NAME
mlock, mlock2, munlock, mlockall, munlockall - lock and unlock memory SYNOPSIS
#include
int mlock(const void *addr, sizet len); int mlock2(const void *addr, sizet len, int flags); int munlock(const void *addr, sizet 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 continu‐ ing 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: MLOCKONFAULT 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 con‐ tinuing 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: MCLCURRENT Lock all pages which are currently mapped into the address space of the process. MCLFUTURE 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. MCLONFAULT (since Linux 4.4) Used together with MCLCURRENT, MCLFUTURE, or both. Mark all current (with MCLCURRENT) or future (with MCLFUTURE) mappings to lock pages when they are faulted in. When used with MCLCURRENT, all present pages are locked, but mlock‐
all() will not fault in non-present pages. When used with MCLFUTURE, all future mappings will be marked to lock pages when they are faulted in, but they will not be popu‐ lated by the lock when the mapping is created. MCLONFAULT must be used with either MCLCURRENT or MCLFUTURE or both. If MCLFUTURE 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 RLIMITMEMLOCK soft resource limit, but tried to lock more memory than the limit permitted. This limit is not enforced if the process is privileged (CAPIPCLOCK). 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 (CAPIPCLOCK) 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. For mlock2(): EINVAL Unknown flags were specified. For mlockall(): EINVAL Unknown flags were specified or MCLONFAULT was specified with‐ out either MCLFUTURE or MCLCURRENT. For munlockall(): EPERM (Linux 2.6.8 and earlier) The caller was not privileged (CAPIPCLOCK). 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, POSIXMEMLOCKRANGE is defined in
and the number of bytes in a page can be determined from the constant PAGESIZE (if defined) in or by calling sysconf(SCPAGESIZE). On POSIX systems on which mlockall() and munlockall() are available, POSIXMEMLOCK is defined in 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 deter‐ ministic timing, and, like scheduling, paging is one major cause of
unexpected program execution delays. Real-time applications will usu‐
ally also switch to a real-time scheduler with schedsetscheduler(2). Cryptographic security software often handles critical bytes like pass‐ words 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 mem‐ ory 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() MCLFUTURE and MCLFUTURE | MCLONFAULT 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 sev‐ eral 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 MCLFUTURE flag is followed by another call that does not specify this flag, the changes made by the MCLFUTURE call will be lost. The mlock2() MLOCKONFAULT flag and the mlockall() MCLONFAULT 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) MAPLOCKED. Limits and permissions In Linux 2.6.8 and earlier, a process must be privileged (CAPIPCLOCK) in order to lock memory and the RLIMITMEMLOCK 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 RLIMITMEMLOCK 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() MCLFUTURE 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(MCLFUTURE) and later drops privileges (loses the CAPIPCLOCK 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 RLIMITMEM‐ LOCK resource limit is encountered. SEE ALSO mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5), capabilities(7) COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
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Linux 2015-08-28 MLOCK(2)