Passing string (char*) back from c function in Swift 3.0

Question

I'm working on sending a bunch of data to and from a program written in c++ to a Swift 3 user interface. I'm trying to do this by just passing a long json string to a simple c function that takes a char string as an input and output.

Here is the only file I include in the bridging header:

#ifndef ModelInterface_h
#define ModelInterface_h

#ifdef __cplusplus
extern "C" {
#endif

    const char* ModelInterface(const char *input);

#ifdef __cplusplus
}
#endif


#endif /* ModelInterface_h */

Here is how I call it in Swift 3.0

let input = "Send In"
let test = ModelInterface(input)
print("Test \(String(cString: test!)) <-- test")

Here is the code in the cpp file:

#include "json.hpp"
#include "ModelInterface.h"
#include "ShellSitzwohl.h"

// for convenience
using json = nlohmann::json;

const char* ModelInterface(const char *input) {

    ShellSitzwohl shell = ShellSitzwohl();
    ShellFullOutput output = shell.Stress(10, 10);

    // conversion: person -> json
    json j = output;
    std::stringstream stringstream;
    stringstream << j;

    const std::string tmp = stringstream.str();
    const char* cstr = tmp.c_str(); // I can see that cstr is populated correctly

    // return "This works!"; // If I just return this then it gets displayed
    // Test This works! <-- test
    return cstr; // This runs but not displayed correctly
    // Test  <-- test
}

The json string appears to be created and if I pass back the static string "This works!" it gets displayed correctly. Not sure if something is being released that shouldn't before the string gets back to swift.