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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
       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.

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 memory lock on an address range is  automatically  removed  if  the
       address range is unmapped via munmap(2).

       Memory  locks  do not stack, i.e., 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  1003.1-2001  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.

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():

       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 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).

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.

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).)

CONFORMING TO

       POSIX.1-2001, SVr4

SEE ALSO

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