Accessing matrices stored inside of cell arrays using MEX files in MATLAB
c, matlab, mex
Solution
Lots of issues in that code sample:
- You're missing trailing semicolons on every single line
- `mxArray`s must be defined as pointers: e.g., `mxArray *C_CELL`.
But based upon the error message you're getting, I suppose you have those items correct in the source you're using.
Your main problem is that you're assigning the output of `mxGetData` to an `mxArray *`. The way `mxGetData` works is that it returns a raw (`void *`) pointer to your matrix contents. In this case, your matrix contains an array of `mxArray` pointers, with two levels of pointer redirection.
As such, `C_CELL` must be defined as `mxArray **C_CELL`. Note that doing this without any sanity checks or bound checking will rapidly lead to segfaults. Using `mxGetCell` as @David Heffernan suggests is a safer and more surefire way to do this (assuming you've checked that your `prhs[0]` contains a cell array).
Problem
I am currently writing a MEX function that will have to work with a cell array in MATLAB. The MEX file is written in C. Essentially, the input to my function will be a cell array where each entry is a numeric matrix with real values. A simple example is: ``` C = cell(1,2); C{1} = ones(10,10); C{2} = zeros(10,4); ``` I would like to be able to access the numeric arrays C{1} and C{2} in my MEX file. Ideally, I would like to do this without having to create a second copy of the data in my MEX File (i.e. get pointers for them). Using the previous example, my current approach is as follows: ``` /* declare a pointer variable to the incoming cell array after it is passed to the MEX function */ mxArray C_CELL = (mxArray *) mxGetData(prhs[0]) /* declare a 2 x 1 array of pointers to access the cell array in C */ double *myarray[2] // /* point towards the contents of C_CELL */ myarray[0] = mxGetPr(C_CELL[0]) myarray[1] = mxGetPr(C_CELL[1]) ``` Unfortunately this seems to yield "invalid use of undefined type 'struct mxArray_tag'" errors.