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  per-
       formed 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  guar-
       anteed 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 mem-
                   ory 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
       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 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  (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 num-
       ber 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 program execution delays.
       Real-time applications will usually also switch to a real-time scheduler with sched_setscheduler(2).  Cryptographic secu-
       rity  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 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 calling process has
       locked using mlock(), mlockall(), shmctl(2) SHM_LOCK, 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
       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
       mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5), capabilities(7)

COLOPHON
       This  page  is  part of release 3.25 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/.



Linux                                                      2010-03-05                                                   MLOCK(2)