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 ad-
       dress range are guaranteed to be resident in RAM when the call  returns
       successfully;  the  pages are guaranteed to stay in RAM until later un-

       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
              so that the remaining nonresident pages are locked when they are
              populated by a page fault.

       If flags is 0, mlock2() behaves exactly the same as mlock().

       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, er-
       rno 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 un-
              locked) exceeding the allowed maximum.  (For example,  unlocking
              a range in the middle of a currently locked mapping would result
              in three mappings: two locked mappings at each end  and  an  un-
              locked 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()  is available since Linux 4.4; glibc support was added in ver-
       sion 2.27.

       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 un-
       expected 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 se-
       crets 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_ON-
       FAULT settings are not inherited by a child created via fork(2) and are
       cleared during an execve(2).

       Note that fork(2) will prepare the address space  for  a  copy-on-write
       operation.   The consequence is that any write access that follows will
       cause a page fault that in turn may cause high latencies  for  a  real-
       time  process.  Therefore, it is crucial not to invoke fork(2) after an
       mlockall() or mlock() operation--not even from a thread which runs at a
       low  priority  within a process which also has a thread running at ele-
       vated priority.

       The memory lock on an address range is automatically removed if the ad-
       dress 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  un-
       locked  by a single call to munlock() for the corresponding range or by
       munlockall().  Pages which are mapped to several locations or  by  sev-
       eral 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 al-
       low efficient memory locking for applications that deal with large map-
       pings 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 sig-
       nificant 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  de-
       fines 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 Linux 4.8 and earlier, a bug in the kernel's  accounting  of  locked
       memory  for  unprivileged  processes (i.e., without CAP_IPC_LOCK) meant
       that if the region specified by addr and  len  overlapped  an  existing
       lock,  then  the  already  locked  bytes in the overlapping region were
       counted twice when checking against the limit.  Such double  accounting
       could  incorrectly  calculate  a  "total  locked  memory" value for the
       process that exceeded the RLIMIT_MEMLOCK limit, with  the  result  that
       mlock() and mlock2() would fail on requests that should have succeeded.
       This bug was fixed in Linux 4.9

       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.

       mincore(2), mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5),  ca-

       This  page  is  part of release 5.05 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                             2018-02-02                          MLOCK(2)
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