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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
       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.

NOTES
       Memory locking has two  main  applications:  real-time  algorithms  and
       high-security data processing. Real-time applications require determin‐
       istic timing, and, like scheduling, paging is one major cause of	 unex‐
       pected  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 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 sus‐
       pend 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, i.e., pages which have been  locked  several
       times  by  calls	 to mlock() 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	 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.

   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.

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 non-zero 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  (Linux   2.6.9   and   later)  the  caller  was  not  privileged
	      (CAP_IPC_LOCK) and its RLIMIT_MEMLOCK soft resource limit was 0.

       EPERM  (Linux 2.6.8 and earlier) The calling process  has  insufficient
	      privilege	 to  call  munlockall().  Under Linux the CAP_IPC_LOCK
	      capability is required.

       For mlock() and munlock():

       EINVAL len was negative.

       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).

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 non-zero	 value),  then	subse‐
       quent  memory  allocations  (e.g.,  mmap(2),  brk(2))  will fail if the
       RLIMIT_MEMLOCK resource limit is encountered.

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).)

CONFORMING TO
       POSIX.1-2001, SVr4

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

Linux 2.6.15			  2006-02-04			      MLOCK(2)
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