Problem with Pointer to const function

Michu · Joined: 05 Dec 2011 Posts: 3

Hi everybody

I have a look-up table with a string and a pointer-to-function. During runtime i want to call these functions. As long as the LUT is in RAM, everything works. As soon as i uncomment the "const", i can't compile it anymore. I obviously identified the problem but am not able to solve it which is kind of frustrating... Sad

This is the corresponding code:

Michu · Joined: 05 Dec 2011 Posts: 3

I forgot to post the error-message i get when i compile it:

RF_Developer · Joined: 07 Feb 2011 Posts: 839

There's a number of possibilities. All come from the fact that PIC have a Harvard architecture which has different busses and address spaces for data (RAM) and instructions. C assumes a more conventional Von Neumann architecture with a single address space for both (though some later PDP-11s used memory mapping to provide seperate I and D spaces which Unix, and maybe other OSs, exploited to give larger virtual task space).

The key here is that const requests that your table is placed in a different address space, in fact a totally different memory area, i.e. program flash. On the 16Fs that space is not even byte wide, its fourteen bit. Because of the difficulties in sorting all this out, it didn't support function pointers at all until fairly recently. It does, but there are limitations. I don't use funtion pointers very often, the sort of command parser thing you're doing is the most likely task which I#d use it for, and as I think you know, its useful to push such tables out to program memory as the strings take up a lot of space.

For the reasons above arrays in program memory are more complex for the compiler to sort out. They are slower and there are restrictions in how they are used, particularly with strings, where unrtil very recently it wasn't possible to form pointers to characters in strings except in certain special cases where CCS had provided routines specifically that dealt with them. The Microchip and Hitec compilers deal with all this differently and Microchip have a load of different versions of the C standard string & memory manipulation functions.

So, you have function pointers, which are tricky for any PIC compiler, and tables in program memory, i.e. const, which are also tricky. Combine the two, tables of function pointers (AND strings!) in program memory and perhaps it should not be surprising that something breaks :-( The key problem is that creating a pointer to a const is not allowed, except in the special string cases.

I've experimented a bit and it seems as though you can't put function pointer tables into program memory :-( I've tried taking out the strings but that doesn't help. Looks like you're stuck with RAM based tables. For that I suspect you'll be needing a bigger PIC.

RF Developer

Ttelmah · Joined: 11 Mar 2010 Posts: 19513

RF_Developer has already explained why this is difficult in the PIC. However the compiler can do it.
If you declare a variable as 'rom', instead of 'const', the compiler adds the extra code to allow pointers to it. In your compiler directory, you should have a 'readme.txt', explaining this.

However there is a second problem. You cannot have pointers inside a variable declared at compile time, so definately not in a const, or rom declared variable...

Best Wishes