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


mlock, munlock - lock or unlock pages in memory


#include <sys/mman.h>

int mlock(caddr_t addr, size_t len);

int munlock(caddr_t addr, size_t len);

Standard conforming

#include <sys/mman.h>

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

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


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() and are not nested.

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


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.


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

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

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.

The system does not support this memory locking interface.

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

The mlock() function will fail if:

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


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


See attributes(7) for descriptions of the following attributes:

|Interface Stability | Standard |
|MT-Level | MT-Safe |


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

April 10, 2007 MLOCK(3C)