man/man2/mlock.2

.\"	$NetBSD: mlock.2,v 1.3 1995/06/24 10:42:03 cgd Exp $
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.\"	@(#)mlock.2	8.2 (Berkeley) 12/11/93
.\"
.Dd June 2, 1993
.Dt MLOCK 2
.Os
.Sh NAME
.Nm mlock ,
.Nm munlock
.Nd lock (unlock) physical pages in memory
.Sh SYNOPSIS
.Fd #include <sys/mman.h>
.Ft int
.Fo mlock
.Fa "const void *addr"
.Fa "size_t len"
.Fc
.Ft int
.Fo munlock
.Fa "const void *addr"
.Fa "size_t len"
.Fc
.Sh DESCRIPTION
The
.Nm mlock
system call
locks a set of physical pages into memory.
The pages are associated with a virtual address range
that starts at
.Fa addr
and extends for
.Fa len
bytes.
The
.Nm munlock
call unlocks pages that were previously locked by one or more
.Nm mlock
calls.
For both calls, the
.Fa addr
parameter should be aligned to a multiple of the page size.
If the
.Fa len
parameter is not a multiple of the page size,
it will be rounded up to be so.
The entire range must be allocated.
.Pp
After an
.Nm mlock
call, the indicated pages will cause neither a non-resident page
nor address-translation fault until they are unlocked.
They may still cause protection-violation faults or TLB-miss faults
on architectures with software-managed TLBs.
The physical pages remain in memory until all locked mappings
for the pages are removed.
.Pp
Multiple processes may have the same physical pages locked
via their own virtual address mappings.
Similarly, a single process may have pages multiply-locked
via different virtual mappings of the same pages or via nested
.Nm mlock
calls on the same address range.
Unlocking is performed explicitly by
.Nm munlock
or implicitly by a call to
.Nm munmap ,
which deallocates the unmapped address range.
Locked mappings are not inherited by the child process after a
.Xr fork 2 .
.Pp
Because physical memory is a potentially scarce resource,
processes are limited in how much memory they can lock down.
A single process can
.Nm mlock
the minimum of
a system-wide ``wired pages'' limit and
the per-process
.Li RLIMIT_MEMLOCK
resource limit.
.Sh RETURN VALUES
A return value of 0 indicates that the call succeeded
and all pages in the range have either been locked or unlocked,
as requested.
A return value of -1 indicates an error occurred
and the locked status of all pages in the range remains unchanged.
In this case, the global location
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn mlock
and
.Fn munlock
will fail if:
.Bl -tag -width Er
.\" ===========
.It Bq Er EINVAL
The address given is not page-aligned or the length is negative.
.\" ===========
.It Bq Er ENOMEM
Part or all of the specified address range
is not mapped to the process.
.El
.Pp
.Fn mlock
will fail if:
.Bl -tag -width Er
.\" ===========
.It Bq Er EAGAIN
Locking the indicated range would exceed either the system or per-process
limit for locked memory.
.\" ===========
.It Bq Er ENOMEM
Some portion of the indicated address range is not allocated.
There was an error faulting/mapping a page.
.El
.Pp
.Fn munlock
will fail if:
.Bl -tag -width Er
.\" ===========
.It Bq Er ENOMEM
Some portion of the indicated address range is not allocated.
Some portion of the indicated address range is not locked.
.El
.Sh LEGACY SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/mman.h>
.Pp
The include file
.In sys/types.h
is necessary.
.Pp
.Ft int
.br
.Fo mlock
.Fa "caddr_t addr"
.Fa "size_t len"
.Fc ;
.Pp
.Ft int
.br
.Fo munlock
.Fa "caddr_t addr"
.Fa "size_t len"
.Fc ;
.Pp
The variable type of
.Fa addr
has changed.
.Sh "SEE ALSO"
.Xr fork 2 ,
.Xr mincore 2 ,
.Xr minherit 2 ,
.Xr mmap 2 ,
.Xr munmap 2 ,
.Xr setrlimit 2 ,
.Xr getpagesize 3 ,
.Xr compat 5
.Sh BUGS
Unlike The Sun implementation, multiple
.Nm mlock
calls on the same address range require the corresponding number of
.Nm munlock
calls to actually unlock the pages, i.e.
.Nm mlock
nests.
This should be considered a consequence of the implementation
and not a feature.
.Pp
The per-process resource limit is a limit on the amount of virtual
memory locked, while the system-wide limit is for the number of locked
physical pages.
Hence a process with two distinct locked mappings of the same physical page
counts as 2 pages against the per-process limit and as only a single page
in the system limit.
.Sh HISTORY
The
.Fn mlock
and
.Fn munlock
functions first appeared in 4.4BSD.