1MLOCK(2) Linux Programmer's Manual MLOCK(2)
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6 mlock, munlock, mlockall, munlockall - lock and unlock memory
7
9 #include <sys/mman.h>
10 int mlock(const void *addr, size_t len);
12 int munlock(const void *addr, size_t len);
13 int mlockall(int flags);
15 int munlockall(void);
16
18 mlock() and mlockall() respectively lock part or all of the calling
19 process's virtual address space into RAM, preventing that memory from
20 being paged to the swap area. munlock() and munlockall() perform the
21 converse operation, respectively unlocking part or all of the calling
22 process's virtual address space, so that pages in the specified virtual
23 address range may once more to be swapped out if required by the kernel
24 memory manager. Memory locking and unlocking are performed in units of
25 whole pages.
26 mlock() and munlock()
28 mlock() locks pages in the address range starting at addr and continu‐
29 ing for len bytes. All pages that contain a part of the specified
30 address range are guaranteed to be resident in RAM when the call
31 returns successfully; the pages are guaranteed to stay in RAM until
32 later unlocked.
33 munlock() unlocks pages in the address range starting at addr and con‐
35 tinuing for len bytes. After this call, all pages that contain a part
36 of the specified memory range can be moved to external swap space again
37 by the kernel.
38 mlockall() and munlockall()
40 mlockall() locks all pages mapped into the address space of the calling
41 process. This includes the pages of the code, data and stack segment,
42 as well as shared libraries, user space kernel data, shared memory, and
43 memory-mapped files. All mapped pages are guaranteed to be resident in
44 RAM when the call returns successfully; the pages are guaranteed to
45 stay in RAM until later unlocked.
46 The flags argument is constructed as the bitwise OR of one or more of
48 the following constants:
49 MCL_CURRENT Lock all pages which are currently mapped into the address
51 space of the process.
52 MCL_FUTURE Lock all pages which will become mapped into the address
54 space of the process in the future. These could be for
55 instance new pages required by a growing heap and stack as
56 well as new memory mapped files or shared memory regions.
57 If MCL_FUTURE has been specified, then a later system call (e.g.,
59 mmap(2), sbrk(2), malloc(3)), may fail if it would cause the number of
60 locked bytes to exceed the permitted maximum (see below). In the same
61 circumstances, stack growth may likewise fail: the kernel will deny
62 stack expansion and deliver a SIGSEGV signal to the process.
63 munlockall() unlocks all pages mapped into the address space of the
65 calling process.
66
68 On success these system calls return 0. On error, -1 is returned,
69 errno is set appropriately, and no changes are made to any locks in the
70 address space of the process.
71
73 ENOMEM (Linux 2.6.9 and later) the caller had a nonzero RLIMIT_MEMLOCK
74 soft resource limit, but tried to lock more memory than the
75 limit permitted. This limit is not enforced if the process is
76 privileged (CAP_IPC_LOCK).
77 ENOMEM (Linux 2.4 and earlier) the calling process tried to lock more
79 than half of RAM.
80 EPERM The caller is not privileged, but needs privilege (CAP_IPC_LOCK)
82 to perform the requested operation.
83 For mlock() and munlock():
85 EAGAIN Some or all of the specified address range could not be locked.
87 EINVAL The result of the addition start+len was less than start (e.g.,
89 the addition may have resulted in an overflow).
90 EINVAL (Not on Linux) addr was not a multiple of the page size.
92 ENOMEM Some of the specified address range does not correspond to
94 mapped pages in the address space of the process.
95 For mlockall():
97 EINVAL Unknown flags were specified.
99 For munlockall():
101 EPERM (Linux 2.6.8 and earlier) The caller was not privileged
103 (CAP_IPC_LOCK).
104
106 POSIX.1-2001, SVr4.
107
109 On POSIX systems on which mlock() and munlock() are available,
110 _POSIX_MEMLOCK_RANGE is defined in <unistd.h> and the number of bytes
111 in a page can be determined from the constant PAGESIZE (if defined) in
112 <limits.h> or by calling sysconf(_SC_PAGESIZE).
