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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 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() 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 calling process.
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 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.
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.
For mlockall():
EINVAL Unknown flags were specified.
For munlockall():
EPERM (Linux 2.6.8 and earlier) The caller was not privileged (CAP_IPC_LOCK).
CONFORMING TO
POSIX.1-2001, SVr4.
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_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).)
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, 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.
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.
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 lock.
BUGS
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.
SEE ALSO
mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5), capabilities(7)
COLOPHON
This page is part of release 3.54 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/.