NAME¶

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

LIBRARY¶

Standard C library (libc, -lc)

SYNOPSIS¶

#include <sys/mman.h>

int mlock(const void addr[.len], size_t len);
int mlock2(const void addr[.len], size_t len, unsigned int flags);
int munlock(const void addr[.len], 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 can be swapped out again if required by the kernel memory manager.

Memory locking and unlocking are performed in units of whole pages.

RETURN VALUE¶

On success, these system calls return 0. On error, -1 is returned, errno is set to indicate the error, and no changes are made to any locks in the address space of the process.

ERRORS¶

EAGAIN

(mlock(), mlock2(), and munlock()) Some or all of the specified address range could not be locked.

EINVAL

(mlock(), mlock2(), and munlock()) The result of the addition addr+len was less than addr (e.g., the addition may have resulted in an overflow).

EINVAL

(mlock2()) Unknown flags were specified.

EINVAL

(mlockall()) Unknown flags were specified or MCL_ONFAULT was specified without either MCL_FUTURE or MCL_CURRENT.

EINVAL

(Not on Linux) addr was not a multiple of the page size.

ENOMEM

(mlock(), mlock2(), and munlock()) Some of the specified address range does not correspond to mapped pages in the address space of the process.

ENOMEM

(mlock(), mlock2(), and munlock()) 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.)

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.

EPERM

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

VERSIONS¶

STANDARDS¶

mlock()

munlock()

mlockall()

munlockall()

POSIX.1-2008.

mlock2()

Linux.

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).)

HISTORY¶

mlock()

munlock()

mlockall()

munlockall()

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

mlock2()

Linux 4.4, glibc 2.27.

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).

Note that fork(2) will prepare the address space for a copy-on-write operation. The consequence is that any write access that follows will cause a page fault that in turn may cause high latencies for a real-time process. Therefore, it is crucial not to invoke fork(2) after an mlockall() or mlock() operation—not even from a thread which runs at a low priority within a process which also has a thread running at elevated priority.

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.

BUGS¶

In Linux 4.8 and earlier, a bug in the kernel's accounting of locked memory for unprivileged processes (i.e., without CAP_IPC_LOCK) meant that if the region specified by addr and len overlapped an existing lock, then the already locked bytes in the overlapping region were counted twice when checking against the limit. Such double accounting could incorrectly calculate a "total locked memory" value for the process that exceeded the RLIMIT_MEMLOCK limit, with the result that mlock() and mlock2() would fail on requests that should have succeeded. This bug was fixed in Linux 4.9.

In Linux 2.4 series of kernels up to and including Linux 2.4.17, a bug caused the mlockall() MCL_FUTURE flag to be inherited across a fork(2). This was rectified in Linux 2.4.18.

Since Linux 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.

SEE ALSO¶

mincore(2), mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5), capabilities(7)