Extract data from uint8 to double

Question

I have a C function receiving a uint8 pointer with another parameter which is its size (number of bytes).

I want to extract double data from this buffer. Here is my code:

Write(uint8* data, uint8 size) /* data and size are given by a callback to my function)*/
{
    double d;
    for (i = 0; i < size; i++)
    {
        d = ((double*)&data)[i];
        printf(" d = %d\n");
    }
}

The problem is that I am not receiving what I am sending within an external hardware. I guess that my cast is wrong. I tried other methods but without any good result. I am still not able to get what I send.

Answer 1

If I understand your question (it's not entirely clear if there's at most one double to extract or there can be many), here's what I would try doing:

double Write(uint8* data, uint8 size)
{
    double d;

    if (size < sizeof(d))
        return 0; // failure; you may want something smarter than this

    memcpy(&d, data, sizeof(d));

    return d;
}

What this avoids is a potential alignment issue in a cast like d = *(double*)data.

This may fail in odd and ugly ways if the data does not represent a valid double, specifically if it's reversed (e.g. your hardware is little-endian and the CPU running this code is big-endian or the other way around).

I don't know yet if aliasing issues apply. Need to read the standard again. Never saw them in practice, though.