373 lines
9.8 KiB
Groff
373 lines
9.8 KiB
Groff
.\" Copyright (c) 1991, 1993
|
|
.\" The Regents of the University of California. All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\" 3. All advertising materials mentioning features or use of this software
|
|
.\" must display the following acknowledgement:
|
|
.\" This product includes software developed by the University of
|
|
.\" California, Berkeley and its contributors.
|
|
.\" 4. Neither the name of the University nor the names of its contributors
|
|
.\" may be used to endorse or promote products derived from this software
|
|
.\" without specific prior written permission.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
|
|
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
|
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
|
|
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
|
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
|
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
|
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
|
.\" SUCH DAMAGE.
|
|
.\"
|
|
.\" @(#)mmap.2 8.4 (Berkeley) 5/11/95
|
|
.\" $FreeBSD$
|
|
.\"
|
|
.Dd November 17, 2001
|
|
.Dt MMAP 2
|
|
.Os
|
|
.Sh NAME
|
|
.Nm mmap
|
|
.Nd allocate memory, or map files or devices into memory
|
|
.Sh LIBRARY
|
|
.Lb libc
|
|
.Sh SYNOPSIS
|
|
.In sys/mman.h
|
|
.Ft void *
|
|
.Fn mmap "void *addr" "size_t len" "int prot" "int flags" "int fd" "off_t offset"
|
|
.Sh DESCRIPTION
|
|
The
|
|
.Fn mmap
|
|
system call causes the pages starting at
|
|
.Fa addr
|
|
and continuing for at most
|
|
.Fa len
|
|
bytes to be mapped from the object described by
|
|
.Fa fd ,
|
|
starting at byte offset
|
|
.Fa offset .
|
|
If
|
|
.Fa len
|
|
is not a multiple of the pagesize, the mapped region may extend past the
|
|
specified range.
|
|
Any such extension beyond the end of the mapped object will be zero-filled.
|
|
.Pp
|
|
If
|
|
.Fa addr
|
|
is non-zero, it is used as a hint to the system.
|
|
(As a convenience to the system, the actual address of the region may differ
|
|
from the address supplied.)
|
|
If
|
|
.Fa addr
|
|
is zero, an address will be selected by the system.
|
|
The actual starting address of the region is returned.
|
|
A successful
|
|
.Fa mmap
|
|
deletes any previous mapping in the allocated address range.
|
|
.Pp
|
|
The protections (region accessibility) are specified in the
|
|
.Fa prot
|
|
argument by
|
|
.Em or Ns 'ing
|
|
the following values:
|
|
.Pp
|
|
.Bl -tag -width PROT_WRITE -compact
|
|
.It Dv PROT_NONE
|
|
Pages may not be accessed.
|
|
.It Dv PROT_READ
|
|
Pages may be read.
|
|
.It Dv PROT_WRITE
|
|
Pages may be written.
|
|
.It Dv PROT_EXEC
|
|
Pages may be executed.
|
|
.El
|
|
.Pp
|
|
The
|
|
.Fa flags
|
|
argument specifies the type of the mapped object, mapping options and
|
|
whether modifications made to the mapped copy of the page are private
|
|
to the process or are to be shared with other references.
|
|
Sharing, mapping type and options are specified in the
|
|
.Fa flags
|
|
argument by
|
|
.Em or Ns 'ing
|
|
the following values:
|
|
.Bl -tag -width MAP_HASSEMAPHORE
|
|
.It Dv MAP_ANON
|
|
Map anonymous memory not associated with any specific file.
|
|
The file descriptor used for creating
|
|
.Dv MAP_ANON
|
|
must be \-1.
|
|
The
|
|
.Fa offset
|
|
argument is ignored.
|
|
.\".It Dv MAP_FILE
|
|
.\"Mapped from a regular file or character-special device memory.
|
|
.It Dv MAP_FIXED
|
|
Do not permit the system to select a different address than the one
|
|
specified.
|
|
If the specified address cannot be used,
|
|
.Fn mmap
|
|
will fail.
|
|
If
|
|
.Dv MAP_FIXED
|
|
is specified,
|
|
.Fa addr
|
|
must be a multiple of the pagesize.
|
|
Use of this option is discouraged.
|
|
.It Dv MAP_HASSEMAPHORE
|
|
Notify the kernel that the region may contain semaphores and that special
|
|
handling may be necessary.
