In Linux, what kinds of files are memory mapped?

Question

What are the different types of Linux files that can be created entirely in memory?

For example, a pipe file may be created, but does the location of where a file is created (or the filesystem type of the file's path) make a difference to whether a disk access is involved? If I create a pipe file in an ext3 file system, could a physical disk access result?

Answer 1

Off the top of my head, and without looking at any books :D, I think it breaks down like this:

mmap-able:

• files (of course)
• soft-links (final target if it's a file, block device or kernel device)
• hard-links (final target if it's a file, block device or kernel device)
• block devices (/dev/ram1, /dev/sda1, etc..)
• character devices (You can mmap character devices, but in some cases it won't make sense (or work right). For instance an easy way to develop a driver in userland is to have a kernel module handle a basic mmap to your hardware and then expose the hardware via a mmapable character device so that a non-privileged user can access it. (USB, audio, flash cards) use this. A lot of embedded stuff does too.
• unix domain sockets? Does zerocopy/sendfile count?

mmap-able but not a file?

• shared memory

un-memmappable?

• directories
• fifos (one reader, one writer) ?