw32-make.exe from cmd.exe you will likely get an error since uname will not be found on your PATH. If you run it through mingw32-make.exe from one of the MSYS shells, it will set msys_version to 1 or 2 as appropriate. On any other system with uname present, it will set it to 0.

msys_version := $(if $(findstring Msys, $(shell uname -o)),$(word 1, $(subst ., ,$(shell uname -r))),0)
$(info The version of MSYS you are running is $(msys_version) (0 meaning not MSYS at all))

Platform checks

You may need to use platform checks to switch between behaviour suited for MSYS2 and the default one. Some useful identifiers:

Identifier Platform(s) Usage
_WIN32 mingw, msvc C code (#ifdef ...)
_WIN64 64-bit mingw, 64-bit msvc C code (#ifdef ...)
__CYGWIN__ msys2, cygwin C code (#ifdef ...)
__MSYS__ msys2 C code (#ifdef ...)
x86_64-pc-msys2 64-bit msys2 Build scripts (if [ $host = '...' ])
i686-pc-msys2 32-bit msys2 Build scripts (if [ $host = '...' ])
x86_64-w64-mingw32 64-bit mingw Build scripts (if [ $host = '...' ])
i686-w64-mingw32 32-bit mingw Build scripts (if [ $host = '...' ])
msys msys2 Python (sys.platform)
win32 mingw Python (sys.platform)

Filesystem namespaces

In MSYS2 there are two different filesystem namespaces in play. If you are building purely msys2 software, you can ignore the Windows filesystem namespace entirely, however, when building native software using MSYS2's tools, you must be mindful that when msys2 executes a native program, it will translate environment variables and command arguments from msys2-form to native-form. To do this, it converts things that look like msys2 paths. Sometimes it gets this wrong, e.g.:

command.exe /switch

Explanation: /switch is a Windows style switch. Note that Windows programs commonly accept unambiguous dash-prefixed switches (-switch) as well.

adb push readme.txt /sdcard/0/

Explanation: /sdcard/0/ is a path on a foreign system.

./configure --root=/

Explanation: The value of root (/) is emitted to a text file via echo (which is an msys2 program and hence not mangled) and also to a Makefile (through autotools) which passes it to a native program, which reads the text file replacing each instance of / with "some-other-path". This will go wrong as / passed to the native program was converted to e.g. C:/msys64/ and therefore the necessary substitutions did not happen. [last sentence needs review or rewording]

To work around this, path conversion can be selectively disabled. MSYS2 reads an environment variable called MSYS2_ARG_CONV_EXCL. This is a ; delimited string each part of which is compared against the front part of each argument and if a match is found, that conversion is skipped. An example of a value for MSYS2_ARG_CONV_EXCL that would inhibit path transformations of the 3 cases above is /switch;/sdcard;--root=.

The development repository for this path conversion code is https://github.com/Alexpux/path_convert. If you find a case that you think is unambiguously being converted incorrectly, please raise an issue there and/or a pull request with the broken test-case.

Hard-coded paths and relocation

When installing MSYS2, the user selects the root folder. POSIX software has a habit of baking the installation paths into the packages at build time, which is okay for msys2 software (which only sees the single-root virtual filesystem), but usually causes failures in native software. When a program tries to load its data files using these absolute paths on another MSYS2 installed to a different root folder, it won't find them. For this reason, adding path relocation patches is a common necessity for mingw packages (but never for msys2 packages). The point is to find the path where the program's executable resides, cutting off the /bin from the end and finding all necessary files using paths relative to that. Since this is a pretty common task and is not exactly trivial to get it right for all cases, we wrote some re-usable C code for that, available at https://github.com/Alexpux/MINGW-packages/tree/master/mingw-w64-pathtools.

Sometimes these paths are baked into shell scripts or pkg-config's .pc files. In that case, you should use sed in the PKGBUILD package() function to correct this back to the msys2 version of the path. [please review last sentence] An example of this can be seen at https://github.com/Alexpux/MINGW-packages/blob/master/mingw-w64-libtool/PKGBUILD#L55.

C *printf Format Specifier issues

The vc6.0 msvcrt.dll that MinGW-w64 targets doesn't implement support for the ANSI standard format specifiers. MinGW-w64 works around this by providing their own implementation. To enable this you should pass -D__USE_MINGW_ANSI_STDIO=1 to the MinGW-w64 C and C++ compilers. All of our C/C++ packages are built with this flag.

Different size of struct timeval

The Windows (and thus mingw-w64) struct timeval is defined as having two long members, while the POSIX specs say it's supposed to have one time_t and one suseconds_t member. This means that on 64-bit, the size of the struct will be different from what you would expect on Linux/Cygwin/MSYS2.

struct timeval on MSDN struct timeval on OpenGroup.org

More 32/64 bit peculiarities

Sorting out the stat thing by LRN: https://gnunet.org/sorting-out-stat-thing

Calling conventions, stdcall, and autotools

Details about Windows' 32bit calling conventions: http://www.willus.com/mingw/yongweiwu_stdcall.html

Problems stdcall causes with autotool's AC_CHECK_FUNC and AC_SEARCH_LIBS: http://lists.gnu.org/archive/html/autoconf/2013-05/msg00090.html

Macro/inline function and autotools

AC_CHECK_FUNCS and friends can't deal with function macros and static inline functions (because the checking code tries to #undef the symbol, doesn't include any headers and declares the symbol as extern instead). Possible solution is replacing them with more elaborate checks using AC_LINK_IFELSE or AC_COMPILE_IFELSE. For example this:

