unmapping

Unmapping is the process of releasing a region of memory or a file that has been mapped into a program's address space. Mapping creates a view or a mapping between a virtual address and a backing store, such as a regular file, a device, or anonymous memory. Unmapping removes that view, after which references to the region may no longer be valid and the resources used by the mapping may be released.

Common contexts include memory-mapped files, shared memory, and anonymous mappings created for inter-process communication or for allocation of large buffers without committing physical pages upfront. In Unix-like systems, the operation is performed by munmap, which takes a starting address and a length; the region must correspond to a previously mapped area and is typically required to be aligned to the system page size. In Windows, UnmapViewOfFile removes a mapped view of a file from a process’s address space, while functions like FlushViewOfFile may be used to ensure changes are written to disk before unmapping.

Unmapping affects program behavior: once unmapped, the address range is no longer accessible, and any access may trigger a fault. The underlying resource is released when no mappings remain, allowing the OS to reclaim memory or file-backed pages. Unmapping is an important aspect of resource management and can be necessary to avoid address space exhaustion, to synchronize changes to storage, or to enable remapping with different protections.

See also: munmap, UnmapViewOfFile, mmap, memory-mapped file, virtual memory.