Copy-on-Write Pages

Copy-on-Write Pages

When fork() is executed, the new process shares the pages of the parent process under a rule of copy-on-write. The pages in the new address space are marked read-only. When the new process attempts to modify a page, a hardware interrupt occurs. The kernel makes a copy of that page, and changes the new address space to point to the copied page. Then the process continues to execute, modifying the page of which it now has a unique copy.

You can apply the copy-on-write discipline to the pages of an arena shared with other processes (see "Mapping a File for Shared Memory").