451 lines
12 KiB
Groff
451 lines
12 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.
|
|
.\" 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 February 18, 2015
|
|
.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_PREFAULT_READ
|
|
.It Dv MAP_32BIT
|
|
Request a region in the first 2GB of the current process's address space.
|
|
If a suitable region cannot be found,
|
|
.Fn mmap
|
|
will fail.
|
|
This flag is only available on 64-bit platforms.
|
|
.It Dv MAP_ALIGNED Ns Pq Fa n
|
|
Align the region on a requested boundary.
|
|
If a suitable region cannot be found,
|
|
.Fn mmap
|
|
will fail.
|
|
The
|
|
.Fa n
|
|
argument specifies the binary logarithm of the desired alignment.
|
|
.It Dv MAP_ALIGNED_SUPER
|
|
Align the region to maximize the potential use of large
|
|
.Pq Dq super
|
|
pages.
|
|
If a suitable region cannot be found,
|
|
.Fn mmap
|
|
will fail.
|
|
The system will choose a suitable page size based on the size of
|
|
mapping.
|
|
The page size used as well as the alignment of the region may both be
|
|
affected by properties of the file being mapped.
|
|
In particular,
|
|
the physical address of existing pages of a file may require a specific
|
|
alignment.
|
|
The region is not guaranteed to be aligned on any specific boundary.
|
|
.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 must be 0.
|
|
.\".It Dv MAP_FILE
|
|
.\"Mapped from a regular file or character-special device memory.
|
|
.It Dv MAP_ANONYMOUS
|
|
This flag is identical to
|
|
.Dv MAP_ANON
|
|
and is provided for compatibility.
|
|
.It Dv MAP_EXCL
|
|
This flag can only be used in combination with
|
|
.Dv MAP_FIXED .
|
|
Please see the definition of
|
|
.Dv MAP_FIXED
|
|
for the description of its effect.
|
|
.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.
|
|
If
|
|
.Dv MAP_EXCL
|
|
is not specified, a successful
|
|
.Dv MAP_FIXED
|
|
request replaces any previous mappings for the process'
|
|
pages in the range from
|
|
.Fa addr
|
|
to
|
|
.Fa addr
|
|
+
|
|
.Fa len .
|
|
In contrast, if
|
|
.Dv MAP_EXCL
|
|
is specified, the request will fail if a mapping
|
|
already exists within the range.
|
|
.It Dv MAP_HASSEMAPHORE
|
|
Notify the kernel that the region may contain semaphores and that special
|
|
handling may be necessary.
|
|
.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 usually not needed 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_PREFAULT_READ
|
|
Immediately update the calling process's lowest-level virtual address
|
|
translation structures, such as its page table, so that every memory
|
|
resident page within the region is mapped for read access.
|
|
Ordinarily these structures are updated lazily.
|
|
The effect of this option is to eliminate any soft faults that would
|
|
otherwise occur on the initial read accesses to the region.
|
|
Although this option does not preclude
|
|
.Fa prot
|
|
from including
|
|
.Dv PROT_WRITE ,
|
|
it does not eliminate soft faults on the initial write accesses to the
|
|
region.
|
|
.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.
|
|
.Sh NOTES
|
|
Although this implementation does not impose any alignment restrictions on
|
|
the
|
|
.Fa offset
|
|
argument, a portable program must only use page-aligned values.
|
|
.Pp
|
|
Large page mappings require that the pages backing an object be
|
|
aligned in matching blocks in both the virtual address space and RAM.
|
|
The system will automatically attempt to use large page mappings when
|
|
mapping an object that is already backed by large pages in RAM by
|
|
aligning the mapping request in the virtual address space to match the
|
|
alignment of the large physical pages.
|
|
The system may also use large page mappings when mapping portions of an
|
|
object that are not yet backed by pages in RAM.
|
|
The
|
|
.Dv MAP_ALIGNED_SUPER
|
|
flag is an optimization that will align the mapping request to the
|
|
size of a large page similar to
|
|
.Dv MAP_ALIGNED ,
|
|
except that the system will override this alignment if an object already
|
|
uses large pages so that the mapping will be consistent with the existing
|
|
large pages.
|
|
This flag is mostly useful for maximizing the use of large pages on the
|
|
first mapping of objects that do not yet have pages present in RAM.
|
|
.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
|
|
An invalid value was passed in the
|
|
.Fa prot
|
|
argument.
|
|
.It Bq Er EINVAL
|
|
An undefined option was set in the
|
|
.Fa flags
|
|
argument.
|
|
.It Bq Er EINVAL
|
|
Both
|
|
.Dv MAP_PRIVATE
|
|
and
|
|
.Dv MAP_SHARED
|
|
were specified.
|
|
.It Bq Er EINVAL
|
|
None of
|
|
.Dv MAP_ANON ,
|
|
.Dv MAP_PRIVATE ,
|
|
.Dv MAP_SHARED ,
|
|
or
|
|
.Dv MAP_STACK
|
|
was specified.
|
|
At least one of these flags must be included.
|
|
.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
|
|
Both
|
|
.Dv MAP_FIXED
|
|
and
|
|
.Dv MAP_32BIT
|
|
were specified and part of the desired address space resides outside
|
|
of the first 2GB of user address space.
|
|
.It Bq Er EINVAL
|
|
The
|
|
.Fa len
|
|
argument
|
|
was equal to zero.
|
|
.It Bq Er EINVAL
|
|
.Dv MAP_ALIGNED
|
|
was specified and the desired alignment was either larger than the
|
|
virtual address size of the machine or smaller than a page.
|
|
.It Bq Er EINVAL
|
|
.Dv MAP_ANON
|
|
was specified and the
|
|
.Fa fd
|
|
argument was not -1.
|
|
.It Bq Er EINVAL
|
|
.Dv MAP_ANON
|
|
was specified and the
|
|
.Fa offset
|
|
argument was not 0.
|
|
.It Bq Er EINVAL
|
|
Both
|
|
.Dv MAP_FIXED
|
|
and
|
|
.Dv MAP_EXCL
|
|
were specified, but the requested region is already used by a mapping.
|
|
.It Bq Er EINVAL
|
|
.Dv MAP_EXCL
|
|
was specified, but
|
|
.Dv MAP_FIXED
|
|
was not.
|
|
.It Bq Er ENODEV
|
|
.Dv MAP_ANON
|
|
has not been specified and
|
|
.Fa fd
|
|
did not reference a regular or character special file.
|
|
.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.
|
|
.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 getpagesizes 3
|