Ubuntu Manpage: mlock, mlock2, munlock, mlockall, munlockall

NAME

       mlock, mlock2, munlock, mlockall, munlockall - lock and unlock memory

SYNOPSIS

       #include <sys/mman.h>

       int mlock(const void *addr, size_t len);
       int mlock2(const void *addr, size_t len, int flags);
       int munlock(const void *addr, size_t len);

       int mlockall(int flags);
       int munlockall(void);

DESCRIPTION

mlock(), mlock2(), and mlockall() 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, 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(), mlock2(), 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.

mlock2() also locks pages in the specified range starting at addr and continuing for len
bytes. However, the state of the pages contained in that range after the call returns
successfully will depend on the value in the flags argument.

The flags argument can be either 0 or the following constant:

MLOCK_ONFAULT
Lock pages that are currently resident and mark the entire range to have pages
locked when they are populated by the page fault.

If flags is 0, mlock2() behaves exactly the same as mlock().

Note: currently, there is not a glibc wrapper for mlock2(), so it will need to be invoked
using syscall(2).

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.

MCL_ONFAULT (since Linux 4.4)
Used together with MCL_CURRENT, MCL_FUTURE, or both. Mark all current (with
MCL_CURRENT) or future (with MCL_FUTURE) mappings to lock pages when they are
faulted in. When used with MCL_CURRENT, all present pages are locked, but
mlockall() will not fault in non-present pages. When used with MCL_FUTURE,
all future mappings will be marked to lock pages when they are faulted in, but
they will not be populated by the lock when the mapping is created.
MCL_ONFAULT must be used with either MCL_CURRENT or MCL_FUTURE or both.

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(), mlock2(), and munlock():

       EAGAIN Some or all of the specified address range could not be locked.

       EINVAL The result of the addition addr+len was less than addr (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.

       ENOMEM Locking or unlocking a region would result in the total  number  of  mappings  with
              distinct  attributes  (e.g., locked versus unlocked) exceeding the allowed maximum.
              (For example, unlocking a range in the middle of a currently locked  mapping  would
              result  in  three mappings: two locked mappings at each end and an unlocked mapping
              in the middle.)

       For mlock2():

       EINVAL Unknown flags were specified.

       For mlockall():

       EINVAL Unknown flags were specified or MCL_ONFAULT was specified without either MCL_FUTURE
              or MCL_CURRENT.

       For munlockall():

       EPERM  (Linux 2.6.8 and earlier) The caller was not privileged (CAP_IPC_LOCK).

VERSIONS

       mlock2(2) is available since Linux 4.4.

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, SVr4.

       mlock2 () is Linux specific.

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 mlockall()
MCL_FUTURE and MCL_FUTURE | MCL_ONFAULT settings are not inherited by a child created via
fork(2) and are cleared during an execve(2).

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(), mlock2(), 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.

If a call to mlockall() which uses the MCL_FUTURE flag is followed by another call that
does not specify this flag, the changes made by the MCL_FUTURE call will be lost.

The mlock2() MLOCK_ONFAULT flag and the mlockall() MCL_ONFAULT flag allow efficient memory
locking for applications that deal with large mappings where only a (small) portion of
pages in the mapping are touched. In such cases, locking all of the pages in a mapping
would incur a significant penalty for memory locking.

Linux notes
Under Linux, mlock(), mlock2(), and munlock() automatically round addr down to the nearest
page boundary. However, the POSIX.1 specification of mlock() and munlock() 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(), mlock2(), 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 4.04 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.