.\" Copyright (C) Michael Kerrisk, 2004

.\"	using some material drawn from earlier man pages

.\"	written by Thomas Kuhn, Copyright 1996

.\"

.\" %%%LICENSE_START(GPLv2+_DOC_FULL)

.\" This is free documentation; you can redistribute it and/or

.\" modify it under the terms of the GNU General Public License as

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.\"

.TH MLOCK 2 2018-02-02 "Linux" "Linux Programmer's Manual"

.SH NAME

mlock, mlock2, munlock, mlockall, munlockall \- lock and unlock memory

.SH SYNOPSIS

.nf

.B #include <sys/mman.h>

.PP

.BI "int mlock(const void *" addr ", size_t " len );

.BI "int mlock2(const void *" addr ", size_t " len ", int " flags );

.BI "int munlock(const void *" addr ", size_t " len );

.PP

.BI "int mlockall(int " flags );

.B int munlockall(void);

.fi

.SH DESCRIPTION

.BR mlock (),

.BR mlock2 (),

and

.BR mlockall ()

lock part or all of the calling process's virtual address

space into RAM, preventing that memory from being paged to the

swap area.

.PP

.BR munlock ()

and

.BR munlockall ()

perform the converse operation,

unlocking part or all of the calling process's virtual

address space, so that pages in the specified virtual address range may

once more to be swapped out if required by the kernel memory manager.

.PP

Memory locking and unlocking are performed in units of whole pages.

.SS mlock(), mlock2(), and munlock()

.BR mlock ()

locks pages in the address range starting at

.I addr

and continuing for

.I len

bytes.

All pages that contain a part of the specified address range are

guaranteed to be resident in RAM when the call returns successfully;

the pages are guaranteed to stay in RAM until later unlocked.

.PP

.BR mlock2 ()

.\" commit a8ca5d0ecbdde5cc3d7accacbd69968b0c98764e

.\" commit de60f5f10c58d4f34b68622442c0e04180367f3f

.\" commit b0f205c2a3082dd9081f9a94e50658c5fa906ff1

also locks pages in the specified range starting at

.I addr

and continuing for

.I len

bytes.

However, the state of the pages contained in that range after the call

returns successfully will depend on the value in the

.I flags

argument.

.PP

The

.I flags

argument can be either 0 or the following constant:

.TP

.B MLOCK_ONFAULT

Lock pages that are currently resident and mark the entire range so

that the remaining nonresident pages locked when they are populated

by a page fault.

.PP

.PP

If

.I flags

is 0,

.BR mlock2 ()

behaves exactly the same as

.BR mlock ().

.PP

.BR munlock ()

unlocks pages in the address range starting at

.I addr

and continuing for

.I len

bytes.

After this call, all pages that contain a part of the specified

memory range can be moved to external swap space again by the kernel.

.SS mlockall() and munlockall()

.BR mlockall ()

locks 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 call returns successfully;

the pages are guaranteed to stay in RAM until later unlocked.

.PP

The

.I flags

argument is constructed as the bitwise OR of one or more 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.

.TP

.BR MCL_ONFAULT " (since Linux 4.4)"

Used together with

.BR MCL_CURRENT ,

.BR MCL_FUTURE ,

or both.

Mark all current (with

.BR MCL_CURRENT )

or future (with

.BR MCL_FUTURE )

mappings to lock pages when they are faulted in.

When used with

.BR MCL_CURRENT ,

all present pages are locked, but

.BR mlockall ()

will not fault in non-present pages.

When used with

.BR MCL_FUTURE ,

all future mappings will be marked to lock pages when they are faulted

in, but they will not be populated by the lock when the mapping is

created.

.B MCL_ONFAULT

must be used with either

.B MCL_CURRENT

or

.B MCL_FUTURE

or both.

.PP

If

.B MCL_FUTURE

has been specified, then a later system call (e.g.,

.BR mmap (2),

.BR sbrk (2),

.BR malloc (3)),

may fail if it would cause the number of locked bytes to exceed

the permitted maximum (see below).

In the same circumstances, stack growth may likewise fail:

the kernel will deny stack expansion and deliver a

.B SIGSEGV

signal to the process.

.PP

.BR munlockall ()

unlocks all pages mapped into the address space of the

calling process.

.SH RETURN VALUE

On success, these system calls return 0.

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 ENOMEM

(Linux 2.6.9 and later) the caller had a nonzero

.B RLIMIT_MEMLOCK

soft resource limit, but tried to lock more memory than the limit

permitted.

This limit is not enforced if the process is privileged

.RB ( CAP_IPC_LOCK ).

.TP

.B ENOMEM

(Linux 2.4 and earlier) the calling process tried to lock more than

half of RAM.

.\" In the case of mlock(), this check is somewhat buggy: it doesn't

.\" take into account whether the to-be-locked range overlaps with

.\" already locked pages.  Thus, suppose we allocate

.\" (num_physpages / 4 + 1) of memory, and lock those pages once using

.\" mlock(), and then lock the *same* page range a second time.

.\" In the case, the second mlock() call will fail, since the check

.\" calculates that the process is trying to lock (num_physpages / 2 + 2)

.\" pages, which of course is not true.  (MTK, Nov 04, kernel 2.4.28)

.TP

.B EPERM

The caller is not privileged, but needs privilege

.RB ( CAP_IPC_LOCK )

to perform the requested operation.

.\"SVr4 documents an additional EAGAIN error code.

.PP

For

.BR mlock (),

.BR mlock2 (),

and

.BR munlock ():

.TP

.B EAGAIN

Some or all of the specified address range could not be locked.

.TP

.B EINVAL

The result of the addition

.IR addr + len

was less than

.IR addr

(e.g., the addition may have resulted in an overflow).

