mirror of https://github.com/mkerrisk/man-pages
Consolidated mlock.2, munlock.2, mlockall.2, and munlockall.2 material into single page to eliminate duplicated material; updated notes for 2.6.9 changes in permissions and limist on memory locking
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Michael Kerrisk
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man2/mlock.2
279
man2/mlock.2
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.\" Hey Emacs! This file is -*- nroff -*- source.
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.\"
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.\" Copyright (C) Markus Kuhn, 1996
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.\" Copyright (C) Michael Kerrisk, 2004
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.\" using some material drawn from earlier man pages
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.\" written by Thomas Kuhn, Copyright 1996
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.\"
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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@ -19,55 +21,102 @@
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual; if not, write to the Free
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.\" Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111,
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.\" USA.
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.\"
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.\" 1995-11-26 Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
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.\" First version written
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.\" 2003-07-09 Michael Kerrisk <mtk-manpages@gmx.net>
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.\" Added note on suspend mode on laptops
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.\"
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.\" Modified, 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
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.\" Added notes on capability requirements
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.\"
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.\" Modified, 2004-11-25, mtk, 2.4 limits locks to half of physical mem.
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.\" Software Foundation, Inc., 59 Temple Place, Suite 330,
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.\" Boston, MA 02111, USA.
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.\"
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.TH MLOCK 2 2004-11-25 "Linux 2.6.9" "Linux Programmer's Manual"
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.SH NAME
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mlock \- disable paging for some parts of memory
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mlock, munlock, mlockall, munlockall \- lock and unlock memory
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.SH SYNOPSIS
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.nf
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.B #include <sys/mman.h>
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.sp
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\fBint mlock(const void *\fIaddr\fB, size_t \fIlen\fB);
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.sp
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\fBint munlock(const void *\fIaddr\fB, size_t \fIlen\fB);
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.sp
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\fBint mlockall(int \fIflags\fB);
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.sp
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\fBint munlockall(void);
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.fi
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.SH DESCRIPTION
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.B mlock
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disables paging for the memory in the range starting at
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.BR mlock ()
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and
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.BR mlockall ()
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respectively lock part or all of the calling process's virtual address
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space into RAM, preventing that memory from being paged to the
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swap area.
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.BR munlock ()
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and
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.BR munlockall ()
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perform the converse operation,
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respectively unlocking part or all of the calling process's virtual
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address space, so that the memory may once more to be swapped out if
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required by the kernel memory manager.
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Memory locking and unlocking is performed in units of whole pages.
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.SS "mlock() and munlock()"
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.BR mlock ()
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locks pages in the address range starting at
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.I addr
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with length
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and continuing for
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.I len
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bytes. All pages which contain a part of the specified memory range
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are guaranteed be resident in RAM when the
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.B mlock
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system call returns successfully and they are guaranteed to stay in RAM
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until the pages are unlocked by
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.B munlock
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or
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.BR munlockall ,
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until the pages are unmapped via
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.BR munmap ,
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or until the process terminates or starts another program with
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.BR exec .
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Child processes do not inherit page locks across a
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.BR fork .
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bytes.
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All pages that contain a part of the specified address range are
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guaranteed to be resident in RAM when the call returns successfully;
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the pages are guaranteed to stay in RAM until later unlocked.
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.BR munlock ()
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unlocks pages in the address range starting at
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.I addr
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and continuing for
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.I len
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bytes.
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After this call, all pages that contain a part of the specified
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memory range can be moved to external swap space again by the kernel.
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.SS "mlockall() and munlockall()"
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.BR mlockall ()
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locks all pages mapped into the address space of the
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calling process. This includes the pages of the code, data and stack
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segment, as well as shared libraries, user space kernel data, shared
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memory, and memory\-mapped files. All mapped pages are guaranteed
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to be resident in RAM when the call returns successfully;
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the pages are guaranteed to stay in RAM until later unlocked.
