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MLOCK(2) Linux Programmer's Manual MLOCK(2)
NAME
mlock, munlock, mlockall, munlockall - lock and unlock memory
SYNOPSIS
#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);
DESCRIPTION
mlock() and mlockall() respectively lock part or all of the calling process's vir-
tual 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() 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 success-
fully; 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 like-
wise 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 pro-
cess.
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
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, i.e., 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.
LINUX NOTES
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.
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.
RETURN VALUE
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.
ERRORS
ENOMEM (Linux 2.6.9 and later) the caller had a non-zero 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():
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).
BUGS
In the 2.4 series Linux kernels up to and including 2.4.17, a bug caused the mlock-
all() MCL_FUTURE flag to be inherited across a fork(2). This was rectified in ker-
nel 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 non-zero value), then subsequent memory allocations (e.g.,
mmap(2), brk(2)) will fail if the RLIMIT_MEMLOCK resource limit is encountered.
AVAILABILITY
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_PAGE-
SIZE).
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).)
CONFORMING TO
POSIX.1-2001, SVr4
SEE ALSO
mmap(2), shmctl(2), setrlimit(2), sysconf(3), capabilities(7)
Linux 2.6.15 2006-02-04 MLOCK(2)
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