Ubuntu Manpage: mlock, mlock2, munlock, mlockall, munlockall

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 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.

AVAILABILITY

       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.

COLOPHON

       This  page  is  part of release 4.15 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/.