matlab crashes without dump file when using fopen for file

I am using gnumex with mingw for compiling mex files in matlab in Windows OS. I am not able to use fopen command to open files. Following is the code which I am using.

#include <stdio.h>
#include "mex.h"
void mexFunction(int nlhs,mxArray *plhs[],int nrhs,const mxArray *prhs[])
{
    FILE  *fp=NULL;
    fp = fopen("test.txt", "w+");
    fclose(fp);
}

The program compiles succesfully but when I try to run the compile mexw64 file, matlab simply closes down without generating any dump file.

Note that I am able to run simple Hello world program and passing and receiving arguments using other mex programs.

I cannot debug using http://www.mathworks.com/help/matlab/matlab_external/debugging-on-microsoft-windows-platforms.html as I am compiling using mingw along with gnumex setup in windows

Output of

mex -v file.cpp

   This is mex, Copyright 1984-2007 The MathWorks, Inc. 

-> Default options filename found in C:\Users\achaudhary\AppData\Roaming\MathWorks\MATLAB\R2009b 
---------------------------------------------------------------- 
->    Options file           = C:\Users\achaudhary\AppData\Roaming\MathWorks\MATLAB\R2009b\mexopts.bat 
      MATLAB                 = C:\PROGRA~1\MATLAB\R2009b 
->    COMPILER               = gcc 
->    Compiler flags: 
         COMPFLAGS           = -c -DMATLAB_MEX_FILE -x c++ 
         OPTIMFLAGS          = -O3 
         DEBUGFLAGS          = -g 
         arguments           =  
         Name switch         = -o 
->    Pre-linking commands   =  
->    LINKER                 = C:\PROGRA~1\MATLAB\R2009b\sys\perl\win32\bin\perl.exe E:\MATLAB~1\linkmex.pl 
->    Link directives: 
         LINKFLAGS           =   -mwindows -LC:\Users\ACHAUD~1\AppData\Roaming\MATHWO~1\MATLAB\R2009b\gnumex GM_ISCPP   -mwindows 
         LINKDEBUGFLAGS      = -g  -Wl,--image-base,0x28000000\n 
         LINKFLAGSPOST       =  
         Name directive      = -o file.mexw64 
         File link directive =  
         Lib. link directive =  
         Rsp file indicator  =  
->    Resource Compiler      = C:\PROGRA~1\MATLAB\R2009b\sys\perl\win32\bin\perl.exe E:\MATLAB~1\rccompile.pl --unix -o mexversion.res 
->    Resource Linker        =  
---------------------------------------------------------------- 

--> gcc  -c -DMATLAB_MEX_FILE -x c++ -oC:\USERS\ACHAUD~1\APPDATA\LOCAL\TEMP\MEX_FZ~1\file.obj -IC:\PROGRA~1\MATLAB\R2009b\extern\include -IC:\PROGRA~1\MATLAB\R2009b\simulink\include -O3 -DMX_COMPAT_32 file.cpp 

--> C:\PROGRA~1\MATLAB\R2009b\sys\perl\win32\bin\perl.exe E:\MATLAB~1\linkmex.pl -o file.mexw64   -mwindows -LC:\Users\ACHAUD~1\AppData\Roaming\MATHWO~1\MATLAB\R2009b\gnumex GM_ISCPP   -mwindows -s  C:\USERS\ACHAUD~1\APPDATA\LOCAL\TEMP\MEX_FZ~1\file.obj   

link command: g++ -shared C:\Users\ACHAUD~1\AppData\Roaming\MATHWO~1\MATLAB\R2009b\gnumex\mex.def -o file.mexw64 -mwindows -LC:\Users\ACHAUD~1\AppData\Roaming\MATHWO~1\MATLAB\R2009b\gnumex  -mwindows -s C:\USERS\ACHAUD~1\APPDATA\LOCAL\TEMP\MEX_FZ~1\file.obj -llibmx -llibmex -llibmat

[Philip Borghesani]

What are the symptoms of the crash? Does MATLAB just disappear? Is there a windows dialog box? Have you looked in your temporary directory for "matlab_crash_dump.*" files?

To debug this I suggest using windbg available for free from Microsoft as part of the Windows Driver Kit (WDK) this debugger is much more powerful and stable then the Visual Studio debugger but has a user interface that is a bit lacking.

I do not see anything that should be causing the problem but 64 bit mex files should not have MX_COMPAT_32 defined are you missing a compile option to disable this?

[amit chaudhary]

The Matlab just disappers. There is no windows dialog box. I have looked into temporary directory and there are no matlab_crash_dump files.

Also, MX_COMPAT_32 is defined in mex.pl file. I have disabled that option and recompiled. The problem still persists.

Looking into windbg as I am unfamiliar with it.

[amit chaudhary]

I am not sure how to use windbg. I can attach the matlab process to windbg but then I do not know how to execute file.mexw64. Please guide me how to proceed further.

[Philip Borghesani]

Once attached use the g (go) command to resume MATLAB. Then just run the mex file normally from MATLAB. The program should break at the first uncaught exception. It is frequently simpler and faster to debug if MATLAB is started with the -nojvm option to disable all Java support reducing the number of threads and removing Java's use of access violations to manage the heap.

[amit chaudhary]

Thank you Philip.

Looked at the debugger and found out that mex file was using libraries from cygwin64 from earlier installation as well. To make the previous installation work, I had to remove -mno-cygwin flag from the mexopts.bat generated by gnumex manually as it was incompatible with the installed gcc version. Somehow, everything apart from FILE open worked.

So did a fresh install of mingw64 and appended environment paths accordingly. Then reinstalled gnumex using mingw64 and now everything works fine.