MLOCK(2) BSD System Calls Manual MLOCK(2)
NAME
mlock, munlock -- lock (unlock) physical pages in memory
SYNOPSIS
#include <sys/mman.h>
int
mlock(const void *addr, size_t len);
int
munlock(const void *addr, size_t len);
DESCRIPTION
The mlock system call locks a set of physical pages into memory. The pages are associated with a vir-tual virtual
tual address range that starts at addr and extends for len bytes. The munlock call unlocks pages that
were previously locked by one or more mlock calls. For both calls, 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 map-pings mappings
pings for the pages are removed.
Multiple processes may have the same physical pages locked via their own virtual address mappings.
Similarly, a single process may 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 map-pings mappings
pings are not inherited by the child process after a fork(2).
Because physical memory is a potentially scarce resource, processes are limited in how much memory 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.
RETURN VALUES
A return value of 0 indicates that the call succeeded and all pages in the range have either been
locked or unlocked, as requested. A return value of -1 indicates an error occurred and the locked sta-tus status
tus of all pages in the range remains unchanged. In this case, the global location errno is set to
indicate the error.
ERRORS
Mlock() and munlock() will fail if:
[EINVAL] The address given is not page-aligned or the length is negative.
[ENOMEM] Part or all of the specified address range is not mapped to the process.
Mlock() will fail if:
[EAGAIN] Locking the indicated range would exceed either the system or per-process limit for
locked memory.
[ENOMEM] Some portion of the indicated address range is not allocated. There was an error
faulting/mapping a page.
Munlock() will fail if:
[ENOMEM] Some portion of the indicated address range is not allocated. Some portion of the
indicated address range is not locked.
LEGACY SYNOPSIS
#include <sys/types.h>
#include <sys/mman.h>
The include file <sys/types.h> is necessary.
int
mlock(caddr_t addr, size_t len);
int
munlock(caddr_t addr, size_t len);
The variable type of addr has changed.
SEE ALSO
fork(2), mincore(2), minherit(2), mmap(2), munmap(2), setrlimit(2), getpagesize(3), compat(5)
BUGS
Unlike The Sun 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.
HISTORY
The mlock() and munlock() functions first appeared in 4.4BSD.
BSD June 2, 1993 BSD
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