|
|
.It Dv MAP_INHERIT
|
|
This flag never operated as advertised and is no longer supported.
|
|
Please refer to
|
|
.Xr minherit 2
|
|
for further information.
|
|
.It Dv MAP_NOCORE
|
|
Region is not included in a core file.
|
|
.It Dv MAP_NOSYNC
|
|
Causes data dirtied via this VM map to be flushed to physical media
|
|
only when necessary (usually by the pager) rather than gratuitously.
|
|
Typically this prevents the update daemons from flushing pages dirtied
|
|
through such maps and thus allows efficient sharing of memory across
|
|
unassociated processes using a file-backed shared memory map.
|
|
Without
|
|
this option any VM pages you dirty may be flushed to disk every so often
|
|
(every 30-60 seconds usually) which can create performance problems if you
|
|
do not need that to occur (such as when you are using shared file-backed
|
|
mmap regions for IPC purposes).
|
|
Note that VM/file system coherency is
|
|
maintained whether you use
|
|
.Dv MAP_NOSYNC
|
|
or not.
|
|
This option is not portable
|
|
across
|
|
.Ux
|
|
platforms (yet), though some may implement the same behavior
|
|
by default.
|
|
.Pp
|
|
.Em WARNING !
|
|
Extending a file with
|
|
.Xr ftruncate 2 ,
|
|
thus creating a big hole, and then filling the hole by modifying a shared
|
|
.Fn mmap
|
|
can lead to severe file fragmentation.
|
|
In order to avoid such fragmentation you should always pre-allocate the
|
|
file's backing store by
|
|
.Fn write Ns ing
|
|
zero's into the newly extended area prior to modifying the area via your
|
|
.Fn mmap .
|
|
The fragmentation problem is especially sensitive to
|
|
.Dv MAP_NOSYNC
|
|
pages, because pages may be flushed to disk in a totally random order.
|
|
.Pp
|
|
The same applies when using
|
|
.Dv MAP_NOSYNC
|
|
to implement a file-based shared memory store.
|
|
It is recommended that you create the backing store by
|
|
.Fn write Ns ing
|
|
zero's to the backing file rather than
|
|
.Fn ftruncate Ns ing
|
|
it.
|
|
You can test file fragmentation by observing the KB/t (kilobytes per
|
|
transfer) results from an
|
|
.Dq Li iostat 1
|
|
while reading a large file sequentially, e.g.\& using
|
|
.Dq Li dd if=filename of=/dev/null bs=32k .
|
|
.Pp
|
|
The
|
|
.Xr fsync 2
|
|
system call will flush all dirty data and metadata associated with a file,
|
|
including dirty NOSYNC VM data, to physical media.
|
|
The
|
|
.Xr sync 8
|
|
command and
|
|
.Xr sync 2
|
|
system call generally do not flush dirty NOSYNC VM data.
|
|
The
|
|
.Xr msync 2
|
|
system call is obsolete since
|
|
.Bx
|
|
implements a coherent file system buffer cache.
|
|
However, it may be
|
|
used to associate dirty VM pages with file system buffers and thus cause
|
|
them to be flushed to physical media sooner rather than later.
|
|
.It Dv MAP_PRIVATE
|
|
Modifications are private.
|
|
.It Dv MAP_SHARED
|
|
Modifications are shared.
|
|
.It Dv MAP_STACK
|
|
.Dv MAP_STACK
|
|
implies
|
|
.Dv MAP_ANON ,
|
|
and
|
|
.Fa offset
|
|
of 0.
|
|
The
|
|
.Fa fd
|
|
argument
|
|
must be -1 and
|
|
.Fa prot
|
|
must include at least
|
|
.Dv PROT_READ
|
|
and
|
|
.Dv PROT_WRITE .
|
|
This option creates
|
|
a memory region that grows to at most
|
|
.Fa len
|
|
bytes in size, starting from the stack top and growing down.
|
|
The
|
|
stack top is the starting address returned by the call, plus
|
|
.Fa len
|
|
bytes.
|
|
The bottom of the stack at maximum growth is the starting
|
|
address returned by the call.
|
|
.El
|
|
.Pp
|
|
The
|
|
.Xr close 2
|
|
system call does not unmap pages, see
|
|
.Xr munmap 2
|
|
for further information.