113 On POSIX systems on which mlockall() and munlockall() are available,
115 _POSIX_MEMLOCK is defined in <unistd.h> to a value greater than 0.
116 (See also sysconf(3).)
117
119 Memory locking has two main applications: real-time algorithms and
120 high-security data processing. Real-time applications require deter‐
121 ministic timing, and, like scheduling, paging is one major cause of
122 unexpected program execution delays. Real-time applications will usu‐
123 ally also switch to a real-time scheduler with sched_setscheduler(2).
124 Cryptographic security software often handles critical bytes like pass‐
125 words or secret keys as data structures. As a result of paging, these
126 secrets could be transferred onto a persistent swap store medium, where
127 they might be accessible to the enemy long after the security software
128 has erased the secrets in RAM and terminated. (But be aware that the
129 suspend mode on laptops and some desktop computers will save a copy of
130 the system's RAM to disk, regardless of memory locks.)
131 Real-time processes that are using mlockall() to prevent delays on page
133 faults should reserve enough locked stack pages before entering the
134 time-critical section, so that no page fault can be caused by function
135 calls. This can be achieved by calling a function that allocates a
136 sufficiently large automatic variable (an array) and writes to the mem‐
137 ory occupied by this array in order to touch these stack pages. This
138 way, enough pages will be mapped for the stack and can be locked into
139 RAM. The dummy writes ensure that not even copy-on-write page faults
140 can occur in the critical section.
141 Memory locks are not inherited by a child created via fork(2) and are
143 automatically removed (unlocked) during an execve(2) or when the
144 process terminates.
145 The memory lock on an address range is automatically removed if the
147 address range is unmapped via munmap(2).
148 Memory locks do not stack, that is, pages which have been locked sev‐
150 eral times by calls to mlock() or mlockall() will be unlocked by a sin‐
151 gle call to munlock() for the corresponding range or by munlockall().
152 Pages which are mapped to several locations or by several processes
153 stay locked into RAM as long as they are locked at least at one loca‐
154 tion or by at least one process.
155 Linux notes
157 Under Linux, mlock() and munlock() automatically round addr down to the
158 nearest page boundary. However, POSIX.1-2001 allows an implementation
159 to require that addr is page aligned, so portable applications should
160 ensure this.
161 The VmLck field of the Linux-specific /proc/PID/status file shows how
163 many kilobytes of memory the process with ID PID has locked using
164 mlock(), mlockall(), and mmap(2) MAP_LOCKED.
165 Limits and permissions
167 In Linux 2.6.8 and earlier, a process must be privileged (CAP_IPC_LOCK)
168 in order to lock memory and the RLIMIT_MEMLOCK soft resource limit
169 defines a limit on how much memory the process may lock.
170 Since Linux 2.6.9, no limits are placed on the amount of memory that a
172 privileged process can lock and the RLIMIT_MEMLOCK soft resource limit
173 instead defines a limit on how much memory an unprivileged process may
174 lock.
175
177 In the 2.4 series Linux kernels up to and including 2.4.17, a bug
178 caused the mlockall() MCL_FUTURE flag to be inherited across a fork(2).
179 This was rectified in kernel 2.4.18.
180 Since kernel 2.6.9, if a privileged process calls mlockall(MCL_FUTURE)
182 and later drops privileges (loses the CAP_IPC_LOCK capability by, for
183 example, setting its effective UID to a nonzero value), then subsequent
184 memory allocations (e.g., mmap(2), brk(2)) will fail if the RLIMIT_MEM‐
185 LOCK resource limit is encountered.
186
188 mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5), capabilities(7)
189
191 This page is part of release 3.53 of the Linux man-pages project. A
192 description of the project, and information about reporting bugs, can
193 be found at http://www.kernel.org/doc/man-pages/.
194Linux 2011-09-14 MLOCK(2)