Mex files: mxCreateXXX only inside main mexFunction()? - matlab

Question

I have a very basic mex file example here:
#include "mex.h"
#include "matrix.h"
void createStructureArray(mxArray* main_array)
{
const char* Title[] = { "first", "second" };
main_array = mxCreateStructMatrix(1,1, 2, Title);
}
void mexFunction(mwSize nlhs, mxArray *plhs[], mwSize nrhs,
const mxArray *prhs[])
{
double* x = mxGetPr(prhs[0]);
if (*x < 1.0)
{
//This works
const char* Title[] = { "first", "second" };
plhs[0] = mxCreateStructMatrix(1,1, 2, Title);
}
else
{
//This does not
createStructureArray(plhs[0]);
}
}
This function should always return a struct with the elements first and second. No matter the input, I expect the same output. However with an input parameter < 1, everything works as expected, but > 1 I get an error message:
>> a = easy_example(0.0)
a =
first: []
second: []
>> a = easy_example(2.0)
One or more output arguments not assigned during call to "easy_example".
Thus, can I not call the mxCreateStructMatrix function outside mexFunction, or did I do something wrong when passing the pointers?

Answer 1

You don't have a problem with mex but with pointers!
Try to change your function to:
void createStructureArray(mxArray** main_array)
{
const char* Title[] = { "first", "second" };
*main_array = mxCreateStructMatrix(1,1, 2, Title);
}
and the function call to
createStructureArray(&plhs[0]);
Your problem is that plhs[0] is a mxArray, but in order to return it, you need to pass the pointer to that mxArray!