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/types.h>
#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-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.
These calls are only available to the super-user.
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
If the call succeeds, all pages in the range become locked (unlocked);
otherwise the locked status of all pages in the range remains unchanged.
ERRORSMlock() will fail if:
[EPERM] The caller is not the super-user.
[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:
[EPERM] The caller is not the super-user.
[EINVAL] The address given is not page aligned or the length is
negative.
[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), minherit(2), mmap(2), munmap(2), setrlimit(2),
getpagesize(3)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.
The per-process resource limit is not currently supported.
BSD May 18, 2004 BSD
