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 seg-
       ment, 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

       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  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

       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 determin-
       istic  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  pass-
       words  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 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 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 implementa-
       tion 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

       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 3.44 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								    2011-09-14								  MLOCK(2)