filemapping

File mapping, commonly called memory-mapped file, is a mechanism that maps a file or a portion of a file into a process’s address space. Once mapped, the file contents can be accessed through ordinary memory reads and writes as if they resided in RAM. The operating system handles paging data in and out of disk, and changes may be propagated back to the file depending on the mapping mode and permissions.

Implementation varies by platform. In Windows, the process uses APIs such as CreateFileMapping and MapViewOfFile to create a mapping object and a view into the file. In POSIX systems, mmap creates the mapping and munmap releases it. Mappings can be read-only, read-write, or executable, and can be shared between processes or private to one process. When multiple processes map the same file with a shared mapping, changes are visible to all mappings.

Benefits include reduced data copying since I/O can occur via the page cache, simplified access to large or random-access data, and the ability to share data between processes through a common mapped region. This technique is often advantageous for large datasets and for zero-copy I/O patterns, where computation overlaps with data loading.

Common considerations or limitations include platform-specific behavior, synchronization requirements for shared mappings, and potential page faults that can cause latency spikes. Large mappings consume virtual address space and may behave differently when the underlying file changes size. Proper use requires attention to access modes, synchronization, and lifecycle of the mapped region to avoid inconsistencies.