Watching mxArray Contents within Visual Studio

mxArray is the main type that Matlab uses to expose functionality to external native code. It is the input/output both to mex files and to compiler-generated functions.

mxArray is an opaque type, defined in matrix.h as -

typedef struct mxArray_tag mxArray;

and so setting a regular watch on it while debugging (either mex files or calls to compiled components) results in a null watch. An intervention is in order.

The code in this project is a natvis file customizing the VS debugger to show some of the mxArray contents when setting a watch on an mxArray variable. It also contains a sample mex code, used for demonstration of this enhanced debugging view. To use this natvis in your own project just include it as one of your project files.

Getting Started

There are various ways to demonstrate debugging with the mxArray watch. Here's one:

1. Build the project (it is recommended in DEBUG as watching variables would be easier),
2. Copy the output ToyMex.mexw64 to somewhere in your Matlab path, OR from Matlab cd to the folder containing this mex.
3. From VS, attach to Matlab (Debug / Attach to Process)
4. Set a breakpoint somewhere in mexFunction
5. in the Matlab command window, type something like -

>> A = rand(3)
>> ToyMex(A)

When hovering above the received mxArray variable during VS debugging, you should see something like: When setting a watch on thesame variable, you should see something like:

If you were to send a matrix of uint8's, you'd get:

.. and similarly for other built in types.

You can also watch complex variables - the 1st array is the real component and the 2nd is the imaginary one:

Strings (although the VS debugger expects them to be null-terminated, and they're not):

Structs - again, the field names and contents are stored in separate buffers and so (at least for now) are displayed separately:

If you understand the internals of sparse matrix storage, you can view sparse matrix contents as well:

Single cells are supported too, cell arrays and struct arrays - not yet. More generally, this work is based on rough reversing that was sufficient for my needs and is still very partial. Moreover, mxArray's layout is known to be prone to change between matlab versions. Current natvis has been tested with Matlab 2016a, 2016b & 2017a, but we believe it should work with at least 2014+ versions.

You're very encouraged to play around with this toy sample. Before you rebuild with your changes remember to 'clear mex' in Matlab - otherwise it might prevent you from replacing the mex file.

(c) Ofek Shilon 2017