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MLOCK(2) Linux Programmer's Manual MLOCK(2)
mlock, munlock, mlockall, munlockall - lock and unlock memory
#include <sys/mman.h>
int mlock(const void *addr, size_t len);
int munlock(const void *addr, size_t len);
int mlockall(int flags);
int munlockall(void);
mlock() and mlockall() respectively 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,
respectively 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. Memory locking and
unlocking are performed in units of whole pages.
mlock() locks pages in the address range starting at addr and continuing for
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.
munlock() unlocks pages in the address range starting at addr and continuing
for 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.
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.
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 appropriately, and no changes are made to any locks in the address space
of the process.
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 (Linux 2.6.9 and later) the caller was not privileged (CAP_IPC_LOCK)
and its RLIMIT_MEMLOCK soft resource limit was 0.
EPERM (Linux 2.6.8 and earlier) The calling process has insufficient
privilege to call munlockall(). Under Linux the CAP_IPC_LOCK
capability is required.
For mlock() and munlock():
EAGAIN Some or all of the specified address range could not be locked.
EINVAL len was negative.
EINVAL (Not on Linux) addr was not a multiple of the page size.
ENOMEM Some of the specified address range does not correspond to mapped pages
in the address space of the process.
For mlockall():
EINVAL Unknown flags were specified.
For munlockall():
EPERM (Linux 2.6.8 and earlier) The caller was not privileged (CAP_IPC_LOCK).
POSIX.1-2001, SVr4.
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).)
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 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() 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.
Under Linux, mlock() and munlock() automatically round addr down to the
nearest page boundary. However, POSIX.1-2001 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(),
mlockall(), and mmap(2) MAP_LOCKED.
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 the 2.4 series Linux kernels up to and including 2.4.17, a bug caused the
mlockall() MCL_FUTURE flag to be inherited across a fork(2). This was
rectified in kernel 2.4.18.
Since kernel 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.
mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5), capabilities(7)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2010-10-30 MLOCK(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface