how to read an integer array in MEX-function

Question

I am passing an integer array of uint8 type from MATLAB to a MEX-function. How do I read these values? I tried using:

int *n;
n = (int * ) mxGetData(prhs[0]);

but the values come out as junk. I also tried

double *n;
n= mxGetPr(prhs[0]);

and in this case junk values also appear.

What is the solution to this?

Basically I want to read the integer value in the MEX-function but mxGetPr returns double type.

Answer 1

You should make sure that the number of bytes you read, and the interpretation of these bytes, is the same in the input data (that comes form Matlab) and in the output array (the array you read into in the mex function). Since uint8 is 8 bits long, both double and int will read wrong number of bytes, and misinterpret these bytes.

Try to include stdint.h header, and use the uint8_t data type for the array you read into.

Answer 2

Take a look at the demo MEX-function explore.c, which you can open in MATLAB using

edit([matlabroot '/extern/examples/mex/explore.c']);

In there you'll find a bunch of functions whose names all start with analyze_ and then a type (for example, analyze_uint8). In those functions you'll see the output of calls to mxGetData being cast to a particular C type, like this:

pr = (unsigned char *)mxGetData(array_ptr);

pr now points to the real part of array_ptr, an array of unsigned char.