mlock - disable paging for some parts of memory
int mlock(const void *addr, size_t len);
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 resi
dent in RAM when the mlock system call returns success
fully and they are guaranteed to stay in RAM until the
pages are unlocked by munlock or munlockall, 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 algo
rithms and high-security data processing. Real-time appli
cations require deterministic timing, and, like schedul
ing, paging is one major cause of unexpected program exe
cution 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 transfered onto
a persistent swap store medium, where they might be acces
sible 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_MEMLOCK_RANGE is defined in <unistd.h> and the
value PAGESIZE from <limits.h> indicates the number of
bytes per page.
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.
ENOMEM Some of the specified address range does not corre
spond to mapped pages in the address space of the
process or the process tried to exceed the maximum
ileges. Only root processes are allowed to lock
EINVAL len was not a positive number.
POSIX.1b, SVr4. SVr4 documents an additional EAGAIN error
munlock(2), mlockall(2), munlockall(2)