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.\" Hey Emacs! This file is -*- nroff -*- source.
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.\"
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2004-11-25 14:39:43 +00:00
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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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.\" 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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2004-11-25 14:39:43 +00:00
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.\" Software Foundation, Inc., 59 Temple Place, Suite 330,
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.\" Boston, MA 02111, USA.
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2004-11-25 13:33:38 +00:00
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.\"
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2008-09-25 09:11:53 +00:00
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.TH MLOCK 2 2008-09-25 "Linux" "Linux Programmer's Manual"
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2004-11-03 13:51:07 +00:00
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.SH NAME
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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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2008-01-12 08:57:58 +00:00
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.BI "int mlock(const void *" addr ", size_t " len );
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.BI "int munlock(const void *" addr ", size_t " len );
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.sp
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2008-01-12 08:57:58 +00:00
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.BI "int mlockall(int " flags );
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2007-12-22 17:10:05 +00:00
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.B int munlockall(void);
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2004-11-03 13:51:07 +00:00
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.fi
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.SH DESCRIPTION
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2004-11-25 14:39:43 +00:00
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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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2007-04-12 22:42:49 +00:00
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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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2004-12-13 07:23:31 +00:00
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address space, so that pages in the specified virtual address range may
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once more to be swapped out if required by the kernel memory manager.
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Memory locking and unlocking are performed in units of whole pages.
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2004-11-25 14:39:43 +00:00
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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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and continuing for
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.I len
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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.
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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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2007-12-26 08:00:10 +00:00
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memory, and memory-mapped files.
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2007-04-12 22:42:49 +00:00
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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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2004-11-03 13:51:07 +00:00
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2004-11-25 14:39:43 +00:00
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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.
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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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2004-12-01 09:43:35 +00:00
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has been specified, then a later system call (e.g.,
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.BR mmap (2),
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.BR sbrk (2),
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.BR malloc (3)),
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may fail if it would cause the number of locked bytes to exceed
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the permitted maximum (see below).
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In the same circumstances, stack growth may likewise fail:
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the kernel will deny stack expansion and deliver a
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2007-09-20 16:26:31 +00:00
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.B SIGSEGV
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signal to the process.
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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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2007-05-18 16:06:42 +00:00
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.SH "RETURN VALUE"
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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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2008-03-19 13:16:39 +00:00
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(Linux 2.6.9 and later) the caller had a non-zero
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2007-05-18 16:06:42 +00:00
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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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.\" In the case of mlock(), this check is somewhat buggy: it doesn't
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.\" take into account whether the to-be-locked range overlaps with
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.\" already locked pages. Thus, suppose we allocate
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.\" (num_physpages / 4 + 1) of memory, and lock those pages once using
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.\" mlock(), and then lock the *same* page range a second time.
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.\" In the case, the second mlock() call will fail, since the check
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.\" calculates that the process is trying to lock (num_physpages / 2 + 2)
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.\" pages, which of course is not true. (MTK, Nov 04, kernel 2.4.28)
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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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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.
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mlock.2: add EAGAIN error.
Summary from mtk: recent work on mlock caused Maxin to notice that
the EAGAIN error was not documented. KOSAKI Motohiro noted
that this behavior is longstanding.
=====
Dear Michael,
As per the mlock(2) implementation bugfix which is present in
Linux 2.6.27-rc2 git commit,
(http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=a477097d9c37c1cf289c7f0257dffcfa42d50197),
the mlock(2) man page should be modified to reflect the latest changes
in the kernel.
See the LKML thread regarding this commit :
http://www.nabble.com/mlock()-return-value-issue-in-kernel-2.6.23.17-td18751601.html
This patch modifies the mlock(2) behaviour as per the SUSv3 specification.
[ENOMEM]
Some or all of the address range specified by the addr and
len arguments does not correspond to valid mapped pages
in the address space of the process.
[EAGAIN]
Some or all of the memory identified by the operation could not
be locked when the call was made.
