using an integer function return value to return a pointer - arduino

Question

I am writing a serial command interpreter. The user will send a text string to the interpreter and it will do stuff and return an integer (either data or a code depending on what the user requested). But I want to expand the interpreter and allow the user to get an array of data or other structure in response to their query.
Can I use the integer return value to return a pointer to EEPROM (or global variable) address? And have the user follow the pointer to the memory location? Based on the query they sent, they would know if the return value is a pointer or data integer.
for example if I want to return
struct curve_t {
int type; // (2 bytes) calibration type indicator
int ref[2]; // (4 bytes) calibration reference point2
float param[11]; // (11*4 bytes) curve fitting parameters
} theCurve;
can I use a function like this?
int serialResponse(char * command) {
// interpret command here
return &theCurve;
}

Answer 1

Can you send a memory address through serial interface?
YES
Can your user access EEPROM through serial interface, using that address?
Not directly. Your MCU has to relay the data between your user and the EEPROM.

Answer 2

I wrote a small test program and confirmed that it is possible. I can pass the address from the function as an integer and then re-cast it in my calling function. It needs to address a global variable or at least on that is available in the calling function.
char res[10];
void loop {
b = function();
Serial.println((char *)b);
}
int function() {
return int(&res[0]);
}

Answer 3

I would not recommend casting a pointer into an integer because it won't work on computer architectures where an int has fewer bits than a pointer.
Lexical Parsers - like what you're writing - often arrange to return a token type, and place the token value in a union that the caller can access. The nice thing about structuring your code in that way is that it's extensible to whatever data types you want, and it will work no matter what C++ platform you're running on.
Here's an example of a token parser that can parse integers and your curve_t:
struct curve_t {
int type; // (2 bytes) calibration type indicator
int ref[2]; // (4 bytes) calibration reference point2
float param[11]; // (11*4 bytes) curve fitting parameters
};
union TokenValue {
int i; // type = TOKEN_TYPE_INT
struct curve_t *pCurve; // type = TOKEN_TYPE_P_CURVE
};
enum TokenType {
TOKEN_TYPE_UNKNOWN = 0,
TOKEN_TYPE_INT,
TOKEN_TYPE_P_CURVE
};
curve_t theCurve;
TokenValue tokenValue;
/*
* Parses the given command,
* setting the parsed value in tokenValue,
* returning the type of value (a TOKEN_TYPE_*).
*/
TokenType serialResponse(char * command) {
if (command[0] == 'a') { // TO DO: your code will test something else.
// We want to return an integer
tokenValue.i = 1234; // TO DO: in your code, instead set the integer value from command
return TOKEN_TYPE_INT;
}
if (command[0] == 'b') { // TO DO: your code will test something else.
// We want to return a pointer to theCurve.
// TO DO: Fill in the values of theCurve, for example theCurve.param[0]
tokenValue.pCurve = &theCurve;
return TOKEN_TYPE_P_CURVE;
}
// Else
return TOKEN_TYPE_UNKNOWN;
}
void setup() {
//TO DO: move this code to where it belongs in your Sketch
//TO DO: parse a command
char command[10] = "and so...";
// TO DO: read the command.
// Process the command
enum TokenType t;
t = serialResponse(command);
if (t == TOKEN_TYPE_INT) {
// The command result is an integer
int i = tokenValue.i;
// TO DO: process the integer.
} else if (t == TOKEN_TYPE_P_CURVE) {
// The command result is a curve
curve_t *pCurve = tokenValue.pCurve;
// TO DO: process the Curve.
} else {
// unrecognized command. TO DO: handle the error.
}
}
void loop() {
// put your main code here, to run repeatedly:
}
If you insist on using the cast of an int to a pointer (which I admit is a lot simpler), you could add a test for int size problems to your setup():
void setup() {
Serial.begin(9600);
if (sizeof(int) < sizeof(curve_t *)) {
Serial.println("cast won't work");
for (;;) {} // hang here forever.
}
}