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The
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.I flags
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argument is constructed as the bitwise OR of one or more of the
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following constants:
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.TP 1.2i
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.B MCL_CURRENT
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Lock all pages which are currently mapped into the address space of
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the process.
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.TP
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.B MCL_FUTURE
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Lock all pages which will become mapped into the address space of the
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process in the future. These could be for instance new pages required
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by a growing heap and stack as well as new memory mapped files or
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shared memory regions.
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.PP
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If
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.B MCL_FUTURE
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has been specified and the number of locked pages exceeds the upper
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limit of allowed locked pages, then the system call which caused the
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new mapping will fail with
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.BR ENOMEM .
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If these new pages have been mapped by the the growing stack, then the
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kernel will deny stack expansion and send a
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.BR SIGSEGV .
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.BR munlockall ()
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unlocks all pages mapped into the address space of the
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calling process.
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.SH "NOTES"
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Memory locking has two main applications: real-time algorithms and
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high-security data processing. Real-time applications require
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deterministic timing, and, like scheduling, paging is one major cause
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of unexpected program execution delays. Real-time applications will
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usually also switch to a real-time scheduler with
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.BR sched_setscheduler .
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usually also switch to a real-time scheduler with
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.BR sched_setscheduler (2).
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Cryptographic security software often handles critical bytes like
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passwords or secret keys as data structures. As a result of paging,
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these secrets could be transferred onto a persistent swap store medium,
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@ -77,47 +126,109 @@ software has erased the secrets in RAM and terminated.
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computers will save a copy of the system's RAM to disk, regardless
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of memory locks.)
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Real-time processes that are using
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.BR mlockall ()
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to prevent delays on page faults should reserve enough
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locked stack pages before entering the time-critical section,
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so that no page fault can be caused by function calls.
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This can be achieved by calling a function that allocates a
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sufficiently large automatic variable (an array) and writes to the
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memory occupied by this array in order to touch these stack pages.
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This way, enough pages will be mapped for the stack and can be
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locked into RAM. The dummy writes ensure that not even copy-on-write
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page faults can occur in the critical section.
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Memory locks are not inherited by a child created via
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.BR fork (2)
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and are automatically removed (unlocked) during an
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.BR execve (2)
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or when the process terminates.
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The memory lock on an address range is automatically removed
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if the address range is unmapped via
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.BR munmap (2).
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Memory locks do not stack, i.e., pages which have been locked several times
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by calls to
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.B mlock
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.BR mlock ()
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or
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.B mlockall
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.BR mlockall ()
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will be unlocked by a single call to
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.B munlock
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.BR munlock ()
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for the corresponding range or by
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.BR munlockall .
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Pages which are mapped to several locations or by several processes stay
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locked into RAM as long as they are locked at least at one location or by
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at least one process.
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On POSIX systems on which
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.B mlock
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.SH "LINUX NOTES"
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Under Linux,
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.BR mlock ()
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and
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.B munlock
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are available,
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.B _POSIX_MEMLOCK_RANGE
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is defined in <unistd.h> and the value
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.B PAGESIZE
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from <limits.h> indicates the number of bytes per page.
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.SH NOTES
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With the Linux system call,
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.BR munlock ()
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automatically round
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.I addr
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is automatically rounded down to the nearest page boundary.
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down to the nearest page boundary.
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However, POSIX 1003.1-2001 allows an implementation to require that
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.I addr
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is page aligned, so portable applications should ensure this.
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.SS "Limits and permissions"
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In Linux 2.6.8 and earlier,
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a process must be privileged
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.RB ( CAP_IPC_LOCK )
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in order to lock memory and the
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.B RLIMIT_MEMLOCK
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soft resource limit defines a limit on how much memory the process may lock.
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In Linux 2.4 and earlier, the kernel prevents a single process
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from locking more than half of RAM.
