mlock2(2) — Linux manual page


MLOCK(2)                  Linux Programmer's Manual                 MLOCK(2)

NAME         top

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

SYNOPSIS         top

       #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         top

       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:

              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:

              Lock all pages which are currently mapped into the address
              space of the process.

              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         top

       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         top

       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

       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

VERSIONS         top

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

CONFORMING TO         top

       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         top

       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

       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         top

       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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.09 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                            2020-04-11                         MLOCK(2)

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