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Maxin B. John <maxin.john@ap.sony.com>
=====
From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: "Maxin John" <maxin.john@gmail.com>
Subject: Re: mlock(2) man page modifications
Cc: kosaki.motohiro@jp.fujitsu.com,
"Michael Kerrisk" <mtk.manpages@googlemail.com>, man@vger.kernel.org
Date: Thu, 25 Sep 2008 15:04:49 +0900 (JST)
Hi Maxin,
Thank you for your attention.
I think your point and your patch are right.
However, my patch is trivial regression fix, not behavior change.
An older kernel can return EAGAIN at memory stavation.
my patch has following hunk.
> +++ b/mm/mlock.c
> @@ -78,8 +78,6 @@ success:
>
> mm->locked_vm -= pages;
> out:
> - if (ret == -ENOMEM)
> - ret = -EAGAIN;
In addition, 2.6.11 (oldest code of git repository) has following code.
static int mlock_fixup(struct vm_area_struct * vma,
unsigned long start, unsigned long end, unsigned int newflags)
{
(snip)
vma->vm_mm->locked_vm -= pages;
out:
if (ret == -ENOMEM)
ret = -EAGAIN;
return ret;
}
that behavior is linux mlock's behavior for long long time.
Thanks!
2008-09-24 14:47:07 +00:00
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.TP
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.B EAGAIN
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Some of the memory identified by the operation could not be locked when
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the function call was made.
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2007-05-18 16:06:42 +00:00
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.LP
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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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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 "CONFORMING TO"
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2008-07-15 13:39:17 +00:00
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POSIX.1-2001, SVr4.
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2007-05-18 16:06:42 +00:00
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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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2007-06-20 21:53:34 +00:00
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is defined in \fI<unistd.h>\fP and the number of bytes in a page
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2007-05-18 16:06:42 +00:00
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can be determined from the constant
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.B PAGESIZE
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2007-06-20 21:53:34 +00:00
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(if defined) in \fI<limits.h>\fP or by calling
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2007-05-18 16:06:42 +00:00
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.IR sysconf(_SC_PAGESIZE) .
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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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2008-06-28 04:57:20 +00:00
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is defined in \fI<unistd.h>\fP to a value greater than 0.
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(See also
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2007-05-18 16:06:42 +00:00
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.BR sysconf (3).)
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.\" POSIX.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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2004-11-25 14:39:43 +00:00
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.SH "NOTES"
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2004-11-03 13:51:07 +00:00
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Memory locking has two main applications: real-time algorithms and
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2007-04-12 22:42:49 +00:00
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high-security data processing.
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Real-time applications require
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2004-11-03 13:51:07 +00:00
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deterministic timing, and, like scheduling, paging is one major cause
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2007-04-12 22:42:49 +00:00
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of unexpected program execution delays.
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Real-time applications will
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2004-11-25 14:39:43 +00:00
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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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2004-11-03 13:51:07 +00:00
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Cryptographic security software often handles critical bytes like
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2007-04-12 22:42:49 +00:00
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passwords or secret keys as data structures.
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As a result of paging,
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2004-11-03 13:51:07 +00:00
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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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2004-11-25 14:39:43 +00:00
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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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2007-04-12 22:42:49 +00:00
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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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2004-11-25 14:39:43 +00:00
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This way, enough pages will be mapped for the stack and can be
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2007-04-12 22:42:49 +00:00
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locked into RAM.
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The dummy writes ensure that not even copy-on-write
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2004-11-25 14:39:43 +00:00
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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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2007-06-08 11:56:22 +00:00
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Memory locks do not stack, that is, pages which have been locked several times
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2004-11-03 13:51:07 +00:00
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by calls to
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2004-11-25 14:39:43 +00:00
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.BR mlock ()
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2004-11-03 13:51:07 +00:00
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or
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2004-11-25 14:39:43 +00:00
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.BR mlockall ()
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2004-11-03 13:51:07 +00:00
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will be unlocked by a single call to
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2004-11-25 14:39:43 +00:00
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.BR munlock ()
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2004-11-03 13:51:07 +00:00
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for the corresponding range or by
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2005-10-19 06:54:38 +00:00
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.BR munlockall ().
