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MLOCK(2)		       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(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()	system	call  unlocks  pages  previously  locked  by one or more
     mlock() calls.  For both, the addr argument should be aligned to a multiple
     of the page size.	If the len argument 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() system 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 architec-
     tures with software-managed TLBs.	The physical pages remain in memory  un-
     til  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 vir-
     tual mappings of the same pages or via nested mlock() calls on the same ad-
     dress 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 lim-
     ited in how much they can lock down.  A single process can mlock() the min-
     imum of a system-wide ``wired  pages''  limit  vm.max_wired  and  the  per-
     process RLIMIT_MEMLOCK resource limit.

     If  security.bsd.unprivileged_mlock is set to 0 these calls are only avail-
     able 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); oth-
     erwise the locked status of all pages in the range remains unchanged.

ERRORS
     The mlock() system call will fail if:

     [EPERM]		security.bsd.unprivileged_mlock is  set  to  0	and  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  the  system
			limit for locked memory.

     [ENOMEM]		Some portion of the indicated address range is not allo-
			cated.	 There	was  an  error	faulting/mapping a page.
			Locking the indicated range would exceed the per-process
			limit for locked memory.
     The munlock() system call will fail if:

     [EPERM]		security.bsd.unprivileged_mlock is  set  to  0	and  the
			caller is not the super-user.

     [EINVAL]		The  address  given is not page aligned or the length is
			negative.

     [ENOMEM]		Some or all of the address range specified by  the  addr
			and  len  arguments  does not correspond to valid mapped
			pages in the address space of the process.

     [ENOMEM]		Locking the pages mapped by the  specified  range  would
			exceed	a limit on the amount of memory that the process
			may lock.

SEE ALSO
     fork(2), mincore(2), minherit(2), mlockall(2), mmap(2), munlockall(2), mun-
     map(2), setrlimit(2), getpagesize(3)

HISTORY
     The mlock() and munlock() system calls first appeared in 4.4BSD.

BUGS
     Allocating too much wired memory can lead to a  memory-allocation	deadlock
     which requires a reboot to recover from.

     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 phys-
     ical  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.

FreeBSD 9.2			 March 18, 2013 			MLOCK(2)
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