Pass struct to C++ DLL library with strings

I have a simple C++ program, where I pass a c_struct by reference and am able to update the values for the doubles. Now I would like to also pass back a string. But it fails on the construction:

---

>> sc = libstruct('c_struct', sm)
Error using feval
Cannot convert data value for field p3 due to error:
Parameter can not be converted to a character vector

Error in libstruct (line 16)
    ptr=feval(['lib.' structtype],initialvalue);
----

How do I pass back a string in the struct. Here is the Header and the Code:

#pragma once
#include "helper.hpp"

struct c_struct {
  double p1;
  double p2;
  char* p3;
};

#ifdef  __cplusplus
extern "C" {
#endif
  EXPORTED_FUNCTION int sq(int x);
  EXPORTED_FUNCTION void lmb(struct c_struct *st);
#ifdef  __cplusplus
}
#endif

----

#include "stdafx.h"
#include "helper.hpp"
#include "simple.hpp"

int sq(int x)
{
  return (x*x);
}

void lmb(struct c_struct *st) {
  st->p1 = 3.0;
  st->p2 = 4.0;
  st->p3 = "goodbye";
}

[Albert Fan]

How did you defined you sm variable?

Answer 1

[Doctor G]

as a struct.

e.g. sm.p1 = 3 sm.p2 = 5 sm.p3 = "foobar"

But anyway, I might have to give this approach up, since I need a C++ class that has two methods. One to open up a TCP connection (nanomsg) and the other to return the next packet.

I am thinking of re-doing this in C#, since it has much better API support with Matlab.