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       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).

       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)


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