The
mlock system call locks into memory the physical pages associated with the virtual address range starting at
addr for
len bytes. The
munlock call unlocks pages previously locked by one or more
mlock calls. For both, the
addr parameter should be aligned to a multiple of the page size. If the
len parameter is not a multiple of the page size, it will be rounded up to be so. The entire range must be allocated.
After an
mlock call, the indicated pages will cause neither a non-resident page nor address-translation fault until they are unlocked. They may still cause protection-violation faults or TLB-miss faults on architectures with software-managed TLBs. The physical pages remain in memory until all locked mappings for the pages are removed. Multiple processes may have the same physical pages locked via their own virtual address mappings. A single process may likewise have pages multiply-locked via different virtual mappings of the same pages or via nested
mlock calls on the same address range. Unlocking is performed explicitly by
munlock or implicitly by a call to
munmap which deallocates the unmapped address range. Locked mappings are not inherited by the child process after a
fork(2).
Since physical memory is a potentially scarce resource, processes are limited in how much they can lock down. A single process can
mlock the minimum of a system-wide ``wired pages'' limit and the per-process
RLIMIT_MEMLOCK resource limit.