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MLOCK(2)		   Linux Programmer's Manual		      MLOCK(2)

NAME
       mlock, munlock, mlockall, munlockall - lock and unlock memory

SYNOPSIS
       #include	<sys/mman.h>

       int mlock(const void *addr, size_t len);
       int munlock(const void *addr, size_t len);

       int mlockall(int	flags);
       int munlockall(void);

DESCRIPTION
       mlock()	and  mlockall()	 respectively  lock part or all	of the calling
       process's virtual address space into RAM, preventing that  memory  from
       being  paged  to	the swap area.	munlock() and munlockall() perform the
       converse	operation, respectively	unlocking part or all of  the  calling
       process's virtual address space,	so that	pages in the specified virtual
       address range may once more to be swapped out if	required by the	kernel
       memory manager.	Memory locking and unlocking are performed in units of
       whole pages.

   mlock() and munlock()
       mlock() locks pages in the address range	starting at addr and  continu-
       ing  for	len bytes.  All	pages that contain a part of the specified ad-
       dress range are guaranteed to be	resident in RAM	when the call  returns
       successfully;  the  pages are guaranteed	to stay	in RAM until later un-
       locked.

       munlock() unlocks pages in the address range starting at	addr and  con-
       tinuing	for len	bytes.	After this call, all pages that	contain	a part
       of the specified	memory range can be moved to external swap space again
       by the kernel.

   mlockall() and munlockall()
       mlockall() locks	all pages mapped into the address space	of the calling
       process.	 This includes the pages of the	code, data and stack  segment,
       as well as shared libraries, user space kernel data, shared memory, and
       memory-mapped files.  All mapped	pages are guaranteed to	be resident in
       RAM  when  the  call  returns successfully; the pages are guaranteed to
       stay in RAM until later unlocked.

       The flags argument is constructed as the	bitwise	OR of one or  more  of
       the following constants:

       MCL_CURRENT Lock	 all pages which are currently mapped into the address
		   space of the	process.

       MCL_FUTURE  Lock	all pages which	will become mapped  into  the  address
		   space of the	process	in the future.	These could be for in-
		   stance new pages required by	a growing heap	and  stack  as
		   well	as new memory-mapped files or shared memory regions.

       If  MCL_FUTURE  has  been  specified,  then  a later system call	(e.g.,
       mmap(2),	sbrk(2), malloc(3)), may fail if it would cause	the number  of
       locked  bytes to	exceed the permitted maximum (see below).  In the same
       circumstances, stack growth may likewise	fail:  the  kernel  will  deny
       stack expansion and deliver a SIGSEGV signal to the process.

       munlockall()  unlocks  all  pages  mapped into the address space	of the
       calling process.

RETURN VALUE
       On success these	system calls return 0.	On error, -1 is	returned,  er-
       rno  is	set appropriately, and no changes are made to any locks	in the
       address space of	the process.

ERRORS
       ENOMEM (Linux 2.6.9 and later) the caller had a nonzero	RLIMIT_MEMLOCK
	      soft  resource  limit,  but  tried  to lock more memory than the
	      limit permitted.	This limit is not enforced if the  process  is
	      privileged (CAP_IPC_LOCK).

       ENOMEM (Linux  2.4  and earlier)	the calling process tried to lock more
	      than half	of RAM.

       EPERM  The caller is not	privileged, but	needs privilege	(CAP_IPC_LOCK)
	      to perform the requested operation.

       For mlock() and munlock():

       EAGAIN Some or all of the specified address range could not be locked.

       EINVAL The  result of the addition start+len was	less than start	(e.g.,
	      the addition may have resulted in	an overflow).

       EINVAL (Not on Linux) addr was not a multiple of	the page size.

       ENOMEM Some of the specified  address  range  does  not	correspond  to
	      mapped pages in the address space	of the process.

       For mlockall():

       EINVAL Unknown flags were specified.

       For munlockall():

       EPERM  (Linux   2.6.8  and  earlier)  The  caller  was  not  privileged
	      (CAP_IPC_LOCK).

CONFORMING TO
       POSIX.1-2001, SVr4.

AVAILABILITY
       On  POSIX  systems  on  which  mlock()  and  munlock()  are  available,
       _POSIX_MEMLOCK_RANGE  is	 defined in _unistd.h_ and the number of bytes
       in a page can be	determined from	the constant PAGESIZE (if defined)  in
       _limits.h_ or by	calling	sysconf(_SC_PAGESIZE).

