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

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

       #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);

       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:

              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.

       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(), 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

       mlock2(2) is available since Linux 4.4.

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

       mlock2 () is Linux specific.

       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  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).

       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

       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 4.04 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

Linux                             2015-12-28                          MLOCK(2)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2021 Hurricane Electric. All Rights Reserved.