DESCRIPTION

mlock() and mlockall() respectively 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, respectively 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.

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

For mlock() and munlock():

For mlockall():

For munlockall():

CONFORMING TO

POSIX.1-2001, SVr4

AVAILABILITY

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).)

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 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() 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.

BUGS

In the 2.4 series Linux kernels up to and including 2.4.17, a bug caused the mlockall() MCL_FUTURE 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(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

mmap(2), shmctl(2), setrlimit(2), sysconf(3), capabilities(7)