.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.

.TP

.B ENOMEM

Locking or unlocking a region would result in the total number of

mappings with distinct attributes (e.g., locked versus unlocked)

exceeding the allowed maximum.

.\" I.e., the number of VMAs would exceed the 64kB maximum

(For example, unlocking a range in the middle of a currently locked

mapping would result in three mappings:

two locked mappings at each end and an unlocked mapping in the middle.)

.PP

For

.BR mlock2 ():

.TP

.B EINVAL

Unknown \fIflags\fP were specified.

.PP

For

.BR mlockall ():

.TP

.B EINVAL

Unknown \fIflags\fP were specified or

.B MCL_ONFAULT

was specified without either

.B MCL_FUTURE

or

.BR MCL_CURRENT .

.PP

For

.BR munlockall ():

.TP

.B EPERM

(Linux 2.6.8 and earlier) The caller was not privileged

.RB ( CAP_IPC_LOCK ).

.SH VERSIONS

.BR mlock2 ()

is available since Linux 4.4;

glibc support was added in version 2.27.

.SH CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4.

.PP

mlock2 ()

is Linux specific.

.SH AVAILABILITY

On POSIX systems on which

.BR mlock ()

and

.BR munlock ()

are available,

.B _POSIX_MEMLOCK_RANGE

is defined in \fI<unistd.h>\fP and the number of bytes in a page

can be determined from the constant

.B PAGESIZE

(if defined) in \fI<limits.h>\fP or by calling

.IR sysconf(_SC_PAGESIZE) .

.PP

On POSIX systems on which

.BR mlockall ()

and

.BR munlockall ()

are available,

.B _POSIX_MEMLOCK

is defined in \fI<unistd.h>\fP to a value greater than 0.

(See also

.BR sysconf (3).)

.\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L.

.\" -1: unavailable, 0: ask using sysconf().

.\" glibc defines it to 1.

.SH NOTES

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 (2).

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.)

.PP

Real-time processes that are using

.BR mlockall ()

to prevent delays on page faults 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 that allocates a

sufficiently large automatic variable (an array) and writes to the

memory occupied by this 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.

.PP

Memory locks are not inherited by a child created via

.BR fork (2)

and are automatically removed (unlocked) during an

.BR execve (2)

or when the process terminates.

The

.BR mlockall ()

.B MCL_FUTURE

and

.B MCL_FUTURE | MCL_ONFAULT

settings are not inherited by a child created via

.BR fork (2)

and are cleared during an

.BR execve (2).

.PP

Note that

.BR fork (2)

will prepare the address space for a copy-on-write operation.

The consequence is that any write access that follows will cause

a page fault that in turn may cause high latencies for a real-time process.

Therefore, it is crucial not to invoke

.BR fork (2)

after an

.BR mlockall ()

or

.BR mlock ()

operation\(emnot even from a thread which runs at a low priority within

a process which also has a thread running at elevated priority.

.PP

The memory lock on an address range is automatically removed

if the address range is unmapped via

.BR munmap (2).

.PP

Memory locks do not stack, that is, pages which have been locked several times

by calls to

.BR mlock (),

.BR mlock2 (),

or

.BR mlockall ()

will be unlocked by a single call to

.BR 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.

.PP

If a call to

.BR mlockall ()

which uses the

.B MCL_FUTURE

flag is followed by another call that does not specify this flag, the

changes made by the

.B MCL_FUTURE

call will be lost.

.PP

The

.BR mlock2 ()

.B MLOCK_ONFAULT

flag and the

.BR mlockall ()

.B MCL_ONFAULT

flag allow efficient memory locking for applications that deal with

large mappings where only a (small) portion of pages in the mapping are touched.

In such cases, locking all of the pages in a mapping would incur

a significant penalty for memory locking.

.SS Linux notes

Under Linux,

.BR mlock (),

.BR mlock2 (),

and

.BR munlock ()

automatically round

.I addr

down to the nearest page boundary.

However, the POSIX.1 specification of

.BR mlock ()

and

.BR munlock ()

allows an implementation to require that

.I addr

is page aligned, so portable applications should ensure this.

.PP

The

.I VmLck

field of the Linux-specific

.I /proc/[pid]/status

file shows how many kilobytes of memory the process with ID

.I PID

has locked using

.BR mlock (),

.BR mlock2 (),

.BR mlockall (),

and

.BR mmap (2)

.BR MAP_LOCKED .

.SS Limits and permissions

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.

.PP

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 Linux 4.8 and earlier,

a bug in the kernel's accounting of locked memory for unprivileged processes

(i.e., without

.BR CAP_IPC_LOCK )

meant that if the region specified by

.I addr

and

.I len

overlapped an existing lock,

then the already locked bytes in the overlapping region were counted twice

when checking against the limit.

Such double accounting could incorrectly calculate a "total locked memory"

value for the process that exceeded the

.BR RLIMIT_MEMLOCK

limit, with the result that

.BR mlock ()

and

.BR mlock2()

would fail on requests that should have succeeded.

This bug was fixed

.\" commit 0cf2f6f6dc605e587d2c1120f295934c77e810e8

in Linux 4.9

.PP

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.

.PP

Since kernel 2.6.9, if a privileged process calls

.I mlockall(MCL_FUTURE)

and later drops privileges (loses the

.B CAP_IPC_LOCK

capability by, for example,

setting its effective UID to a nonzero value),

then subsequent memory allocations (e.g.,

.BR mmap (2),

.BR brk (2))

will fail if the

.B RLIMIT_MEMLOCK

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

.SH SEE ALSO

.BR mincore (2),

.BR mmap (2),

.BR setrlimit (2),

.BR shmctl (2),

.BR sysconf (3),

.BR proc (5),

.BR capabilities (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.