man-pages/man2/mlockall.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
.\" Modified, 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
.\"     Added notes on capability requirements
.\" Modified, 2004-11-23, mtk, Noted buggy MCL_FUTURE behaviour in 2.4.x
.\" Modified, 2004-11-25, mtk, 2.4 limits locks to half of physical mem.
.\" Modified, 2004-11-25, mtk, Noted MCL_FUTURE bug in 2.4.x, x < 18
.\"
.TH MLOCKALL 2 2004-11-25 "Linux 2.6.9" "Linux Programmer's Manual"
.SH NAME
mlockall \- disable paging for calling process
.SH SYNOPSIS
.nf
.B #include <sys/mman.h>
.sp
\fBint mlockall(int \fIflags\fB);
.fi
.SH DESCRIPTION
.B mlockall
disables paging for all pages mapped into the address space of the
calling process. This includes the pages of the code, data and stack
segment, 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
.B mlockall
system call returns successfully and they are guaranteed to stay in RAM
until the pages are unlocked again by
.B munlock
or
.B munlockall
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 transfered 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. For security
applications, only small parts of memory have to be locked, for which
.B mlock
is available.

The 
.I flags
parameter can be constructed from the bitwise OR of the following
constants:
.TP 1.2i
.B MCL_CURRENT
Lock all pages which are currently mapped into the address space of
the process.
.TP
.B 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.
.PP
If
.B MCL_FUTURE
has been specified and the number of locked pages exceeds the upper
limit of allowed locked pages, then the system call which caused the
new mapping will fail with
.BR ENOMEM .
If these new pages have been mapped by the the growing stack, then the
kernel will deny stack expansion and send a 
.BR SIGSEGV .

Real-time processes 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
which has a sufficiently large automatic variable and which writes to
the memory occupied by this large 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 do not stack, i.e., pages which have been locked several times
by calls to
.B mlockall
or
.B mlock
will be unlocked by a single call to
.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.
.SH "NOTES"
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 mlockall
returns zero.  On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EINVAL
Unknown flags were specified.
.TP
.B ENOMEM
The process tried to exceed the maximum number of allowed locked
pages.
.TP
.B EPERM
The calling process has insufficient privilege to call 
.BR mlockall .
Under Linux the
.B CAP_IPC_LOCK
capability is required.
.SH "BUGS"
In the 2.4 series Linux kernels up to and including 2.4.17,
a bug caused the
.B MCL_FUTURE
flag to be inherited across a
.BR fork (2).
This was rectified in kernel 2.4.18.
.SH AVAILABILITY
On POSIX systems on which
.B mlockall
and
.B munlockall
are available,
.B _POSIX_MEMLOCK
is defined in <unistd.h> to a value greater than 0. (See also
.BR sysconf (3).)
.\" POSIX 1003.1-2001: It shall be defined to -1 or 0 or 200112L.
.\" -1: unavailable, 0: ask using sysconf().
.\" glibc defines it to 1.
.SH "CONFORMING TO"
POSIX.1b, SVr4.   SVr4 documents an additional EAGAIN error code.
.SH "SEE ALSO"
.BR mlock (2),
.BR munlock (2),
.BR munlockall (2),
.BR setrlimit (2),
.BR sysconf (3),
.BR capabilities (7)