It does a memory map (mmap). Your file is addressed in virtual memory. See https://en.wikipedia.org/wiki/Mmap https://en.wikipedia.org/wiki/Mmap and https://en.

It does a memory map (mmap). Your file is addressed in virtual memory. See https://en.wikipedia.org/wiki/Mmap https://en.wikipedia.org/wiki/Mmap and https://en.wikipedia.org/wiki/Memory-mapped_file https://en.wikipedia.org/wiki/Memory-mapped_file