|MLOCK(2)||Linux Programmer's Manual||MLOCK(2)|
NAMEmlock, munlock, mlockall, munlockall - lock and unlock memory
int mlock(const void * addr , size_t len );
int munlock(const void * addr , size_t len );
int mlockall(int flags ); int
DESCRIPTIONmlock() 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.
mlock() and munlock()mlock() locks pages in the address range starting at addr and continuing 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.
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.
- Lock all pages which are currently mapped into the address space of the process.
- 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.
If MCL_FUTURE 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 VALUEOn 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.
- (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).
- (Linux 2.4 and earlier) the calling process tried to lock more than half of RAM.
- The caller is not privileged, but needs privilege ( CAP_IPC_LOCK) to perform the requested operation.
For mlock() and munlock():
- Some or all of the specified address range could not be locked.
- The result of the addition start+ len was less than start (e.g., the addition may have resulted in an overflow).
- (Not on Linux) addr was not a multiple of the page size.
- Some of the specified address range does not correspond to mapped pages in the address space of the process.
- Unknown flags were specified.
- (Linux 2.6.8 and earlier) The caller was not privileged ( CAP_IPC_LOCK).
CONFORMING TOPOSIX.1-2001, SVr4.
AVAILABILITYOn 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).
NOTESMemory 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.)
Linux notesUnder Linux, mlock() and munlock() automatically round addr down to the nearest page boundary. However, POSIX.1-2001 allows an implementation to require that addr is page aligned, so portable applications should ensure this.
Limits and permissionsIn Linux 2.6.8 and earlier, a process must be privileged ( CAP_IPC_LOCK) in order to lock memory and the RLIMIT_MEMLOCK soft resource limit defines a limit on how much memory the process may lock.
BUGSIn 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.
SEE ALSOmmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5), capabilities(7)
COLOPHONThis page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.