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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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mlock(2) System Calls Manual mlock(2)
mlock, mlock2, munlock, mlockall, munlockall - lock and unlock
memory
Standard C library (libc, -lc)
#include <sys/mman.h>
int mlock(size_t size;
const void addr[size], size_t size);
int mlock2(size_t size;
const void addr[size], size_t size, unsigned int flags);
int munlock(size_t size;
const void addr[size], size_t size);
int mlockall(int flags);
int munlockall(void);
mlock(), mlock2(), and 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.
munlock() and 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 can be
swapped out again if required by the kernel memory manager.
Memory locking and unlocking are performed in units of whole
pages.
mlock(), mlock2(), and munlock()
mlock() locks pages in the address range starting at addr and
continuing for size 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.
mlock2() also locks pages in the specified range starting at addr
and continuing for size bytes. However, the state of the pages
contained in that range after the call returns successfully will
depend on the value in the flags argument.
The flags argument can be either 0 or the following constant:
MLOCK_ONFAULT
Lock pages that are currently resident and mark the entire
range so that the remaining nonresident pages are locked
when they are populated by a page fault.
If flags is 0, mlock2() behaves exactly the same as mlock().
munlock() unlocks pages in the address range starting at addr and
continuing for size 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.
mlockall() and munlockall()
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.
The flags argument is constructed as the bitwise OR of one or more
of the following constants:
MCL_CURRENT
Lock all pages which are currently mapped into the address
space of the process.
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.
MCL_ONFAULT (since Linux 4.4)
Used together with MCL_CURRENT, MCL_FUTURE, or both. Mark
all current (with MCL_CURRENT) or future (with MCL_FUTURE)
mappings to lock pages when they are faulted in. When used
with MCL_CURRENT, all present pages are locked, but
mlockall() will not fault in non-present pages. When used
with 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.
MCL_ONFAULT must be used with either MCL_CURRENT or
MCL_FUTURE or both.
If MCL_FUTURE has been specified, then a later system call (e.g.,
mmap(2), sbrk(2), 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 SIGSEGV
signal to the process.
munlockall() unlocks all pages mapped into the address space of
the calling process.
On success, these system calls return 0. On error, -1 is
returned, errno is set to indicate the error, and no changes are
made to any locks in the address space of the process.
EAGAIN (mlock(), mlock2(), and munlock()) Some or all of the
specified address range could not be locked.
EINVAL (mlock(), mlock2(), and munlock()) The result of the
addition addr+size was less than addr (e.g., the addition
may have resulted in an overflow).
EINVAL (mlock2()) Unknown flags were specified.
EINVAL (mlockall()) Unknown flags were specified or MCL_ONFAULT
was specified without either MCL_FUTURE or MCL_CURRENT.
EINVAL (Not on Linux) addr was not a multiple of the page size.
ENOMEM (mlock(), mlock2(), and munlock()) Some of the specified
address range does not correspond to mapped pages in the
address space of the process.
ENOMEM (mlock(), mlock2(), and munlock()) 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. (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.)
ENOMEM (Linux 2.6.9 and later) the caller had a nonzero
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 (CAP_IPC_LOCK).
ENOMEM (Linux 2.4 and earlier) the calling process tried to lock
more than half of RAM.
EPERM The caller is not privileged, but needs privilege
(CAP_IPC_LOCK) to perform the requested operation.
EPERM (munlockall()) (Linux 2.6.8 and earlier) The caller was not
privileged (CAP_IPC_LOCK).
Linux
Under Linux, mlock(), mlock2(), and munlock() automatically round
addr down to the nearest page boundary. However, the POSIX.1
specification of mlock() and munlock() allows an implementation to
require that addr is page aligned, so portable applications should
ensure this.
The VmLck field of the Linux-specific /proc/pid/status file shows
how many kilobytes of memory the process with ID PID has locked
using mlock(), mlock2(), mlockall(), and mmap(2) MAP_LOCKED.
mlock()
munlock()
mlockall()
munlockall()
POSIX.1-2008.
mlock2()
Linux.
On POSIX systems on which mlock() and munlock() are available,
_POSIX_MEMLOCK_RANGE is defined in <unistd.h> and the number of
bytes in a page can be determined from the constant PAGESIZE (if
defined) in <limits.h> or by calling sysconf(_SC_PAGESIZE).
On POSIX systems on which mlockall() and munlockall() are
available, _POSIX_MEMLOCK is defined in <unistd.h> to a value
greater than 0. (See also sysconf(3).)
mlock()
munlock()
mlockall()
munlockall()
POSIX.1-2001, POSIX.1-2008, SVr4.
mlock2()
Linux 4.4, glibc 2.27.
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 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.)
Real-time processes that are using 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.
Memory locks are not inherited by a child created via fork(2) and
are automatically removed (unlocked) during an execve(2) or when
the process terminates. The mlockall() MCL_FUTURE and MCL_FUTURE
| MCL_ONFAULT settings are not inherited by a child created via
fork(2) and are cleared during an execve(2).
Note that 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 fork(2) after an mlockall() or mlock() operation—not
even from a thread which runs at a low priority within a process
which also has a thread running at elevated priority.
The memory lock on an address range is automatically removed if
the address range is unmapped via munmap(2).
Memory locks do not stack, that is, pages which have been locked
several times by calls to mlock(), mlock2(), or mlockall() will be
unlocked by a single call to munlock() for the corresponding range
or by 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.
If a call to mlockall() which uses the MCL_FUTURE flag is followed
by another call that does not specify this flag, the changes made
by the MCL_FUTURE call will be lost.
The mlock2() MLOCK_ONFAULT flag and the mlockall() 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.
Limits and permissions
In Linux 2.6.8 and earlier, a process must be privileged
(CAP_IPC_LOCK) in order to lock memory and the RLIMIT_MEMLOCK soft
resource limit defines a limit on how much memory the process may
lock.
Since Linux 2.6.9, no limits are placed on the amount of memory
that a privileged process can lock and the RLIMIT_MEMLOCK soft
resource limit instead defines a limit on how much memory an
unprivileged process may lock.
In Linux 4.8 and earlier, a bug in the kernel's accounting of
locked memory for unprivileged processes (i.e., without
CAP_IPC_LOCK) meant that if the region specified by addr and size
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
RLIMIT_MEMLOCK limit, with the result that mlock() and mlock2()
would fail on requests that should have succeeded. This bug was
fixed in Linux 4.9.
In Linux 2.4 series of kernels up to and including Linux 2.4.17, a
bug caused the mlockall() MCL_FUTURE flag to be inherited across a
fork(2). This was rectified in Linux 2.4.18.
Since Linux 2.6.9, if a privileged process calls
mlockall(MCL_FUTURE) and later drops privileges (loses the
CAP_IPC_LOCK capability by, for example, setting its effective UID
to a nonzero value), then subsequent memory allocations (e.g.,
mmap(2), brk(2)) will fail if the RLIMIT_MEMLOCK resource limit is
encountered.
mincore(2), mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5),
capabilities(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-06-28 mlock(2)
Pages that refer to this page: execve(2), fork(2), getrlimit(2), madvise(2), memfd_secret(2), mincore(2), mmap(2), mremap(2), perf_event_open(2), shmctl(2), syscalls(2), proc_pid_status(5), systemd.exec(5), capabilities(7), sched(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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