return array from c++ dll to matlab

Question

I am trying to build a DLL in C++ and call it from MATLAB by using loadlibrary and calllib instructions. It works for one value and it return a value normally, but now I am trying to return a whole array from C++ DLL to MATLAB as an output of a function.

As you know C++ usually return the arrays as pointers but this don't work with MATLAB ... I searched for that at the internet and they are using some MEX function but it is not clear ...

Can you explain how to return an array from C++ DLL to MATLAB calllib and how should we return it from C++ code ?

Answer 1

Consider a DLL that exposes the following C function:

void getData(double *x, const int len)
{
    for(int i=0; i<len; i++) {
        x[i] = i;
    }
}

It takes an array already allocated an its length, and fills it with incremental values.

In MATLAB, first we load the library:

>> loadlibrary('mydll.dll', 'mydll.h')
>> libfunctions mydll -full

Functions in library mydll:

doublePtr getData(doublePtr, int32)

To call the exposed function, we use libpointer:

>> p = libpointer('doublePtr', zeros(1,10))  % initialize array of 10 elements
p =
libpointer
>> get(p)
       Value: [0 0 0 0 0 0 0 0 0 0]
    DataType: 'doublePtr'
>> calllib('mydll', 'getData', p, 10)        % call C function
>> get(p)
       Value: [0 1 2 3 4 5 6 7 8 9]
    DataType: 'doublePtr'

we could also simply pass regular vectors and MATLAB will take care of marshalling:

>> x = calllib('mydll', 'getData', zeros(1,10), 10)
x =
     0     1     2     3     4     5     6     7     8     9

note that in this case, the modified array will be returned as an output (since builtin types will not be modified in-place).