mlock(3C) causes the pages in the specified address range to be locked in physical memory. References to locked pages in this process or in other processes do not result in page faults that require an I/O operation. Because this I/O operation interferes with normal operation of virtual memory, as well as slowing other processes, the use of mlock is limited to the superuser. The limit to the number of pages that can be locked in memory is dependent on system configuration. The call to mlock fails if this limit is exceeded.
munlock releases the locks on physical pages. If multiple mlock calls are made on an address range of a single mapping, a single munlock call releases the locks. However, if different mappings to the same pages are locked by mlock, the pages are not unlocked until the locks on all the mappings are released.
The copy-on-write event that is associated with a MAP_PRIVATE mapping transfers a lock on the source page to the destination page. Thus locks on an address range that includes MAP_PRIVATE mappings are retained transparently along with the copy-on-write redirection. For a discussion of this redirection, see Creating and Using Mappings.