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arrayscpointersconstants

typedef for constant pointer to constant data function array

I have a C header file (.h):

typedef uint8_t paraFunction(uint8_t paraVal, uint8_t paraNum);
paraFunction *paraCallTable[256];

And I have a C source file (.c):

paraFunction *paraCallTable[] = {   fn1, fn2, ... fn255, fn256 };

However, the array ends up in the data section. I want it to be in code/flash because the function addresses (the array data) are constant. The array is used by functions in other source files.

How do I rewrite these declarations to make it all constant? And where do they need to be (.c or .h)?

I googled and tried a gazillion different ways and I obviously just don't get it.

Solution

  • Since you are already using the clearest (IMHO) form of function pointer typedef, it's simply a matter of adding const to the declaration at the right side of the *, like you would with an object pointer you'd like to end up in ROM:

    paraFunction *const paraCallTable[]

    Had you used the form typedef uint8_t (*paraFunction)(uint8_t paraVal, uint8_t paraNum); then you'd have to use the far less obvious and confusing form const paraFunction paraCallTable[]. This is a read-only pointer to data, not read-only data or pointer to read-only data as one might think.

    This is a solid argument for sticking to the good typedef style you are already using.

    We may also note that it's good practice to always declare function pointer tables as * const, ROM or no ROM, since changing function pointers in run-time is most often undesired.