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 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(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.
RETURN VALUES
The mlock() and
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.
ERRORS
mlock() will fail if:
- [
EINVAL] - The address given is not page aligned or the length is negative.
- [
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:
- [
EINVAL] - The address given is not page aligned or addr and size specify a region that would extend beyond the end of the address space.
- [
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), minherit(2), mlockall(2), mmap(2), munmap(2), setrlimit(2), getpagesize(3)
HISTORY
The mlock() and
munlock() functions first appeared in
4.4BSD.
BUGS
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.