mlock, munlock - lock or unlock pages in memory
int mlock(caddr_t addr, size_t len);
int munlock(caddr_t addr, size_t len);
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
A given page may be locked multiple times by executing an
mlock() through different mappings. That is, if two dif-
ferent processes lock the same page, then the page will
remain locked until both processes remove their locks. How-
ever, within a given mapping, page locks do not nest - mul-
tiple 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 pro-
cess) is still locked in memory. This fact can be used to
create applications that do nothing other than lock impor-
tant data in memory, thereby avoiding page I/O faults on
references from other processes in the system.
If the mapping through which an mlock() has been performed
is removed, an munlock() is implicitly performed. An mun-
lock() 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
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 inter-
EPERM The process's effective user ID is not superuser.
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.
Because of the impact on system resources, the use of
mlock() and munlock() is restricted to the superuser.
See attributes(5) for descriptions of the following attri-
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| Interface Stability | Standard |
| MT-Level | MT-Safe |
fork(2), memcntl(2), mmap(2), plock(3C), mlockall(3C),
sysconf(3C), attributes(5), standards(5)
Man(1) output converted with