MLOCK(3C) Standard C Library Functions MLOCK(3C)

NAME

mlock, munlock - lock or unlock pages in memory

SYNOPSIS

#include <sys/mman.h>

int
mlock(const void *addr, size_t len);

int
munlock(const void *addr, size_t len);

DESCRIPTION

The mlock() function uses the mappings established for the address
range [addr, addr + len) to identify pages to be locked in memory. If
the page identified by a mapping changes, such as occurs when a copy of
a writable MAP_PRIVATE page is made upon the first store, the lock will
be transferred to the newly copied private page.

The munlock() function removes locks established with mlock().

A given page may be locked multiple times by executing an mlock()
through different mappings. That is, if two different processes lock
the same page, then the page will remain locked until both processes
remove their locks. However, within a given mapping, page locks do not
nest -- multiple mlock() operations on the same address in the same
process will all be removed with a single munlock(). Of course, a page
locked in one process and mapped in another (or visible through a
different mapping in the locking process) is still locked in memory.
This fact can be used to create applications that do nothing other than
lock important data in memory, thereby avoiding page I/O faults on
references from other processes in the system.

The contents of the locked pages will not be transferred to or from
disk except when explicitly requested by one of the locking processes.
This guarantee applies only to the mapped data, and not to any
associated data structures (file descriptors and on-disk metadata,
among others).

If the mapping through which an mlock() has been performed is removed,
an munlock() is implicitly performed. An munlock() is also performed
implicitly when a page is deleted through file removal or truncation.

Locks established with mlock() are not inherited by a child process
after a fork(2) and are not nested.

Attempts to mlock() more memory than a system-specific limit will fail.

RETURN VALUES

Upon successful completion, the mlock() and munlock() functions return
0. Otherwise, no changes are made to any locks in the address space of
the process, the functions return -1 and set errno to indicate the
error.

ERRORS

The mlock() and munlock() functions will fail if:

EINVAL The addr argument is not a multiple of the page size
as returned by sysconf(3C).

ENOMEM Addresses in the range [addr, addr + len) are
invalid for the address space of a process, or
specify one or more pages which are not mapped.

ENOSYS The system does not support this memory locking
interface.

EPERM The {PRIV_PROC_LOCK_MEMORY} privilege is not
asserted in the effective set of the calling
process.

The mlock() function will fail if:

EAGAIN Some or all of the memory identified by the range
[addr, addr + len) could not be locked because of
insufficient system resources or because of a limit
or resource control on locked memory.

USAGE

Because of the impact on system resources, the use of mlock() and
munlock() is restricted to users with the {PRIV_PROC_LOCK_MEMORY}
privilege.

INTERFACE STABILITY

Committed

MT-LEVEL
MT-Safe

SEE ALSO

fork(2), memcntl(2), mmap(2), mlockall(3C), plock(3C), sysconf(3C),
attributes(7), standards(7)

illumos March 13, 2022 illumos