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mlock, munlock, mlockall, munlockall - lock and unlock memory
int mlock(const void *addr, size_t len);
int munlock(const void *addr, size_t len);
int mlockall(int flags);
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
mlock() and munlock()
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() 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.
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
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
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.
For mlock() and munlock():
EAGAIN Some or all of the specified address range could not be locked.
EINVAL The result of the addition start+len was less than start (e.g.,
the addition may have resulted in an overflow).
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.
EINVAL Unknown flags were specified.
EPERM (Linux 2.6.8 and earlier) The caller was not privileged
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
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
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.
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
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)
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