sizeof(long) returns 8 bytes but the &along (address of a long) is 12 hex digits (48-bits) or 6 bytes. On OS X 64-bit compiled with clang. Is there a discrepancy here or is this a genuine 64-bit address space?
In effect modern systems use 64 bit for addresses but not all of these are used by the system since 2 to the 64 is an amount of memory no one will use in a near future. Under linux you can find out what your machine actually uses by doing cat /proc/cpuinfo. On my machine I have e.g
address sizes : 36 bits physical, 48 bits virtual
so this would allow for 64 GiB of physical memory and 256 TiB of virtual memory. This is largely enough for what this machine will ever encounter.