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NAME

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

SYNOPSIS

       #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);

DESCRIPTION

       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:

       MLOCK_ONFAULT
              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  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.

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(), 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 (CAP_IPC_LOCK).

VERSIONS

       mlock2() is available since Linux 4.4; glibc support was added in version 2.27.

CONFORMING TO

       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).)

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  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 lock.

BUGS

       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_MEMLOCK resource limit is encountered.

SEE ALSO

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

COLOPHON

       This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.