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
   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.

   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
   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.

   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
               mlockall() 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
               populated   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,
          unlocking 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
          without 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
   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  and  MCL_FUTURE  |
   MCL_ONFAULT 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
   elevated priority.

   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(), 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_MEMLOCK resource limit is encountered.


   mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5), capabilities(7)


   This  page  is  part of release 4.09 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


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