Ubuntu Manpage: mlock, munlock, mlockall, munlockall

Provided by: manpages-dev_3.54-1ubuntu1_all

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.

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/.