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



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

SYNOPSIS
       #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
       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  mem-
       ory-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  pro-
       cess.

       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 non-zero 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  (Linux 2.6.9 and later) the caller  was  not  privileged  (CAP_IPC_LOCK)  and  its  RLIMIT_MEMLOCK  soft
              resource limit was 0.

       EPERM  (Linux  2.6.8  and  earlier) The calling process has insufficient privilege to call munlockall().  Under
              Linux the CAP_IPC_LOCK capability is required.

       For mlock() and munlock():

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

       EINVAL len was negative.

       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 pro-
              cess.

       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
       POSIX.1-2001, SVr4.

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 pro-
       gram 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 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 mlock-
       all() 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.

   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 non-zero value), then subsequent memory
       allocations (e.g., mmap(2), brk(2)) will fail if the RLIMIT_MEMLOCK resource limit is encountered.

SEE ALSO
       mmap(2), setrlimit(2), shmctl(2), sysconf(3), capabilities(7)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-09-25                          MLOCK(2)