mlock
MLOCK(2) Linux Programmer's Manual MLOCK(2)
NAME
mlock - disable paging for some parts of memory
SYNOPSIS
#include <sys/mman.h>
int mlock(const void *addr, size_t len);
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 mem-
ory 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 determin-
istic timing, and, like scheduling, paging is one major cause of unex-
pected program execution delays. Real-time applications will usually
also switch to a real-time scheduler with sched_setscheduler. Crypto-
graphic security software often handles critical bytes like passwords
or secret keys as data structures. As a result of paging, these secrets
could be transfered 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.
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_MEM-
LOCK_RANGE is defined in <unistd.h> and the value PAGESIZE from <lim-
its.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 implemen-
tation 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 len was not a positive number.
CONFORMING TO
POSIX.1b, SVr4. SVr4 documents an additional EAGAIN error code.
SEE ALSO
mlockall(2), munlock(2), munlockall(2), munmap(2), setrlimit(2)
Linux 1.3.43 1995-11-26 MLOCK(2)
