You are confusing ABIs with what the C standards mandate. A compiler emitting code for a particular architecture need to conform to that architecture's ABI. Or else it won't be able to interface with syscalls or other libraries. This goes for other things too like calling conventions and setup of stack frames. As klodolph states, long is 64 bit on x86_64, (32 on x86_64-win) and 32 on x86.