C memcpy in reverse

Question

I am working with audio data. I'd like to play the sample file in reverse. The data is stored as unsigned ints and packed nice and tight. Is there a way to call memcpy that will copy in reverse order. i.e. if I had 1,2,3,4 stored in an array, could I call memcpy and magically reverse them so I get 4,3,2,1.

Answer 1

This works for copying ints in reverse:

void reverse_intcpy(int *restrict dst, const int *restrict src, size_t n)
{
    size_t i;

    for (i=0; i < n; ++i)
        dst[n-1-i] = src[i];

}

Just like memcpy(), the regions pointed-to by dst and src must not overlap.

If you want to reverse in-place:

void reverse_ints(int *data, size_t n)
{
    size_t i;

    for (i=0; i < n/2; ++i) {
        int tmp = data[i];
        data[i] = data[n - 1 - i];
        data[n - 1 - i] = tmp;
    }
}

Both the functions above are portable. You might be able to make them faster by using hardware-specific code.

(I haven't tested the code for correctness.)

Answer 2

No, memcpy won't do that backwards. If you're working in C, write a function to do it. If you're really working in C++ use std::reverse or std::reverse_copy.