...call an assembly-coded function from C or C++?

How do I go about writing a function in assembly code and calling it from C or C++?

The main thing to be aware of is the ARM's procedure call standard. This is described in detail at http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042c/IHI0042C_aapcs.pdf

For the assembly code function's entry point to be visible to other modules you should use the .global directive

For example, here is a very simple function written in assembly code, it takes one parameter which it doubles and returns:

    .text  32
    global func
func:
    add    r0, r0, r0
    bx     lr

From C, you would call this function as follows:

unsigned func(unsigned n);

int main(void) {
  unsigned n = func(10);
  return 0;
}

To call this function from C++, the only difference is that you would need to declare the function as an external C function:

extern "C" unsigned func(unsigned n);

int main(void) {
  unsigned n = func(10);
  return 0;
}