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, unsigned 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 to indicate the error, and no changes are made to any  locks
100       in the 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       mlock(),  munlock(),  mlockall(),   and   munlockall():   POSIX.1-2001,
153       POSIX.1-2008, SVr4.
154
155       mlock2() is Linux specific.
156
157       On  POSIX  systems  on  which  mlock()  and  munlock()  are  available,
158       _POSIX_MEMLOCK_RANGE is defined in <unistd.h> and the number  of  bytes
159       in  a page can be determined from the constant PAGESIZE (if defined) in
160       <limits.h> or by calling sysconf(_SC_PAGESIZE).
161
162       On POSIX systems on which mlockall() and  munlockall()  are  available,
163       _POSIX_MEMLOCK  is  defined  in  <unistd.h>  to a value greater than 0.
164       (See also sysconf(3).)
165

NOTES

167       Memory locking has two  main  applications:  real-time  algorithms  and
168       high-security  data  processing.  Real-time applications require deter‐
169       ministic timing, and, like scheduling, paging is one major cause of un‐
170       expected program execution delays.  Real-time applications will usually
171       also switch to a real-time scheduler with sched_setscheduler(2).  Cryp‐
172       tographic security software often handles critical bytes like passwords
173       or secret keys as data structures.  As a result of  paging,  these  se‐
174       crets  could  be transferred onto a persistent swap store medium, where
175       they might be accessible to the enemy long after the security  software
176       has  erased  the secrets in RAM and terminated.  (But be aware that the
177       suspend mode on laptops and some desktop computers will save a copy  of
178       the system's RAM to disk, regardless of memory locks.)
179
180       Real-time processes that are using mlockall() to prevent delays on page
181       faults should reserve enough locked stack  pages  before  entering  the
182       time-critical  section, so that no page fault can be caused by function
183       calls.  This can be achieved by calling a  function  that  allocates  a
184       sufficiently large automatic variable (an array) and writes to the mem‐
185       ory occupied by this array in order to touch these stack  pages.   This
186       way,  enough  pages will be mapped for the stack and can be locked into
187       RAM.  The dummy writes ensure that not even copy-on-write  page  faults
188       can occur in the critical section.
189
190       Memory  locks  are not inherited by a child created via fork(2) and are
191       automatically removed  (unlocked)  during  an  execve(2)  or  when  the
192       process terminates.  The mlockall() MCL_FUTURE and MCL_FUTURE | MCL_ON‐
193       FAULT settings are not inherited by a child created via fork(2) and are
194       cleared during an execve(2).
195
196       Note  that  fork(2)  will prepare the address space for a copy-on-write
197       operation.  The consequence is that any write access that follows  will
198       cause  a  page  fault that in turn may cause high latencies for a real-
199       time process.  Therefore, it is crucial not to invoke fork(2) after  an
200       mlockall()  or mlock() operation—not even from a thread which runs at a
201       low priority within a process which also has a thread running  at  ele‐
202       vated priority.
203
204       The memory lock on an address range is automatically removed if the ad‐
205       dress range is unmapped via munmap(2).
206
207       Memory locks do not stack, that is, pages which have been  locked  sev‐
208       eral  times  by  calls  to mlock(), mlock2(), or mlockall() will be un‐
209       locked by a single call to munlock() for the corresponding range or  by
210       munlockall().   Pages  which are mapped to several locations or by sev‐
211       eral processes stay locked into RAM as long as they are locked at least
212       at one location or by at least one process.
213
214       If  a  call to mlockall() which uses the MCL_FUTURE flag is followed by
215       another call that does not specify this flag, the changes made  by  the
216       MCL_FUTURE call will be lost.
217
218       The mlock2() MLOCK_ONFAULT flag and the mlockall() MCL_ONFAULT flag al‐
219       low efficient memory locking for applications that deal with large map‐
220       pings where only a (small) portion of pages in the mapping are touched.
221       In such cases, locking all of the pages in a mapping would incur a sig‐
222       nificant penalty for memory locking.
223
224   Linux notes
225       Under  Linux, mlock(), mlock2(), and munlock() automatically round addr
226       down to the nearest page boundary.  However, the POSIX.1  specification
227       of  mlock() and munlock() allows an implementation to require that addr
228       is page aligned, so portable applications should ensure this.
229
230       The VmLck field of the Linux-specific /proc/[pid]/status file shows how
231       many  kilobytes  of  memory  the  process  with ID PID has locked using
232       mlock(), mlock2(), mlockall(), and mmap(2) MAP_LOCKED.
233
234   Limits and permissions
235       In Linux 2.6.8 and earlier, a process must be privileged (CAP_IPC_LOCK)
236       in  order to lock memory and the RLIMIT_MEMLOCK soft resource limit de‐
237       fines a limit on how much memory the process may lock.
238
239       Since Linux 2.6.9, no limits are placed on the amount of memory that  a
240       privileged  process can lock and the RLIMIT_MEMLOCK soft resource limit
241       instead defines a limit on how much memory an unprivileged process  may
242       lock.
243

BUGS

245       In  Linux  4.8  and earlier, a bug in the kernel's accounting of locked
246       memory for unprivileged processes (i.e.,  without  CAP_IPC_LOCK)  meant
247       that  if  the  region  specified by addr and len overlapped an existing
248       lock, then the already locked bytes  in  the  overlapping  region  were
249       counted  twice when checking against the limit.  Such double accounting
250       could incorrectly calculate a  "total  locked  memory"  value  for  the
251       process  that  exceeded  the RLIMIT_MEMLOCK limit, with the result that
252       mlock() and mlock2() would fail on requests that should have succeeded.
253       This bug was fixed in Linux 4.9.
254
255       In  the  2.4  series  Linux  kernels  up to and including 2.4.17, a bug
256       caused the mlockall() MCL_FUTURE flag to be inherited across a fork(2).
257       This was rectified in kernel 2.4.18.
258
259       Since  kernel 2.6.9, if a privileged process calls mlockall(MCL_FUTURE)
260       and later drops privileges (loses the CAP_IPC_LOCK capability  by,  for
261       example, setting its effective UID to a nonzero value), then subsequent
262       memory allocations (e.g., mmap(2), brk(2)) will fail if the RLIMIT_MEM‐
263       LOCK resource limit is encountered.
264

SEE ALSO

266       mincore(2),  mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5), ca‐
267       pabilities(7)
268

COLOPHON

270       This page is part of release 5.12 of the Linux  man-pages  project.   A
271       description  of  the project, information about reporting bugs, and the
272       latest    version    of    this    page,    can     be     found     at
273       https://www.kernel.org/doc/man-pages/.
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277Linux                             2021-03-22                          MLOCK(2)
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