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2004-11-03 13:51:07 +00:00
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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.
|
2007-05-18 10:39:45 +00:00
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.SS "Linux Notes"
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2004-11-25 14:39:43 +00:00
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Under Linux,
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.BR mlock ()
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2004-11-03 13:51:07 +00:00
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and
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2004-11-25 14:39:43 +00:00
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|
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.BR munlock ()
|
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automatically round
|
2004-11-03 13:51:07 +00:00
|
|
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.I addr
|
2004-11-25 14:39:43 +00:00
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down to the nearest page boundary.
|
2006-08-03 13:57:17 +00:00
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|
However, POSIX.1-2001 allows an implementation to require that
|
2004-11-03 13:51:07 +00:00
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|
|
.I addr
|
|
|
|
is page aligned, so portable applications should ensure this.
|
2004-11-25 14:39:43 +00:00
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|
|
.SS "Limits and permissions"
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|
|
|
In Linux 2.6.8 and earlier,
|
|
|
|
a process must be privileged
|
|
|
|
.RB ( CAP_IPC_LOCK )
|
|
|
|
in order to lock memory and the
|
|
|
|
.B RLIMIT_MEMLOCK
|
|
|
|
soft resource limit defines a limit on how much memory the process may lock.
|
2004-11-25 13:33:38 +00:00
|
|
|
|
2004-11-25 14:39:43 +00:00
|
|
|
Since Linux 2.6.9, no limits are placed on the amount of memory
|
|
|
|
that a privileged process can lock and the
|
|
|
|
.B RLIMIT_MEMLOCK
|
|
|
|
soft resource limit instead defines a limit on how much memory an
|
|
|
|
unprivileged process may lock.
|
|
|
|
.SH "BUGS"
|
|
|
|
In the 2.4 series Linux kernels up to and including 2.4.17,
|
|
|
|
a bug caused the
|
|
|
|
.BR mlockall ()
|
|
|
|
.B MCL_FUTURE
|
|
|
|
flag to be inherited across a
|
|
|
|
.BR fork (2).
|
|
|
|
This was rectified in kernel 2.4.18.
|
2006-02-03 20:56:21 +00:00
|
|
|
|
2007-04-12 22:42:49 +00:00
|
|
|
Since kernel 2.6.9, if a privileged process calls
|
2006-02-03 20:56:21 +00:00
|
|
|
.I mlockall(MCL_FUTURE)
|
2007-04-12 22:42:49 +00:00
|
|
|
and later drops privileges (loses the
|
|
|
|
.B CAP_IPC_LOCK
|
|
|
|
capability by, for example,
|
2008-03-19 13:16:39 +00:00
|
|
|
setting its effective UID to a non-zero value),
|
2007-04-12 22:42:49 +00:00
|
|
|
then subsequent memory allocations (e.g.,
|
2006-02-03 20:56:21 +00:00
|
|
|
.BR mmap (2),
|
|
|
|
.BR brk (2))
|
2007-04-12 22:42:49 +00:00
|
|
|
will fail if the
|
|
|
|
.B RLIMIT_MEMLOCK
|
2006-02-03 20:56:21 +00:00
|
|
|
resource limit is encountered.
|
|
|
|
.\" See the following LKML thread:
|
|
|
|
.\" http://marc.theaimsgroup.com/?l=linux-kernel&m=113801392825023&w=2
|
|
|
|
.\" "Rationale for RLIMIT_MEMLOCK"
|
|
|
|
.\" 23 Jan 2006
|
2004-11-03 13:51:07 +00:00
|
|
|
.SH "SEE ALSO"
|
2004-11-25 14:39:43 +00:00
|
|
|
.BR mmap (2),
|
2004-11-03 13:51:07 +00:00
|
|
|
.BR setrlimit (2),
|
2008-07-14 15:52:21 +00:00
|
|
|
.BR shmctl (2),
|
2004-11-25 14:39:43 +00:00
|
|
|
.BR sysconf (3),
|
2004-11-03 13:51:07 +00:00
|
|
|
.BR capabilities (7)
|