1MLOCK(2)                   Linux Programmer's Manual                  MLOCK(2)
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NAME

6       mlock, mlock2, munlock, mlockall, munlockall - lock and unlock memory
7

SYNOPSIS

9       #include <sys/mman.h>
10
11       int mlock(const void *addr, size_t len);
12       int mlock2(const void *addr, size_t len, int flags);
13       int munlock(const void *addr, size_t len);
14
15       int mlockall(int flags);
16       int munlockall(void);
17

DESCRIPTION

19       mlock(),  mlock2(),  and  mlockall()  lock  part  or all of the calling
20       process's virtual address space into RAM, preventing that  memory  from
21       being paged to the swap area.
22
23       munlock()  and  munlockall()  perform the converse operation, unlocking
24       part or all of the calling process's virtual  address  space,  so  that
25       pages  in  the  specified  virtual  address  range  may once more to be
26       swapped out if required by the kernel memory manager.
27
28       Memory locking and unlocking are performed in units of whole pages.
29
30   mlock(), mlock2(), and munlock()
31       mlock() locks pages in the address range starting at addr and  continu‐
32       ing  for len bytes.  All pages that contain a part of the specified ad‐
33       dress range are guaranteed to be resident in RAM when the call  returns
34       successfully;  the  pages are guaranteed to stay in RAM until later un‐
35       locked.
36
37       mlock2() also locks pages in the specified range starting at  addr  and
38       continuing for len bytes.  However, the state of the pages contained in
39       that range after the call returns successfully will depend on the value
40       in the flags argument.
41
42       The flags argument can be either 0 or the following constant:
43
44       MLOCK_ONFAULT
45              Lock pages that are currently resident and mark the entire range
46              so that the remaining nonresident pages are locked when they are
47              populated by a page fault.
48
49       If flags is 0, mlock2() behaves exactly the same as mlock().
50
51       munlock()  unlocks pages in the address range starting at addr and con‐
52       tinuing for len bytes.  After this call, all pages that contain a  part
53       of the specified memory range can be moved to external swap space again
54       by the kernel.
55
56   mlockall() and munlockall()
57       mlockall() locks all pages mapped into the address space of the calling
58       process.   This includes the pages of the code, data and stack segment,
59       as well as shared libraries, user space kernel data, shared memory, and
60       memory-mapped files.  All mapped pages are guaranteed to be resident in
61       RAM when the call returns successfully; the  pages  are  guaranteed  to
62       stay in RAM until later unlocked.
63
64       The  flags  argument is constructed as the bitwise OR of one or more of
65       the following constants:
66
67       MCL_CURRENT
68              Lock all pages which are currently mapped into the address space
69              of the process.
70
71       MCL_FUTURE
72              Lock  all  pages which will become mapped into the address space
73              of the process in the future.  These could be, for instance, new
74              pages  required  by a growing heap and stack as well as new mem‐
75              ory-mapped files or shared memory regions.
76
77       MCL_ONFAULT (since Linux 4.4)
78              Used together with MCL_CURRENT, MCL_FUTURE, or both.   Mark  all
79              current  (with MCL_CURRENT) or future (with MCL_FUTURE) mappings
80              to lock pages when they are faulted in.  When used with MCL_CUR‐
81              RENT,  all  present  pages  are  locked, but mlockall() will not
82              fault in non-present pages.  When used with MCL_FUTURE, all  fu‐
83              ture mappings will be marked to lock pages when they are faulted
84              in, but they will not be populated by the lock when the  mapping
85              is created.  MCL_ONFAULT must be used with either MCL_CURRENT or
86              MCL_FUTURE or both.
87
88       If MCL_FUTURE has been specified,  then  a  later  system  call  (e.g.,
89       mmap(2),  sbrk(2), malloc(3)), may fail if it would cause the number of
90       locked bytes to exceed the permitted maximum (see below).  In the  same
91       circumstances,  stack  growth  may  likewise fail: the kernel will deny
92       stack expansion and deliver a SIGSEGV signal to the process.
93
94       munlockall() unlocks all pages mapped into the  address  space  of  the
95       calling process.
96

RETURN VALUE

98       On success, these system calls return 0.  On error, -1 is returned, er‐
99       rno is set appropriately, and no changes are made to any locks  in  the
100       address space of the process.
101

