In C, there’s no such thing as a function being const or otherwise, so a pointer to a const function is meaningless (shouldn’t compile, though I haven’t checked with any particular compiler).
Note that although it’s different, you can have a const pointer to a function, a pointer to function returning const, etc. Essentially everything but the function itself can be const. Consider a few examples:
// normal pointer to function
int (*func)(int);
// pointer to const function -- not allowed
int (const *func)(int);
// const pointer to function. Allowed, must be initialized.
int (*const func)(int) = some_func;
// Bonus: pointer to function returning pointer to const
void const *(*func)(int);
// triple bonus: const pointer to function returning pointer to const.
void const *(*const func)(int) = func.
As far as passing a pointer to a function as a parameter goes, it’s pretty simple. You normally want to just pass a pointer to the correct type. However, a pointer to any type of function can be converted to a pointer to some other type of function, then back to its original type, and retain the original value.