mmap - How big can a memory-mapped file be?

Question

What limits the size of a memory-mapped file? I know it can't be bigger than the largest continuous chunk of unallocated address space, and that there should be enough free disk space. But are there other limits?

Answer 1

You're being too conservative: A memory-mapped file can be larger than the address space. The view of the memory-mapped file is limited by OS memory constraints, but that's only the part of the file you're looking at at one time. (And I guess technically you could map multiple views of discontinuous parts of the file at once, so aside from overhead and page length constraints, it's only the total # of bytes you're looking at that poses a limit. You could look at bytes [0 to 1024] and bytes [2⁴⁰ to 2⁴⁰ + 1024] with two separate views.)

In MS Windows, look at the MapViewOfFile function. It effectively takes a 64-bit file offset and a 32-bit length.