Using Function Pointer from struct in PROGMEM in C on Arduino

Question

I am trying to read a function pointer from a structure stored in PROGMEM, then passing a value (input) to the corresponding function and save the returned value, but i can not find the correct syntax.

uint8_t (*pStateFunc) (uint8_t);
uint8_t input; 
uint8_t nextstate;

enum MENUSTATES {STATE1, STATE2};

typedef struct PROGMEM {
  unsigned char state;
  uint16_t someNumber;   // Just arbitrary information
  uint8_t (*pFunc) (uint8_t input);
} MENU_STATE;

const MENU_STATE menu_state[] PROGMEM = {
  //  state  someNumber  pFunc
  {STATE1,   2,          NULL}, 
  {STATE2,   4,          doSomething},
  {0,        0,          NULL}
  };


// Get the Function 
pStateFunc = (PGM_VOID_P) pgm_read_byte(&menu_state[1].pFunc); 

// Execute the Function and save the returned state
nextstate = pStateFunc(input);

// Function Definition
uint8_t doSomething(u8int_t input){
    return STATE1;
}

All i get is the following error from the Arduino IDE 1.6.5:

invalid conversion from 'const void*' to 'uint8_t (*)(uint8_t) {aka unsigned char (*)(unsigned char)}' [-fpermissive]

How can i read the function from PROGMEM and execute it correctly?

What is this pStateFunc = (PGM_VOID_P) pgm_read_byte(&menu_state[1].pFunc); supposed to do? — alk
– alk, Commented Jan 3, 2016 at 13:49
The initialiser to menu_state[] seems to be missing a closing brace (}). — alk
– alk, Commented Jan 3, 2016 at 13:50
It should read the doSomething Pointer from the menu_statestruct progmem. I added the missing closing brace... sorry forgot that before. — Catherine Deringer
– Catherine Deringer, Commented Jan 3, 2016 at 13:51

Alan Stokes · Accepted Answer · 2016-01-03 13:53:29Z

1

You seem to be reading one byte - I would have thought pgm_read_ptr would be more appropriate. And you need to cast it to the right type:

typedef uint8_t (*StateFunc) (uint8_t);
pStateFunc = (StateFunc) pgm_read_ptr(&menu_state[1].pFunc);

answered Jan 3, 2016 at 13:53

Alan Stokes

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1 Comment

Catherine Deringer Over a year ago

That did it! Thanks for your support!

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