ERRORS

103       ENOMEM (Linux  2.6.9 and later) the caller had a nonzero RLIMIT_MEMLOCK
104              soft resource limit, but tried to  lock  more  memory  than  the
105              limit  permitted.   This limit is not enforced if the process is
106              privileged (CAP_IPC_LOCK).
107
108       ENOMEM (Linux 2.4 and earlier) the calling process tried to  lock  more
109              than half of RAM.
110
111       EPERM  The caller is not privileged, but needs privilege (CAP_IPC_LOCK)
112              to perform the requested operation.
113
114       For mlock(), mlock2(), and munlock():
115
116       EAGAIN Some or all of the specified address range could not be locked.
117
118       EINVAL The result of the addition addr+len was less  than  addr  (e.g.,
119              the addition may have resulted in an overflow).
120
121       EINVAL (Not on Linux) addr was not a multiple of the page size.
122
123       ENOMEM Some  of  the  specified  address  range  does not correspond to
124              mapped pages in the address space of the process.
125
126       ENOMEM Locking or unlocking a region would result in the  total  number
127              of  mappings  with  distinct attributes (e.g., locked versus un‐
128              locked) exceeding the allowed maximum.  (For example,  unlocking
129              a range in the middle of a currently locked mapping would result
130              in three mappings: two locked mappings at each end  and  an  un‐
131              locked mapping in the middle.)
132
133       For mlock2():
134
135       EINVAL Unknown flags were specified.
136
137       For mlockall():
138
139       EINVAL Unknown  flags were specified or MCL_ONFAULT was specified with‐
140              out either MCL_FUTURE or MCL_CURRENT.
141
142       For munlockall():
143
144       EPERM  (Linux  2.6.8  and  earlier)  The  caller  was  not   privileged
145              (CAP_IPC_LOCK).
146

VERSIONS

148       mlock2()  is available since Linux 4.4; glibc support was added in ver‐
149       sion 2.27.
150

CONFORMING TO

152       POSIX.1-2001, POSIX.1-2008, SVr4.
153
154       mlock2() is Linux specific.
155
156       On  POSIX  systems  on  which  mlock()  and  munlock()  are  available,
157       _POSIX_MEMLOCK_RANGE  is  defined in <unistd.h> and the number of bytes
158       in a page can be determined from the constant PAGESIZE (if defined)  in
159       <limits.h> or by calling sysconf(_SC_PAGESIZE).
160
161       On  POSIX  systems  on which mlockall() and munlockall() are available,
162       _POSIX_MEMLOCK is defined in <unistd.h> to  a  value  greater  than  0.
163       (See also sysconf(3).)
164

