structure alignement problem ?

Your variable 'buffer' is an array of bytes (alt_u8), so when you declare it, you are not guaranteed it will be aligned on word boundaries, only byte boundaries since that's it's type. You then assign your structure pointer an address based on where 'buffer' was put. Therefore, you can only expect that your structure pointer is going to be aligned on a byte boundary. If you declare your buffer as an array of words (alt_u32), not bytes, I imagine it's starting address will be on a word boundary, and you'll be in good shape.