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Manpage of MLOCK

MLOCK

Index
 

NAME

mlock - disable paging for some parts of memory  

SYNOPSIS

#include <sys/mman.h>

fBint mlock(const void *fIaddrfB, size_t fIlenfB);
 

DESCRIPTION

mlock disables paging for the memory in the range starting at addr with length len bytes. All pages which contain a part of the specified memory range are guaranteed be resident in RAM when the mlock system call returns successfully and they are guaranteed to stay in RAM until the pages are unlocked by munlock or munlockall, until the pages are unmapped via munmap, or until the process terminates or starts another program with exec. Child processes do not inherit page locks across a fork.

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. 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.)

Memory locks do not stack, i.e., 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.

On POSIX systems on which mlock and munlock are available, _POSIX_MEMLOCK_RANGE is defined in <unistd.h> and the value PAGESIZE from <limits.h> indicates the number of bytes per page.  

NOTES

With the Linux system call, addr is automatically rounded down to the nearest page boundary. However, POSIX 1003.1-2001 allows an implementation to require that addr is page aligned, so portable applications should ensure this.  

RETURN VALUE

On success, mlock returns zero. 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
Some of the specified address range does not correspond to mapped pages in the address space of the process or the process tried to exceed the maximum number of allowed locked pages.
EPERM
The calling process does not have appropriate privileges. Only root processes are allowed to lock pages.
EINVAL
(Not on Linux) addr was not a multiple of the page size.

Linux adds

EINVAL
len was negative.
 

CONFORMING TO

POSIX.1b, SVr4. SVr4 documents an additional EAGAIN error code.  

SEE ALSO

mlockall(2), munlock(2), munlockall(2), munmap(2), setrlimit(2)


 

Index

NAME
SYNOPSIS
DESCRIPTION
NOTES
RETURN VALUE
ERRORS
CONFORMING TO
SEE ALSO