Pointer to function in ROM

Question

I have microcontroler that I am working with. When debugging it is necessary to call a function from that is hard coded in ROM. Technical Reference shows how to do this:

# define Device_cal (void(*)(void))0x3D7C80

and calling procedure looks like this:

(*Device_cal)()

I can't understand what actually happens here, so my question is: How does it work?

Answer 1

void (*) (void) is a type. It's a pointer to a function that takes no parameter and returns void.

(void(*)(void)) 0x3D7C80 casts the 0x3D7C80 integer to this function pointer.

(*Device_cal)() calls the function.

(Device_cal)() would do the exactly the same.

The parentheses around *Device_cal and Device_cal are required because otherwise the cast to the integer would not have the higher precedence.

Answer 2

The #define causes (*Device_cal)() to be expanded into this immediately before compiling:

(*(void(*)(void))0x3D7C80)()

The void(*)(void) is a declaration for a function pointer that takes void and returns void types. The (*()) represents a cast for the next token in the expression (0x3D7C80). Thus this asks to treat the data at location 0x3D7C80 as a function. The final () calls the function with no arguments.

Answer 3

well, you "define" a pointer to function, and call it. void(*)(void) mean a pointer to function, that gets no arguments, and return void. If you cast 0x3D7C80 to that type, and call it, you basically call the function that its address is 0x3D7C80.