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 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.

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(2) is available since Linux 4.4.

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).

       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  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  4.04  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
       http://www.kernel.org/doc/man-pages/.