MLOCK(2)                  (2020-04-11)                   MLOCK(2)

     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 con-
          stant:

          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().

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          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 expan-
          sion 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

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          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 cur-
               rently 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():

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          EINVAL
               Unknown flags were specified or MCL_ONFAULT was speci-
               fied 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 avail-
          able, _POSIX_MEMLOCK_RANGE is defined in <unistd.h> and the
          number of bytes in a page can be determined from the con-
          stant 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 algo-
          rithms and high-security data processing.  Real-time appli-
          cations 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).  Crypto-
          graphic 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 sus-
          pend 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

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          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 applica-
          tions 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 signifi-
          cant 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

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          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 effec-
          tive UID to a nonzero value), then subsequent memory alloca-
          tions (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/.

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