NOTES

166       Memory  locking  has  two  main  applications: real-time algorithms and
167       high-security data processing.  Real-time applications  require  deter‐
168       ministic timing, and, like scheduling, paging is one major cause of un‐
169       expected program execution delays.  Real-time applications will usually
170       also switch to a real-time scheduler with sched_setscheduler(2).  Cryp‐
171       tographic security software often handles critical bytes like passwords
172       or  secret  keys  as data structures.  As a result of paging, these se‐
173       crets could be transferred onto a persistent swap store  medium,  where
174       they  might be accessible to the enemy long after the security software
175       has erased the secrets in RAM and terminated.  (But be aware  that  the
176       suspend  mode on laptops and some desktop computers will save a copy of
177       the system's RAM to disk, regardless of memory locks.)
178
179       Real-time processes that are using mlockall() to prevent delays on page
180       faults  should  reserve  enough  locked stack pages before entering the
181       time-critical section, so that no page fault can be caused by  function
182       calls.   This  can  be  achieved by calling a function that allocates a
183       sufficiently large automatic variable (an array) and writes to the mem‐
184       ory  occupied  by this array in order to touch these stack pages.  This
185       way, enough pages will be mapped for the stack and can be  locked  into
186       RAM.   The  dummy writes ensure that not even copy-on-write page faults
187       can occur in the critical section.
188
189       Memory locks are not inherited by a child created via fork(2)  and  are
190       automatically  removed  (unlocked)  during  an  execve(2)  or  when the
191       process terminates.  The mlockall() MCL_FUTURE and MCL_FUTURE | MCL_ON‐
192       FAULT settings are not inherited by a child created via fork(2) and are
193       cleared during an execve(2).
194
195       Note that fork(2) will prepare the address space  for  a  copy-on-write
196       operation.   The consequence is that any write access that follows will
197       cause a page fault that in turn may cause high latencies  for  a  real-
198       time  process.  Therefore, it is crucial not to invoke fork(2) after an
199       mlockall() or mlock() operation—not even from a thread which runs at  a
200       low  priority  within a process which also has a thread running at ele‐
201       vated priority.
202
203       The memory lock on an address range is automatically removed if the ad‐
204       dress range is unmapped via munmap(2).
205
206       Memory  locks  do not stack, that is, pages which have been locked sev‐
207       eral times by calls to mlock(), mlock2(), or  mlockall()  will  be  un‐
208       locked  by a single call to munlock() for the corresponding range or by
209       munlockall().  Pages which are mapped to several locations or  by  sev‐
210       eral processes stay locked into RAM as long as they are locked at least
211       at one location or by at least one process.
212
213       If a call to mlockall() which uses the MCL_FUTURE flag is  followed  by
214       another  call  that does not specify this flag, the changes made by the
215       MCL_FUTURE call will be lost.
216
217       The mlock2() MLOCK_ONFAULT flag and the mlockall() MCL_ONFAULT flag al‐
218       low efficient memory locking for applications that deal with large map‐
219       pings where only a (small) portion of pages in the mapping are touched.
220       In such cases, locking all of the pages in a mapping would incur a sig‐
221       nificant penalty for memory locking.
222
223   Linux notes
224       Under Linux, mlock(), mlock2(), and munlock() automatically round  addr
225       down  to the nearest page boundary.  However, the POSIX.1 specification
226       of mlock() and munlock() allows an implementation to require that  addr
227       is page aligned, so portable applications should ensure this.
228
229       The VmLck field of the Linux-specific /proc/[pid]/status file shows how
230       many kilobytes of memory the process  with  ID  PID  has  locked  using
231       mlock(), mlock2(), mlockall(), and mmap(2) MAP_LOCKED.
232
233   Limits and permissions
234       In Linux 2.6.8 and earlier, a process must be privileged (CAP_IPC_LOCK)
235       in order to lock memory and the RLIMIT_MEMLOCK soft resource limit  de‐
236       fines a limit on how much memory the process may lock.
237
238       Since  Linux 2.6.9, no limits are placed on the amount of memory that a
239       privileged process can lock and the RLIMIT_MEMLOCK soft resource  limit
240       instead  defines a limit on how much memory an unprivileged process may
241       lock.
242

BUGS

244       In Linux 4.8 and earlier, a bug in the kernel's  accounting  of  locked
245       memory  for  unprivileged  processes (i.e., without CAP_IPC_LOCK) meant
246       that if the region specified by addr and  len  overlapped  an  existing
247       lock,  then  the  already  locked  bytes in the overlapping region were
248       counted twice when checking against the limit.  Such double  accounting
249       could  incorrectly  calculate  a  "total  locked  memory" value for the
250       process that exceeded the RLIMIT_MEMLOCK limit, with  the  result  that
251       mlock() and mlock2() would fail on requests that should have succeeded.
252       This bug was fixed in Linux 4.9.
253
254       In the 2.4 series Linux kernels up  to  and  including  2.4.17,  a  bug
255       caused the mlockall() MCL_FUTURE flag to be inherited across a fork(2).
256       This was rectified in kernel 2.4.18.
257
258       Since kernel 2.6.9, if a privileged process calls  mlockall(MCL_FUTURE)
259       and  later  drops privileges (loses the CAP_IPC_LOCK capability by, for
260       example, setting its effective UID to a nonzero value), then subsequent
261       memory allocations (e.g., mmap(2), brk(2)) will fail if the RLIMIT_MEM‐
262       LOCK resource limit is encountered.
263

SEE ALSO

265       mincore(2), mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5),  ca‐
266       pabilities(7)
267

COLOPHON

269       This  page  is  part of release 5.10 of the Linux man-pages project.  A
270       description of the project, information about reporting bugs,  and  the
271       latest     version     of     this    page,    can    be    found    at
272       https://www.kernel.org/doc/man-pages/.
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276Linux                             2020-04-11                          MLOCK(2)
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