       On  POSIX  systems  on which mlockall() and munlockall()	are available,
       _POSIX_MEMLOCK is defined in _unistd.h_ to  a  value  greater  than  0.
       (See also sysconf(3).)

NOTES
       Memory  locking	has  two  main	applications: real-time	algorithms and
       high-security data processing.  Real-time applications  require	deter-
       ministic	timing,	and, like scheduling, paging is	one major cause	of un-
       expected	program	execution delays.  Real-time applications will usually
       also switch to a	real-time scheduler with sched_setscheduler(2).	 Cryp-
       tographic security software often handles critical bytes	like passwords
       or  secret  keys	 as data structures.  As a result of paging, these se-
       crets could be transferred onto a persistent swap store	medium,	 where
       they  might be accessible to the	enemy long after the security software
       has erased the secrets in RAM and terminated.  (But be aware  that  the
       suspend	mode on	laptops	and some desktop computers will	save a copy of
       the system's RAM	to disk, regardless of memory locks.)

       Real-time processes that	are using mlockall() to	prevent	delays on page
       faults  should  reserve	enough	locked stack pages before entering the
       time-critical section, so that no page fault can	be caused by  function
       calls.	This  can  be  achieved	by calling a function that allocates a
       sufficiently large automatic variable (an array)	and writes to the mem-
       ory  occupied  by this array in order to	touch these stack pages.  This
       way, enough pages will be mapped	for the	stack and can be  locked  into
       RAM.   The  dummy writes	ensure that not	even copy-on-write page	faults
       can occur in the	critical section.

       Memory locks are	not inherited by a child created via fork(2)  and  are
       automatically  removed  (unlocked)  during  an  execve(2)  or  when the
       process terminates.  The	mlockall() MCL_FUTURE setting is not inherited
       by a child created via fork(2) and is cleared during an execve(2).

       The memory lock on an address range is automatically removed if the ad-
       dress range is unmapped via munmap(2).

       Memory locks do not stack, that is, pages which have been  locked  sev-
       eral times by calls to mlock() or mlockall() will be unlocked by	a sin-
       gle call	to munlock() for the corresponding range or  by	 munlockall().
       Pages  which  are  mapped  to several locations or by several processes
       stay locked into	RAM as long as they are	locked at least	at  one	 loca-
       tion or by at least one process.

   Linux notes
       Under Linux, mlock() and	munlock() automatically	round addr down	to the
       nearest page boundary.  However,	POSIX.1-2001 allows an	implementation
       to  require  that addr is page aligned, so portable applications	should
       ensure this.

       The VmLck field of the Linux-specific /proc/PID/status file  shows  how
       many  kilobytes	of  memory  the	 process  with ID PID has locked using
       mlock(),	mlockall(), and	mmap(2)	MAP_LOCKED.

   Limits and permissions
       In Linux	2.6.8 and earlier, a process must be privileged	(CAP_IPC_LOCK)
       in  order to lock memory	and the	RLIMIT_MEMLOCK soft resource limit de-
       fines a limit on	how much memory	the process may	lock.

       Since Linux 2.6.9, no limits are	placed on the amount of	memory that  a
       privileged  process can lock and	the RLIMIT_MEMLOCK soft	resource limit
       instead defines a limit on how much memory an unprivileged process  may
       lock.

BUGS
       In  the	2.4  series  Linux  kernels  up	to and including 2.4.17, a bug
       caused the mlockall() MCL_FUTURE	flag to	be inherited across a fork(2).
       This was	rectified in kernel 2.4.18.

       Since  kernel 2.6.9, if a privileged process calls mlockall(MCL_FUTURE)
       and later drops privileges (loses the CAP_IPC_LOCK capability  by,  for
       example,	setting	its effective UID to a nonzero value), then subsequent
       memory allocations (e.g., mmap(2), brk(2)) will fail if the RLIMIT_MEM-
       LOCK resource limit is encountered.

SEE ALSO
       mmap(2),	setrlimit(2), shmctl(2), sysconf(3), proc(5), capabilities(7)

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-04-14			      MLOCK(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | AVAILABILITY | NOTES | BUGS | SEE ALSO | COLOPHON

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