mmap

mmap is a POSIX system call that creates a new mapping in the virtual address space of the calling process. It can map a regular file or device into memory, or create an anonymous mapping that is not backed by any file. Mapped regions behave like arrays and can be accessed via pointers, with the actual I/O performed by the kernel as pages are faulted in or out.

Signature and parameters: void* mmap(void* addr, size_t length, int prot, int flags, int fd, off_t offset). The addr hint is the desired address or NULL to let the kernel choose. The length is the mapping size; offset must be a multiple of the system page size. Prot flags specify allowed access: PROT_READ, PROT_WRITE, PROT_EXEC, PROT_NONE. Flags include MAP_SHARED, MAP_PRIVATE, MAP_ANONYMOUS. If MAP_ANONYMOUS is used, fd and offset are ignored; if not, fd refers to an open file descriptor.

Behavior: File-backed mappings can be shared between processes if MAP_SHARED; changes may be written back to the underlying file, depending on flags. MAP_PRIVATE provides copy-on-write semantics; writes are not visible to other processes nor written to the file. Anonymous mappings are not tied to files and are typically zero-initialized. On unmapping with munmap, resources are released; changes to a file-based mapping may be synchronized with the file via msync or automatically on unmapping in some cases.

Notes: The offset must be a multiple of the page size, and the mapping length should be at least one page. The function returns a pointer to the mapped area on success, or MAP_FAILED on error with errno.

Usage and portability: mmap is standard in POSIX; Windows provides a different API (MapViewOfFile). Performance benefits include zero-copy I/O and efficient interprocess communication via shared memory, but it requires careful synchronization. Common uses include loading large files for random access and implementing shared memory regions.