AC_CHECK_FUNCS([localtime_r],
[AM_CONDITIONAL([HAVE_LOCALTIME_R], true)],
[AM_CONDITIONAL([HAVE_LOCALTIME_R], false)])

... can be replaced with this:

AC_MSG_CHECKING([for localtime_r])
AC_LINK_IFELSE([AC_LANG_PROGRAM([[
  #include <time.h>
]], [[
  time_t t; struct tm r;
  localtime_r(&t, &r);
]])],
[AC_MSG_RESULT([yes])
 AM_CONDITIONAL([HAVE_LOCALTIME_R], true)
 AC_DEFINE([HAVE_LOCALTIME_R], [1], [Define to 1 if you have the `localtime_r' function or macro.])],
[AC_MSG_RESULT([no])
 AM_CONDITIONAL([HAVE_LOCALTIME_R], false)])

Don't forget to autoreconf -fi after patching configure.ac.

A tidy workaround from flameeyes for asprintf/vasprintf (but generally applicable): https://mailman.videolan.org/pipermail/vlc-devel/2015-March/101802.html

Guarded time functions, ANSI format specifiers

Some things in mingw-w64 are (maybe unexpectedly) guarded by #ifdefs. Check out _POSIX_C_SOURCE, __USE_MINGW_ANSI_STDIO, the time.h file. Note that (at the time of writing) pthread.h contains some defines that affect the definitions in time.h.

Undefined references and linking to DLLs/SOs

[this section may contain misinformations; it needs review from a knowledgeable person]

Linux/ELF platforms generally don't do anything special to link to shared objects, they just leave the undefined references in the binary. Windows however requires all references to be resolved at link time. In case of DLLs, this is solved by the .dll.a import libraries that add the relevant .dll to the binary's import table and insert correct calls into the code, but it needs that correct linker flags be passed when linking binaries. Note that the linker is aware of these files and will use them automatically when using the standard -l arguments, for example -lfoo will make the linker check for libfoo.dll.a and libfoo.a, in this order (unless specified otherwise).

Libtool generally refuses to create DLLs unless -no-undefined is passed to the linker invocation (library_la_LDFLAGS = -no-undefined). See: https://lists.gnu.org/archive/html/libtool/2007-04/msg00066.html

Library prefixes

mingw DLLs follow the convention of prefixing libraries with lib. This affects shared libraries (.dll), static object archives (.a), and DLL import libraries (.dll.a). Because msys2 DLLs are generally ABI-incompatible with everything from outside of msys2, they are prefixed with msys2- instead. For completeness, we note that Cygwin DLLs are prefixed with cyg.

[does libtool or the linker handle this automatically?]

Standard streams in mintty

mintty is primarily designed to be a good terminal emulator (in the POSIX sense of the word) and it works well with bash, ssh and other Cygwin/MSYS2 programs. Because native Windows console programs use a fundamentally different way of handling console input/output, mintty uses pipes to connect to their standard streams (stdin, stdout, stderr). Unfortunately, this has the effect that isatty and similar APIs (used to check whether a stream is attached to an interactive console or a pipe/file) will return incorrect values for native programs running under mintty.

One way of fixing this problem is to run native programs inside a real Windows console, hide the console and use the console API to communicate with the program. This approach has obvious disadvantages, but it's good enough. Actually, this is the way most Windows console emulators work (e.g. ConsoleZ, ConEmu). There is also a wrapper program called winpty that does exactly this and translates the I/O to/from standard terminal sequences which mintty understands.

Running a program under winpty (by prefixing the command line with winpty) will make the program think it is running interactively, but it will also break any special features depending on terminal sequences, possibly including colored text output and TUIs.

MSYS2 includes wrappers for some affected programs, so that they will work correctly most of the time. Examples can be seen in packages containing REPLs (python3, lua, nodejs, ...).

mintty issue #56, original on Google Code

winpty on GitHub

Signals in mintty

When running a native Windows program in mintty (or urxvt, apparently), Ctrl+C will not be correctly passed to the process. Instead, it seems to be eaten by the terminal and the running process is terminated.

ticket #135

Paths in mingw Python

The mingw python provided by MSYS2 will try to produce paths that are right for the environment it's running in. According to the value of MSYSTEM, it will use either forward or backward slashes as os.path.sep.

Response files

When passing arguments with absolute paths to native Windows programs (such as /mingw64/bin/gcc), these paths get (in most cases) automatically converted by MSYS2 if they're in POSIX format. However, if you're passing these paths inside a file (e.g. a response file for GCC -- gcc @somefile), they need to be pre-converted to Windows native format.

discussion topic: gcc -I (eye) uses msys2 path or windows path?

Setting floating point precision

Floating point precision issues with multiple threads: gfortran, open mp and floating point precision by Carl Kleffner:

Command line parsing

Windows programs parse the command line themselves, it isn't parsed for them by the calling process, as on Linux. This means that if wildcards (glob patterns) are to be accepted by the program, it has to be able to expand them somehow. MinGW-w64 supplies the correct start-up code, so it happens automatically, in a manner compatible with MSVC-compiled programs. If undesirable, the behavior can be disabled at program build.

Cygwin/MSYS2 programs have to deal with a mix of both approaches, but they can apparently deal with it. Note that they don't behave exactly like native programs, for example they understand single-quoted arguments.

"How Command Line Parameters Are Parsed" by David Deley

"Everyone quotes command line arguments the wrong way" by Daniel Colascione

Other resources

A collection of articles on general C and C++ topics. http://locklessinc.com/articles/

The MinGW-w64 project's wiki. http://sourceforge.net/p/mingw-w64/wiki2/browse_pages/

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