MLOCK(2) BSD System Calls Manual MLOCK(2)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-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. 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.
ERRORSmlock() will fail if:
[EAGAIN] Locking the indicated range would exceed either the
system or per-process limit for locked memory.
[EINVAL] The length is negative; or 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 length is negative; or 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 ALSOfork(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.1”).
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.
BSD February 28, 2011 BSD
