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Copy-on-write defers copying until a modification needs private storage. With paging, separate address spaces can map their memory pages to the same page frames, using memory protection to prevent writes to the common contents. Reads need no copy. A permitted write to a shared page frame first separates the writer's storage, so the other operating-system process keeps the old contents. Sharing is safe because the common contents remain unchanged while shared; the logical private copies can still evolve independently.^[ref]

Consider a fork-like model with parent A and child B. Each has a separate page table, with three inherited private, logically writable memory pages. Both initially map virtual page numbers 0x12, 0x13, 0x14 to page frames F0, F1, F2. The page table entries temporarily deny writes for copy-on-write; a read of x at offset 0x34 in virtual page 0x12 returns 7 in either address space. Separate page tables do not imply separate stored bytes.

Now B writes x = 42. In this model, copy-on-write fault handling allocates F3, copies all of F0's contents into F3, redirects only B's page table entry, enables its write, and retries the store. A still reads 7 through F0; B reads 42 through F3 at the same virtual address. F1 and F2 remain shared. The reference counting labels count mappings in this example: F0 drops from two to one, while F3 starts at one. Updating one byte can therefore copy a whole memory page.

The attraction is avoiding work that may never be needed. If B only reads, this model makes no data copies; if B replaces its program and discards these mappings, copying them beforehand would have been wasted. Copy-on-write trades eager copying for protected sharing and later page faults. The page tables, reference counting, and operating-system process records still cost time and computer memory.^[ref]

For a concrete operating system contract, the fork(2) manual for the Linux kernel documents copy-on-write, while its mmap(2) manual distinguishes MAP_PRIVATE from MAP_SHARED. Writes through MAP_PRIVATE are not propagated to other mappings or the underlying file; writes through MAP_SHARED are visible to other operating-system processes mapping that region. An inherited shared mapping therefore retains shared-write behavior. For a private memory-mapped file, visibility of later changes to the underlying file is unspecified: private writes alone do not guarantee a frozen snapshot of the file. ^[ref][ref]

Nor does fork make every resource independent. With the Linux kernel, inherited file descriptors refer to the same open file descriptions, including shared file positions. Special mapping policies can omit a region from inheritance or zero it in the child. Thus the diagrams describe ordinary inherited private memory pages, not every resource reachable by a parent. ^[ref]

The POSIX mapping interface. Ordinary Linux kernel file mappings require a positive length and an aligned file offset. A null address lets the operating-system kernel choose an aligned base; length need not be aligned. Check for MAP_FAILED. After success the file descriptor may be closed without invalidating the mapping; munmap or process termination removes it.^[ref]

Mapping does not extend the backing file. Under POSIX, for an initially partial last memory page, the tail beyond the file's end is zero-filled and is not written back as file data; accesses to whole mapped memory pages beyond the end raise SIGBUS. POSIX leaves access to portions added or removed by later resizing unspecified. Establish the required file size first and coordinate resizing with readers; a mapping's length is not proof that those bytes exist in the file.^[ref]

The Linux kernel documents a tail-cache exception: modified beyond-end bytes can survive in the page cache and appear in later mappings, although never written to the file. Do not use that tail as reliable storage.^[ref]

With MAP_SHARED, updates are visible to other operating-system processes mapping the same region and are carried through to the backing file. With MAP_PRIVATE, updates instead stay private, and later backing file changes have unspecified visibility in that mapping.^[ref]

In the toy model, both shared file mappings initially read backing bytes $x=7,y=8$ at file offsets 8229 and 8230. Both memory pages have been accessed, so their translations are installed. A shared store changes the common page cache contents and creates a dirty memory page eligible for page writeback. That change can be observed before it reaches persistent storage. Cooperating readers need synchronization to know when an update is ready; MAP_SHARED alone supplies no mutual exclusion or indivisible multi-field update.