doubts about bin_at

[Bin Chen]

Hi,

In malloc.c defines bin_at macro, but I don't know why this way bin_at 
can find the bin address,

#define bin_at(m, i) ((mbinptr)((char*)&((m)->bins[(i)<<1]) - (SIZE_SZ<<1)))

Anyone help to explain this?

Thanks a lot.
Bin Chen