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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
       continuing  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
       continuing  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.

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).

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
       deterministic  timing,  and, like scheduling, paging is one major cause
       of unexpected program execution delays.   Real-time  applications  will
       usually     also    switch    to    a    real-time    scheduler    with
       sched_setscheduler(2).  Cryptographic 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 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
       memory 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
       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 location 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.

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
       subsequent memory allocations (e.g., mmap(2), brk(2)) will fail if  the
       RLIMIT_MEMLOCK resource limit is encountered.

SEE ALSO

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

COLOPHON

       This  page  is  part of release 3.01 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.