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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 dif‐
ferent 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 refer‐
ences 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 associ‐
ated data structures (file descriptors and on-disk metadata, among oth‐
ers).
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.
mlock() and munlock() have no effect when applied to shared memory seg‐
ments created by shmget_osm(2). To lock and unlock these segments, the
MC_LOCK_GRANULE and MC_UNLOCK_GRANULE interfaces of memcntl() should be
used instead.
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} priv‐
ilege.
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ Interface StabilityCommitted _ MT-LevelMT-Safe _ Stan‐
dardSee standards(7).
SEE ALSO
fork(2), memcntl(2), mmap(2), shmget_osm(2), mlockall(3C), plock(3C),
sysconf(3C), attributes(7), standards(7)
Oracle Solaris 11.4 16 Feb 2015 mlock(3C)