FILEMAPREAD

FILEMAPREAD is a function described in some memory-mapped file APIs that reads data from a mapped region of a file into a user-provided buffer. It is designed to supply explicit input semantics for applications that prefer a read-style interface over direct pointer access to a mapped view.

A typical description of the function is that it takes a mapping view or handle, an offset within the mapped region, a requested byte count, and a destination buffer. It returns the actual number of bytes read or an error indicator if the operation fails. The data read reflects the current contents of the mapped region as seen by the process, subject to the operating system’s paging and consistency rules.

Behavior and constraints vary by implementation, but common characteristics include: reads do not modify the underlying file or the mapped view’s state beyond advancing the user buffer; if the requested range extends beyond the mapped region, the function may return a partial result or signal an error depending on the API. Reads can incur page faults if the needed pages are not resident, and performance depends on mapping granularity, paging behavior, and concurrent access patterns. Some APIs synchronize with flush or cache-coherence mechanisms, while others rely on the operating system’s natural paging.

In practice, FILEMAPREAD is not a universal standard; it appears in certain legacy or specialized libraries and is often superseded by direct access to the mapped memory or by higher-level I/O abstractions. See also memory-mapped files, MapViewOfFile, mmap, and related file-mapping functions.