|MLOCK(2)||System Calls Manual||MLOCK(2)|
— lock (unlock) physical pages in memory
void *addr, size_t
void *addr, size_t
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
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(2) 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.
munlock() functions return the value 0 if
successful; otherwise the value -1 is returned and the global
variable errno is set to indicate the error.
mlock() will fail if:
munlock() will fail if:
munlock() functions first appeared in
Unlike Sun's implementation, multiple
mlock() calls on the same address range require the
corresponding number of
munlock() calls to actually
unlock the pages, i.e.,
mlock() nests. This should
be considered a consequence of the implementation and not a feature.
The per-process resource limit is a limit on the amount of virtual memory locked, while the system-wide limit is for the number of locked physical pages. Hence a process with two distinct locked mappings of the same physical page counts as 2 pages against the per-process limit and as only a single page in the system limit.
|January 11, 2019||OpenBSD-current|