|
|
.Pp
|
|
The current design does not allow a process to specify the location of
|
|
swap space.
|
|
In the future we may define an additional mapping type,
|
|
.Dv MAP_SWAP ,
|
|
in which
|
|
the file descriptor argument specifies a file or device to which swapping
|
|
should be done.
|
|
.Sh RETURN VALUES
|
|
Upon successful completion,
|
|
.Fn mmap
|
|
returns a pointer to the mapped region.
|
|
Otherwise, a value of
|
|
.Dv MAP_FAILED
|
|
is returned and
|
|
.Va errno
|
|
is set to indicate the error.
|
|
.Sh ERRORS
|
|
The
|
|
.Fn mmap
|
|
system call
|
|
will fail if:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EACCES
|
|
The flag
|
|
.Dv PROT_READ
|
|
was specified as part of the
|
|
.Fa prot
|
|
argument and
|
|
.Fa fd
|
|
was not open for reading.
|
|
The flags
|
|
.Dv MAP_SHARED
|
|
and
|
|
.Dv PROT_WRITE
|
|
were specified as part of the
|
|
.Fa flags
|
|
and
|
|
.Fa prot
|
|
argument and
|
|
.Fa fd
|
|
was not open for writing.
|
|
.It Bq Er EBADF
|
|
The
|
|
.Fa fd
|
|
argument
|
|
is not a valid open file descriptor.
|
|
.It Bq Er EINVAL
|
|
.Dv MAP_FIXED
|
|
was specified and the
|
|
.Fa addr
|
|
argument was not page aligned, or part of the desired address space
|
|
resides out of the valid address space for a user process.
|
|
.It Bq Er EINVAL
|
|
The
|
|
.Fa len
|
|
argument
|
|
was negative.
|
|
.It Bq Er EINVAL
|
|
.Dv MAP_ANON
|
|
was specified and the
|
|
.Fa fd
|
|
argument was not -1.
|
|
.It Bq Er EINVAL
|
|
.Dv MAP_ANON
|
|
has not been specified and
|
|
.Fa fd
|
|
did not reference a regular or character special file.
|
|
.It Bq Er EINVAL
|
|
The
|
|
.Fa offset
|
|
argument
|
|
was not page-aligned.
|
|
(See
|
|
.Sx BUGS
|
|
below.)
|
|
.It Bq Er ENOMEM
|
|
.Dv MAP_FIXED
|
|
was specified and the
|
|
.Fa addr
|
|
argument was not available.
|
|
.Dv MAP_ANON
|
|
was specified and insufficient memory was available.
|
|
The system has reached the per-process mmap limit specified in the
|
|
.Va vm.max_proc_mmap
|
|
sysctl.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr madvise 2 ,
|
|
.Xr mincore 2 ,
|
|
.Xr minherit 2 ,
|
|
.Xr mlock 2 ,
|
|
.Xr mprotect 2 ,
|
|
.Xr msync 2 ,
|
|
.Xr munlock 2 ,
|
|
.Xr munmap 2 ,
|
|
.Xr getpagesize 3 ,
|
|
.Xr make.conf 5
|
|
.Sh BUGS
|
|
The
|
|
.Fa len
|
|
argument
|
|
is limited to 2GB.
|
|
Mmapping slightly more than 2GB does not work, but
|
|
it is possible to map a window of size (filesize % 2GB) for file sizes
|
|
of slightly less than 2G, 4GB, 6GB and 8GB.
|
|
.Pp
|
|
The limit is imposed for a variety of reasons.
|
|
Most of them have to do
|
|
with
|
|
.Fx
|
|
not wanting to use 64 bit offsets in the VM system due to
|
|
the extreme performance penalty.
|
|
So
|
|
.Fx
|
|
uses 32bit page indexes and
|
|
this gives
|
|
.Fx
|
|
a maximum of 8TB filesizes.
|
|
It is actually bugs in
|
|
the file system code that causes the limit to be further restricted to
|
|
1TB (loss of precision when doing blockno calculations).
|
|
.Pp
|
|
Another reason for the 2GB limit is that file system metadata can
|
|
reside at negative offsets.
|
|
.Pp
|
|
Note that an attempt to
|
|
.Fn mmap
|
|
zero bytes has no effect and succeeds, while an attempt to
|
|
.Fn munmap
|
|
zero bytes will return
|
|
.Bq Er EINVAL .
|