mlockall


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  continu-
       ing  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  con-
       tinuing  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
       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 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
       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 deter-
       ministic timing, and, like scheduling, paging is  one  major  cause  of
       unexpected  program execution delays.  Real-time applications will usu-
       ally also switch to a real-time scheduler  with  sched_setscheduler(2).
       sufficiently large automatic variable (an array) and writes to the mem-
       ory 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  sev-
       eral times by calls to mlock() or mlockall() will be unlocked by a sin-
       gle 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  loca-
       tion 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 subse-
       quent 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 3.23 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.
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