C coding design - function pointers?

Question

I have a PIC18F46K22 and program it with the XC8 compiler. In the end, I'll have a system like a pc with stdin and stdout. So in the main loop there will be a function which is checking if there is new input. If there's input, a function will be called accordingly. So for example when I input an A on stdin, the PIC will run a function like function_A instead of function_B which is called when I input a B.

When the PIC is done with the function, I want that the new input is sent to the function. So when pressing A opens the RS232 transmitter, from that moment every input will be sent over RS232. In the end the project is a stand-alone text editor. So when pressing A opens the file system, from that moment you aren't text editing anymore but looking through a list of files. That means that pressing Up and Down means something different than in the text-editing environment.

I've done a lot thinking on how to program this in C. I thought this up last night and would like to know if it's possible and if so, how. What I want to do is:

• The main function calls a function like function_A
• function_A changes a global variable function_addr to the address pointer of function in_function_A
• From that moment, main calls the function at function_addr when there is new input.

So what I'd need is a main function which checks if function_addr is zero. If it is, a 'normal' function should be called, like function_A. If it isn't, the function at function_addr should be called. I also need a function_A which changes the function_addr to a pointer to in_function_A.

Note: when the filesystem function should be closed, is_function_A should just change function_addr to 0.

So basically my question is how can I

• Get the address of a function (and store it in a variable)
• Call a function at a specified address

Answer 1

A function

int f(int x){ .. }

Get the address of a function (and store it in a variable)

int (*fp)(int) = f;

Call a function at a specified address

int x = (*fp)( 12 );

Answer 2

While Wouters answer is absolutely correct this maybe more beginner friendly and a better example regarding your question.

int (*fp)(int) = NULL;

int FunctionA(int x){ 
    return x + x;
}

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

void Example(void) {
    int x = 0;

    fp = FunctionA;
    x = fp(3);  /* after this, x == 6 */

    fp = FunctionB;
    x = fp(3);  /* after this, x == 9 */
}

If it is not obvious that the function pointer actually has a target function address assigned, its wise to perform a NULL check.

if (fp != NULL)
    fp(); /* with parameters, if applicable */