NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | AVAILABILITY | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

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

SYNOPSIS         top

       #include <sys/mman.h>

       int mlock(const void *addr, size_t len);
       int munlock(const void *addr, size_t len);

       int mlockall(int flags);
       int munlockall(void);

DESCRIPTION         top

       mlock() and mlockall() respectively 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, respectively 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() 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.

       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.

       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() and munlock():

       EAGAIN Some or all of the specified address range could not be
              locked.

       EINVAL The result of the addition start+len was less than start
              (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.

       For mlockall():

       EINVAL Unknown flags were specified.

       For munlockall():

       EPERM  (Linux 2.6.8 and earlier) The caller was not privileged
              (CAP_IPC_LOCK).

CONFORMING TO         top

       POSIX.1-2001, SVr4.

AVAILABILITY         top

       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 setting is not
       inherited by a child created via fork(2) and is 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() 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.

   Linux notes
       Under Linux, mlock() and munlock() automatically round addr down to
       the nearest page boundary.  However, POSIX.1-2001 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(), 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         top

       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.

SEE ALSO         top

       mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5),
       capabilities(7)

COLOPHON         top

       This page is part of release 3.70 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/.

Linux                            2014-04-14                         MLOCK(2)