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Since Linux 2.6.9, no limits are placed on the amount of memory
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that a privileged process can lock and the
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.B RLIMIT_MEMLOCK
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soft resource limit instead defines a limit on how much memory an
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unprivileged process may lock.
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.SH "RETURN VALUE"
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On success,
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.B mlock
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returns zero. On error, \-1 is returned,
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On success these system calls return 0.
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On error, \-1 is returned,
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.I errno
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is set appropriately, and no changes are made to any locks in the
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address space of the process.
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.SH ERRORS
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.TP
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.B ENOMEM
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(Linux 2.6.9 and later) the caller had a non-zero
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.B RLIMIT_MEMLOCK
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soft resource limit, but tried to lock more memory than the limit
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permitted.
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This limit is not enforced if the process is privileged
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.RB ( CAP_IPC_LOCK ).
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.TP
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.B ENOMEM
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(Linux 2.4 and earlier) the calling process tried to lock more than
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half of RAM.
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.TP
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.B EPERM
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(Linux 2.6.9 and later) the caller was not privileged
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.RB ( CAP_IPC_LOCK )
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and its
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.B RLIMIT_MEMLOCK
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soft resource limit was 0.
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.TP
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.B EPERM
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(Linux 2.6.8 and earlier)
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The calling process has insufficient privilege to call
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.BR munlockall .
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Under Linux the
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.B CAP_IPC_LOCK
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capability is required.
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.\"SVr4 documents an additional EAGAIN error code.
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.LP
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For
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.BR mlock ()
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and
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.BR munlock ():
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.TP
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.B EINVAL
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.I len
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was negative.
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.TP
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.B EINVAL
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(Not on Linux)
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.I addr
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@ -125,27 +236,55 @@ was not a multiple of the page size.
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.TP
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.B ENOMEM
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Some of the specified address range does not correspond to mapped
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pages in the address space of the process or the process tried to
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exceed the maximum number of allowed locked pages.
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.TP
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.B EPERM
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The calling process has insufficient privilege to call
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.BR mlock .
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Under Linux the
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.B CAP_IPC_LOCK
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capability is required.
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pages in the address space of the process.
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.LP
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Linux adds:
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For
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.BR mlockall ():
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.TP
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.B EINVAL
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.I len
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was negative.
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Unknown \fIflags\fP were specified.
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.LP
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For
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.BR munlockall ():
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.TP
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.B EPERM
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(Linux 2.6.8 and earlier) The caller was not privileged
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.RB ( CAP_IPC_LOCK ).
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.SH "BUGS"
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In the 2.4 series Linux kernels up to and including 2.4.17,
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a bug caused the
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.BR mlockall ()
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.B MCL_FUTURE
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flag to be inherited across a
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.BR fork (2).
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This was rectified in kernel 2.4.18.
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.SH AVAILABILITY
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On POSIX systems on which
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.BR mlock ()
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and
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.BR munlock ()
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are available,
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.B _POSIX_MEMLOCK_RANGE
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is defined in <unistd.h> and the value
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.B PAGESIZE
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from <limits.h> indicates the number of bytes per page.
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On POSIX systems on which
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.BR mlockall ()
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and
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.BR munlockall ()
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are available,
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.B _POSIX_MEMLOCK
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is defined in <unistd.h> to a value greater than 0. (See also
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.BR sysconf (3).)
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.\" POSIX 1003.1-2001: It shall be defined to -1 or 0 or 200112L.
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.\" -1: unavailable, 0: ask using sysconf().
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.\" glibc defines it to 1.
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.SH "CONFORMING TO"
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POSIX.1b, SVr4. SVr4 documents an additional EAGAIN error code.
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POSIX.1-2001, SVr4
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.SH "SEE ALSO"
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.BR mlockall (2),
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.BR munlock (2),
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.BR munlockall (2),
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.BR munmap (2),
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.BR mmap (2),
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.BR shmctl (2),
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.BR setrlimit (2),
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.BR sysconf (3),
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.BR capabilities (7)
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