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



NAME
       mlock, mlock2, munlock, mlockall, munlockall - lock and unlock memory

SYNOPSIS
       #include <sys/mman.h>

       int mlock(const void *addr, size_t len);
       int mlock2(const void *addr, size_t len, int flags);
       int munlock(const void *addr, size_t len);

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

DESCRIPTION
       mlock(),	 mlock2(),  and	 mlockall()  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, 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(), mlock2(), 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
       address range are guaranteed to	be  resident  in  RAM  when  the  call
       returns	successfully;  the  pages  are guaranteed to stay in RAM until
       later unlocked.

       mlock2() also locks pages in the specified range starting at  addr  and
       continuing for len bytes.  However, the state of the pages contained in
       that range after the call returns successfully will depend on the value
       in the flags argument.

       The flags argument can be either 0 or the following constant:

       MLOCK_ONFAULT
	      Lock pages that are currently resident and mark the entire range
	      to have pages locked when they are populated by the page fault.

       If flags is 0, mlock2() behaves exactly the same as mlock().

       Note: currently, there is not a glibc wrapper for mlock2(), so it  will
       need to be invoked using syscall(2).

       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
		   instance, new pages required by a growing heap and stack as
		   well as new memory-mapped files or shared memory regions.

       MCL_ONFAULT (since Linux 4.4)
		   Used together with MCL_CURRENT, MCL_FUTURE, or both.	  Mark
		   all	current (with MCL_CURRENT) or future (with MCL_FUTURE)
		   mappings to lock pages when they are faulted in.  When used
		   with	 MCL_CURRENT, all present pages are locked, but mlock-
		   all() will not fault in non-present pages.  When used  with
		   MCL_FUTURE,	all  future  mappings  will  be marked to lock
		   pages when they are faulted in, but they will not be	 popu-
		   lated by the lock when the mapping is created.  MCL_ONFAULT
		   must be used with either MCL_CURRENT or MCL_FUTURE or both.

       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,
       errno 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(), mlock2(), and munlock():

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

       EINVAL The result of the addition addr+len was less  than  addr	(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.

       ENOMEM Locking or unlocking a region would result in the	 total	number
	      of  mappings  with  distinct  attributes	(e.g.,	locked	versus
	      unlocked) exceeding the allowed maximum.	(For example,  unlock-
	      ing  a  range  in the middle of a currently locked mapping would
	      result in three mappings: two locked mappings at each end and an
	      unlocked mapping in the middle.)

       For mlock2():

       EINVAL Unknown flags were specified.

       For mlockall():

       EINVAL Unknown  flags were specified or MCL_ONFAULT was specified with-
	      out either MCL_FUTURE or MCL_CURRENT.

       For munlockall():

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

VERSIONS
       mlock2() is available since Linux 4.4.

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, SVr4.

       mlock2 () is Linux specific.

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
       unexpected  program execution delays.  Real-time applications will usu-
       ally also switch to a real-time scheduler  with	sched_setscheduler(2).
       Cryptographic security software often handles critical bytes like pass-
       words or secret keys as data structures.	 As a result of paging,	 these
       secrets 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  and	 MCL_FUTURE  |
       MCL_ONFAULT settings are not inherited by a child created  via  fork(2)
       and are cleared during an execve(2).

       Note  that  fork(2)  will prepare the address space for a copy-on-write
       operation.  The consequence is that any write access that follows  will
       cause  a	 page  fault that in turn may cause high latencies for a real-
       time process.  Therefore, it is crucial not to invoke fork(2) after  an
       mlockall() or mlock() operation--not even from a thread which runs at a
       low priority within a process which also has a thread running  at  ele-
       vated priority.

       The  memory  lock  on  an address range is automatically removed if the
       address 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(),  mlock2(),	or  mlockall() will be
       unlocked by a single 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 location or by at least one process.

       If  a  call to mlockall() which uses the MCL_FUTURE flag is followed by
       another call that does not specify this flag, the changes made  by  the
       MCL_FUTURE call will be lost.

       The  mlock2()  MLOCK_ONFAULT  flag  and the mlockall() MCL_ONFAULT flag
       allow efficient memory locking for applications that  deal  with	 large
       mappings	 where	only  a	 (small)  portion  of pages in the mapping are
       touched.	 In such cases, locking all of the pages in  a	mapping	 would
       incur a significant penalty for memory locking.

   Linux notes
       Under  Linux, mlock(), mlock2(), and munlock() automatically round addr
       down to the nearest page boundary.  However, the POSIX.1	 specification
       of  mlock() and munlock() 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(), mlock2(), 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
       defines 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  Linux  4.8  and earlier, a bug in the kernel's accounting of locked
       memory for unprivileged processes (i.e.,	 without  CAP_IPC_LOCK)	 meant
       that  if	 the  region  specified by addr and len overlapped an existing
       lock, then the already locked bytes  in	the  overlapping  region  were
       counted	twice when checking against the limit.	Such double accounting
       could incorrectly calculate a  "total  locked  memory"  value  for  the
       process	that  exceeded	the RLIMIT_MEMLOCK limit, with the result that
       mlock() and mlock2() would fail on requests that should have succeeded.
       This bug was fixed in Linux 4.9

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

COLOPHON
       This page is part of release 4.10 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
       https://www.kernel.org/doc/man-pages/.



Linux				  2017-03-13			      MLOCK(2)