MATLAB: MATLAB returns empty string from C-MEX file

Question

I have an external C function which takes a double pointer and returns a char. The problem is that when I interface that function, I cannot extract the character. Always an empty string is returned by MATLAB. What is the problem? The mex compiler returns no errors and warnings.

Here is a minimum working example:

#include "mex.h"
char mwe(double* a)
{
    return 'h';
}
/* Gateway function */
void mexFunction( int nlhs, mxArray *plhs[],
                  int nrhs, const mxArray *prhs[])
{
    char *code;
    double *input;
    
    input = mxGetPr(prhs[0]);
    
    *code = mwe(input);
    
    plhs[0] = mxCreateString(code);
}

Answer 1

It is hard to believe that this code did not crash MATLAB. The issues:

    char *code;  <-- This is an uninitialized pointer ... it contains a garbage address
    double *input;
    input = mxGetPr(prhs[0]);
    *code = mwe(input); <-- this copies a char into the garbage address (should have crashed MATLAB here)
    plhs[0] = mxCreateString(code); <-- Attempts to create a MATLAB char array from the bytes at that garbage address

That mxCreateString( ) call could easily have crashed MATLAB as well. If you want to copy characters into a pointer, then you need to allocate that pointer first. E.g.,

code = mxMalloc(2);
    :
code[0] = mwe(input);
code[1] = '\0';  /* Null terminate the string */
    :
mxFree(code);

It is not really clear what your ultimate intent is passing a double pointer into mwe and getting a char passed back, so I can't give you any advice there unless you give more details.