MLOCK(2) Linux Programmer's Manual MLOCK(2)
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 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 pro-
cesses do not inherit page locks across a fork.
Memory locking has two main applications: real-time algorithms and high-security data pro-
cessing. 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 secu-
rity software often handles critical bytes like passwords or secret keys as data struc-
tures. 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
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
With the Linux system call, addr is automatically rounded down to the nearest page bound-
ary. However, POSIX 1003.1-2001 allows an implementation to require that addr is page
aligned, so portable applications should ensure this.
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 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.
POSIX.1b, SVr4. SVr4 documents an additional EAGAIN error code.
mlockall(2), munlock(2), munlockall(2), munmap(2), setrlimit(2)
Linux 1.3.43 1995-11-26 MLOCK(2)