man-pages/man2/mlock.2

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.\" Copyright (C) Markus Kuhn, 1996
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.\" 1995-11-26  Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
.\"	First version written
.\" 2003-07-09  Michael Kerrisk <mtk-manpages@gmx.net>
.\"	Added note on suspend mode on laptops
.\"
.\" Modified, 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
.\"     Added notes on capability requirements
.\"
.\" Modified, 2004-11-25, mtk, 2.4 limits locks to half of physical mem.
.\"
.TH MLOCK 2 2004-11-25 "Linux 2.6.9" "Linux Programmer's Manual"
.SH NAME
mlock \- disable paging for some parts of memory
.SH SYNOPSIS
.nf
.B #include <sys/mman.h>
.sp
\fBint mlock(const void *\fIaddr\fB, size_t \fIlen\fB);
.fi
.SH DESCRIPTION
.B mlock
disables paging for the memory in the range starting at
.I addr
with length
.I len
bytes. All pages which contain a part of the specified memory range
are guaranteed be resident in RAM when the
.B mlock
system call returns successfully and they are guaranteed to stay in RAM
until the pages are unlocked by
.B munlock
or
.BR munlockall ,
until the pages are unmapped via
.BR munmap ,
or until the process terminates or starts another program with
.BR exec .
Child processes do not inherit page locks across a
.BR fork .

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 
.BR sched_setscheduler .
Cryptographic security 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.)

Memory locks do not stack, i.e., pages which have been locked several times
by calls to
.B mlock
or
.B mlockall
will be unlocked by a single call to
.B munlock
for the corresponding range or by
.BR 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.

On POSIX systems on which
.B mlock
and
.B munlock
are available,
.B _POSIX_MEMLOCK_RANGE
is defined in <unistd.h> and the value
.B PAGESIZE
from <limits.h> indicates the number of bytes per page.
.SH NOTES
With the Linux system call,
.I addr
is automatically rounded down to the nearest page boundary.  
However, POSIX 1003.1-2001 allows an implementation to require that
.I addr
is page aligned, so portable applications should ensure this.

In Linux 2.4 and earlier, the kernel prevents a single process
from locking more than half of RAM.
.SH "RETURN VALUE"
On success,
.B mlock
returns zero.  On error, \-1 is returned,
.I errno
is set appropriately, and no changes are made to any locks in the
address space of the process.
.SH ERRORS
.TP
.B EINVAL
(Not on Linux)
.I addr
was not a multiple of the page size.
.TP
.B ENOMEM
Some of the specified address range does not correspond to mapped
pages in the address space of the process or the process tried to
exceed the maximum number of allowed locked pages.
.TP
.B EPERM
The calling process has insufficient privilege to call 
.BR mlock .
Under Linux the
.B CAP_IPC_LOCK
capability is required.
.LP
Linux adds:
.TP
.B EINVAL
.I len
was negative.
.SH "CONFORMING TO"
POSIX.1b, SVr4.  SVr4 documents an additional EAGAIN error code.
.SH "SEE ALSO"
.BR mlockall (2),
.BR munlock (2),
.BR munlockall (2),
.BR munmap (2),
.BR setrlimit (2),
.BR capabilities (7)