A special gcc type, which is, not surprisingly, twice the size of a long. On a 32-bit architecture, such as Intel, this means 64 bits.
By the way, the following will break g++ (or gcc in c++ mode):
long long x=0x1234567812345678;
Since g++ says that any number above 32 bits on Intel is too large! It is quite easy to patch gcc around this, though I made an ugly gcc patch to get Pulse/IT3/Audiomate/OVE to compile. But I hope someone makes a clean one soonish that is fit for maintainer consumption, because it's a pain!