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 virtual address range that starts at addr and
extends for len bytes. The munlock call unlocks pages that were previ-
ously locked by one or more mlock calls. For both calls, the addr param-
eter should be aligned to a multiple of the page size. If the len param-
eter 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-resi-
dent page nor address-translation fault until they are unlocked. They
may still cause protection-violation faults or TLB-miss faults on archi-
tectures 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. 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 deallo-
cates the unmapped address range. Locked mappings 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 status 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 conse-
quence 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
Mac OS X 10.9.1 - Generated Mon Jan 6 09:25:45 CST 2014