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 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
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:
- [
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 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.