2004-11-03 13:51:07 +00:00
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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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.\"
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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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.\" published by the Free Software Foundation; either version 2 of
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.\" the License, or (at your option) any later version.
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.\"
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.\" The GNU General Public License's references to "object code"
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.\" and "executables" are to be interpreted as the output of any
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.\" document formatting or typesetting system, including
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.\" intermediate and printed output.
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.\"
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.\" This manual is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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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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2004-11-03 14:43:40 +00:00
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.\" 2003-07-09 Michael Kerrisk <mtk-manpages@gmx.net>
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2004-11-03 13:51:07 +00:00
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.\" Added note on suspend mode on laptops
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.\"
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2004-11-03 14:43:40 +00:00
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.\" Modified, 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
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2004-11-03 13:51:07 +00:00
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.\" Added notes on capability requirements
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.\"
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.TH MLOCK 2 2004-05-27 "Linux 2.6.6" "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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.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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.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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.I addr
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with length
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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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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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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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where they might be accessible to the enemy long after the security
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software has erased the secrets in RAM and terminated.
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(But be aware that the suspend mode on laptops and some desktop
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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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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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or
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.B mlockall
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will be unlocked by a single call to
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.B 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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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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.I addr
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is automatically rounded 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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.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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.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 EINVAL
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(Not on Linux)
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.I addr
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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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.LP
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Linux adds
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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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.SH "CONFORMING TO"
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POSIX.1b, SVr4. SVr4 documents an additional EAGAIN error code.
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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 setrlimit (2),
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.BR capabilities (7)
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