An issue with Matlab for a mex file compiled with GNU CYGWIN g++

An issue with Matlab for a mex file compiled with GNU CYGWIN g++

[Emad Gad]

Hello

I apologize that this message is a long one. But you may skip to the
question at the end, if you wish to know the troubles I am having.

I have been using LINUX for a very long time.
I decided to compile my work on CYGWIN. Furthermore, I wanted to compile a
Matlab interface to be able to use it from Matlab under Windows.

I succeeded, after plodding for a whole weekend, in compiling everything,
without errors.

The problem came when I tried to run the matlab interface, which is the
usual .mexw64 (on the recent versions of Matlab) file that is produced by
the mex uttility in Matlab. At that moment, Matlab complained of “Invalid
mex file: [the file name].mexw64 is not a valid Win32 application”

I know that, up until now, the problem does not seem even remotely related
to CYGWIN.

However, investigating and reading around the web about this issue further
has led me to conclude that Matlab will generate this error if the .mexw64
the dll in question refers or uses other 32 bit dll. Matlab presumably
cannot run a hybrid dll, that is a dll with 64 and 32 bits components.

I then used the “Dependency Walker” tool to examine the components of my
.mexw64. And I found out that my “.mexw64” file is linked to the following
library C:\Windows\system32\kernel32.dll, which is a flagrantly 32 bit
Windows system dll. So I assumed that this must be the reason that Matlab
failed to run the mex file, and wanted to know why this library got linked
in the first place.

That brings me to the CYGWIN connection: The CYGWIN linker that I invoked to
finish the job is the g++-4, which has added the library kernel32.dll, with
the linker flag –lkernel32.dll. Notice that this linker flag has been aded
by the linker, and not by me. It also added the 32-bit Windows path on the
Library paths, and that is how I ended up putting a 32-bit dll in the
structure of 64 bit mex file.

And that finally brings me to the main point of my post, and my question.

Is there a way to make the CYGWIN g++ linker choose the Windows 64 bit
system libraries instead of the 32 bit?





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Re: An issue with Matlab for a mex file compiled with GNU CYGWIN g++

[David Billinghurst]

On 1/07/2013 12:27 PM, Emad Gad wrote:
> Hello
>
> I apologize that this message is a long one. But you may skip to the
> question at the end, if you wish to know the troubles I am having.
>
> I have been using LINUX for a very long time.
> I decided to compile my work on CYGWIN. Furthermore, I wanted to compile a
> Matlab interface to be able to use it from Matlab under Windows.

Try one of the supported compilers for your version of Matlab.  Getting 
an unsupported compiler to work could be difficult, and may be impossible.

See http://www.mathworks.com.au/support/compilers/R2013a/index.html
and http://www.mathworks.com.au/support/sysreq/previous_releases.html

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Re: An issue with Matlab for a mex file compiled with GNU CYGWIN g++

[Tim Prince]

On 6/30/2013 10:27 PM, Emad Gad wrote:
>
> Is there a way to make the CYGWIN g++ linker choose the Windows 64 bit
> system libraries instead of the 32 bit?
>
cygwin64.  If you mean to link against Microsoft X64 libraries in place 
of cygwin ones, x86_64-w64-mingw32.

-- 
Tim Prince


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Re: An issue with Matlab for a mex file compiled with GNU CYGWIN g++

[Emad Gad]

I do really appreciate your help.
At least it put me on the right path.

So I decided just to test the basic idea: I used the compiler
i686-w64-mingw32-g++ to compile a simple C file with mexFunction, and see to
it being run, or called, from the Matlab command line and return the
expected value without errors. Of course I put a small mexopt.bat file to
guide the compilation process, etc.

And it happened! 
And that led me to conclude that the i686-w64-mingw32-g++ is the way to go
when it comes to Matlab, and that it is one of those approved compilers that
the Windows-based Matlab can invoke to compile a mex-based C function, and
will not complain of any missing library etc.

Now I shifted my strategy, where I decided to port my LINUX project to
Windows. The goal was to compile this project using i686-w64-mingw32-g++ so
that I can link it with the Matlab interface. As I said before, I succeeded
in doing just that, but with the cygwin g++ compiler. Now, having been
enlightened to the differences between cygwin and MinGW, I tried to restart
compiling everything with mingw species.
I am still using the cygwin shell to accomplish my scheme.

The troubles I am having now is how to build the cmake-based project with
i686-w64-mingw32-g++ compiler, through the cygwin shell? Somehow, I feel
that this may be a contradiction in terms, but let me first report my
observations.

The main issue here is that the cmake that comes with cygwin will not accept
the compiler i686-w64-mingw32-g++. It will complain that that compiler
"cannot compile a simple file" which is a lie, because I know that it does.
I actually tested, and Matlab used successfully. So how come cmake found out
that it fails to compile?

