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

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

       #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);

       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
       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.

       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.

       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.

       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.

       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

       POSIX.1-2001, SVr4.

       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).)

       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 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() 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
       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

       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.

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

       This page is part of release 3.63 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2011-09-14			      MLOCK(2)

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