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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
            so that the remaining nonresident pages  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,
     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()  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
     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).
     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
     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 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),


     This  page  is  part of release 4.16 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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