The Unix system call which establishes a mapping between a range of addresses in a user process's address space and a portion of some "memory object" (typically a file, one of the special "devices" /dev/mem or /dev/kmem or some memory-mapped peripheral). This allows the process to access a file at random byte offsets without using the seek system call or to access physical addresses or kernel's virtual address space. It can also be used as an alternative to writing a device driver since it is usually simpler to code and faster to use.