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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 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
       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 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
       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.

       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
       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_MEMLOCK 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 3.65 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/.



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