I found out the reason by invoking cmake with the option --debug-trycompile,
and there I saw what cmake is trying to do and how it failed.

cmake tests the compiler by requesting it to compile a very small file,
called textCCompile.c, that it creates on the fly. However, its uses the
absolute path of this file. To be more precise, here is the literal command
that the /cygwinish/ cmake issues 

/cygdrive/c/MinGW64/mingw64/i686-w64-mingw32-g++ -o
cmTryxxxxxxxx/testCCompile.c.o -c
/home/usrername/path/to/the/file/build/CMakeFiles/CMakeTmp/testCCompile.c.

The main problem here is that i686-w64-mingw32-g++ will not take the
absolute path of the C file /home/usrername/path/to/the/file/testCCompile.c.
Instead, it returns a "file not found" kind of error. If I invoke the same
command that cmake did invoke, it returns the same error. However, if I use
the relative path, say like the following

/cygdrive/c/MinGW64/mingw64/i686-w64-mingw32-g++ -o
cmTryxxxxxxxx/testCCompile.c.o -c ../CMakeFiles/CMakeTmp/testCCompile.c.

then the command is executed without any troubles.
So I feel now that the main problem is that mingw compiler, which worked
with Matlab, the i686-w64-mingw32-g++, does not like UNIX-style absolute
paths, and will not be able to grab any file specified using this mode.

So my question is am I in the right place? Is it normal to use a
cygwin-based cmake to compile a project with i686-w64-mingw32-g++? 

Your help is indeed appreciated.

Thanks 
 

 



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Re: An issue with Matlab for a mex file compiled with GNU CYGWIN g++

[Csaba Raduly]

Hi Emad,

On Fri, Jul 12, 2013 at 5:17 AM, Emad Gad  wrote:
(snip)
> And that led me to conclude that the i686-w64-mingw32-g++ is the way to go
(snip)

> The troubles I am having now is how to build the cmake-based project with
> i686-w64-mingw32-g++ compiler, through the cygwin shell? Somehow, I feel
> that this may be a contradiction in terms, but let me first report my
> observations.
>
> The main issue here is that the cmake that comes with cygwin will not accept
> the compiler i686-w64-mingw32-g++. It will complain that that compiler
> "cannot compile a simple file" which is a lie, because I know that it does.
> I actually tested, and Matlab used successfully. So how come cmake found out
> that it fails to compile?
>
> I found out the reason by invoking cmake with the option --debug-trycompile,
> and there I saw what cmake is trying to do and how it failed.
>
> cmake tests the compiler by requesting it to compile a very small file,
> called textCCompile.c, that it creates on the fly. However, its uses the
> absolute path of this file. To be more precise, here is the literal command
> that the /cygwinish/ cmake issues
>
> /cygdrive/c/MinGW64/mingw64/i686-w64-mingw32-g++ -o
> cmTryxxxxxxxx/testCCompile.c.o -c
> /home/usrername/path/to/the/file/build/CMakeFiles/CMakeTmp/testCCompile.c.
>
> The main problem here is that i686-w64-mingw32-g++ will not take the
> absolute path of the C file /home/usrername/path/to/the/file/testCCompile.c.
> Instead, it returns a "file not found" kind of error.

The problem is that /cygdrive/c/MinGW64/mingw64/i686-w64-mingw32-g++
is not a Cygwin program, so it interprets
/home/usrername/path/to/the/file/testCCompile.c differently from
Cygwin's cmake : as a path on the current drive, starting with a
directory named "home". However, Cygwin's /home is Windows'
C:\cygwin\home

Cygwin contains a utility called "cygpath" which can convert between
POSIX paths and Windows paths:

$ cygpath -w /home/usrername/path/to/the/file/testCCompile.c
C:\cygwin\home\usrername\path\to\the\file\testCCompile.c

Unfortunately, I don't think you can force cmake to pass all paths through it.

> So I feel now that the main problem is that mingw compiler, which worked
> with Matlab, the i686-w64-mingw32-g++, does not like UNIX-style absolute
> paths, and will not be able to grab any file specified using this mode.
>
> So my question is am I in the right place? Is it normal to use a
> cygwin-based cmake to compile a project with i686-w64-mingw32-g++?

It may be possible to force cmake to accept the compiler you specify
without checking, e.g. in the environment, like this:

CXX=/cygdrive/c/MinGW64/mingw64/i686-w64-mingw32-g++  cmake
--trust-the-compiler --the-compiler-is-your-friend path/to/sources

But it's likely that the makefiles it generates would also use POSIX
paths, and i686-w64-mingw32-g++ will not understand them unless you
hack the makefiles to use cygpath to translate filenames before make
passes them to i686-w64-mingw32-g++

I suspect it would be easier to use the native Windows version of
cmake with the mingw compiler.

Csaba
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