NAME
mlock,
munlock —
lock
(unlock) physical pages in memory
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/mman.h>
int
mlock(
void
*addr,
size_t len);
int
munlock(
void
*addr,
size_t len);
DESCRIPTION
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. The entire range of memory 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.
Portable code should ensure that the
addr and
len parameters are aligned to a multiple of the page
size, even though the
NetBSD implementation will round
as necessary.
RETURN VALUES
A return value of 0 indicates that the call succeeded and all pages in the range
have either been locked or unlocked. 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() will fail if:
-
-
- [
EAGAIN
]
- Locking the indicated range would exceed either the system
or per-process limit for locked memory.
-
-
- [
EINVAL
]
- The address or length given is not page aligned and the
implementation does not round.
-
-
- [
ENOMEM
]
- Some portion of the indicated address range is not
allocated. There was an error faulting/mapping a page.
-
-
- [
EPERM
]
- mlock() was called by non-root on an
architecture where locked page accounting is not implemented.
munlock() will fail if:
-
-
- [
EINVAL
]
- The address or length given is not page aligned and the
implementation does not round.
-
-
- [
ENOMEM
]
- Some portion of the indicated address range is not
allocated. Some portion of the indicated address range is not locked.
SEE ALSO
fork(2),
mincore(2),
mmap(2),
munmap(2),
setrlimit(2),
getpagesize(3)
STANDARDS
The
mlock() and
munlock() functions conform
to
IEEE Std 1003.1b-1993 (“POSIX.1b”).
HISTORY
The
mlock() and
munlock() functions first
appeared in
4.4BSD.
BUGS
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.