mmap (2)

Particularly useful when the file is a device file! Allows to access device I/O memory and ports without having to go through (expensive) read, write or ioctl calls!

X server example (maps excerpt)

start end perm offset major:minor inode mapped file name 08047000-081be000 r-xp 00000000 03:05 310295 /usr/X11R6/bin/Xorg 081be000-081f0000 rw-p 00176000 03:05 310295 /usr/X11R6/bin/Xorg ... f4e08000-f4f09000 rw-s e0000000 03:05 655295 /dev/dri/card0 f4f09000-f4f0b000 rw-s 4281a000 03:05 655295 /dev/dri/card0 f4f0b000-f6f0b000 rw-s e8000000 03:05 652822 /dev/mem f6f0b000-f6f8b000 rw-s fcff0000 03:05 652822 /dev/mem

A more user friendly way to get such information: pmap <pid>