Mex dynamic memory management issue with std::vector in linked external DLL; Segmentation error

Question

I am trying to create a mex file that interfaces MATLAB with an external C++ library that communicates with some hardware. An imported library and precompiled DLL (.lib and .dll) are provided by the hardware vendor for my version of VC++ and I was able to implement them in C++ without any issue.

However, I ran into segmentation error at run time when the code is written as a mex(compiled with the same version of VC++). After some investigation with the VC++ debugger, the likely culprit seems to be the fact that one of the external dll functions returns the data type std::vector, and probably tries to dynamically allocate memory for the vector container somewhere inside the function. I know that if I use std::vector in my own mex function, everything works fine, but I suspect that the mex header itself wraps the std::vector container in my own code for memory management(?) as required for all dynamically allocated memory in mex codes, whereas it can't do the same for the pre-compiled .dll.

Now the question is: since I cannot modify the external .dll file and have no access to its source files, are there any ways to work with this external dll such that the dynamic memory becomes managed by MATLAB(perhaps a wrapper of some sort..?)...and thereby avoid the segmentation error and return the correct data? Or if my analysis is wrong please correct me too!

Please let me know if there are any ideas or hacks, thanks!

My system: Windows 7 SP1 32 bit, MATLAB 2009b, Visual C++ 2008 Pro.

I also posted the same question at: http://www.mathworks.com/matlabcentral/answers/9294-mex-dynamic-memory-management-issue-with-std-vector-in-linked-external-dll-segmentation-error .You also can share your insights there if you have an account, thanks!

Answer 1

Thanks everyone for the answers and comments. I was able to resolve the issue with some help from the friendly folks at MathWorks.

From the original post at http://www.mathworks.com/matlabcentral/answers/9294-mex-dynamic-memory-management-issue-with-std-vector-in-linked-external-dll-segmentation-error :

You are probably seeing an incompatibility between the stl library and or compiler options used by your pre-compiled dll and those used by MATLAB and the MEX command. MATLAB 2009b was built with MSVC 2005.

You may be able to fix the problem by changing the options used by mex or by building your mex file directly with MSVC. One example of an option that may effect things is SECURE_SCL=0. I would start by building your test program with the options MATLAB is using to find the problematic option then try removing that option when building the mex file.

Because of this sort of incompatibility use of stl objects in the api's of third party compiled libraries is usually a bad idea.

Following his advice, I removed the SECURE_SCL=0 option from the mex options file at

C:\Users\(username)\AppData\Roaming\MathWorks\MATLAB\R2009b\mexopts.bat

Then recompiled the mex file, now everything works like a charm - the function is returning the correct data and segmentation error no longer occurs.

Answer 2

The MEX API doesn't do anything special with STL containers, since they cannot be passed between MATLAB and a MEX-function (the only non-primitive data type that can do that is the mxArray). It's basically up to the MEX-function to make sure that the memory used by the STL container is handled properly; MATLAB doesn't track it.

Passing a std::vector across a DLL boundary is somewhat tricky. I'd assume the vendor would be aware of this, and provide you with an appropriate header file with the correct declspecs and such, but in case they didn't, you might want to refer to this Microsoft support link to read more about what is required.