libc/sys/mmap.2

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.\"	@(#)mmap.2	8.4 (Berkeley) 5/11/95
.\" $FreeBSD: src/lib/libc/sys/mmap.2,v 1.22.2.12 2002/02/27 03:40:13 dd Exp $
.\" $DragonFly: src/lib/libc/sys/mmap.2,v 1.9 2007/05/17 08:19:00 swildner Exp $
.\"
.Dd December 11, 2006
.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/types.h
.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
function 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
parameter 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
parameter 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 contains other mappings those mappings will
be replaced.
If the specified address cannot otherwise be used,
.Fn mmap
will fail.
If
.Dv MAP_FIXED
is specified,
.Fa addr
must be a multiple of the pagesize.
.It Dv MAP_TRYFIXED
Try to do a fixed mapping but fail if another mapping already exists in
the space instead of overwriting the mapping.
.Pp
When used with MAP_STACK this flag allows one MAP_STACK mapping to be
made within another (typically the master user stack), as long as
no pages have been faulted in the area requested.
.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/filesystem 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
function 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 filesystem buffer cache.  However, it may be
used to associate dirty VM pages with filesystem 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
Map the area as a stack.
.Dv MAP_ANON
is implied.
.Fa Offset
should be 0,
.Fa fd
must be -1, and
.Fa prot
should 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.
.Pp
The entire area is reserved from the point of view of other
.Fn mmap
calls, even if not faulted in yet.
.Pp
WARNING.  We currently allow
.Dv MAP_STACK
mappings to provide a hint that points within an existing
.Dv MAP_STACK
mapping's space, and this will succeed as long as no page have been
faulted in the area specified, but this behavior is no longer supported
unless you also specify the
.Dv MAP_TRYFIXED
flag.
.Pp
Note that unless
.Dv MAP_FIXED
or
.Dv MAP_TRYFIXED
is used, you cannot count on the returned address matching the hint
you have provided.
.It Dv MAP_VPAGETABLE
Memory accessed via this map is not linearly mapped and will be governed
by a virtual page table.  The base address of the virtual page table may
be set using
.Xr mcontrol 2
with
.Dv MADV_SETMAP .
Virtual page tables work with anonymous memory but there
is no way to populate the page table so for all intents and purposes
.Dv MAP_VPAGETABLE
can only be used when mapping file descriptors.  Since the kernel will
update the VPTE_M bit in the virtual page table, the mapping must R+W
even though actual access to the memory will be properly governed by
the virtual page table.
.Pp
Addressable backing store is limited by the range supported in the virtual
page table entries.  The kernel may implement a page table abstraction capable
of addressing a larger range within the backing store then could otherwise
be mapped into memory.
.El
.Pp
The
.Xr close 2
function 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
.Fn Mmap
will fail if:
.Bl -tag -width Er
.It Bq Er EACCES
The flag
.Dv PROT_READ
was specified as part of the
.Fa prot
parameter 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
parameters and
.Fa fd
was not open for writing.
.It Bq Er EBADF
.Fa fd
is not a valid open file descriptor.
.It Bq Er EINVAL
.Dv MAP_FIXED
was specified and the
.Fa addr
parameter 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
.Fa Len
was negative.
.It Bq Er EINVAL
.Dv MAP_ANON
was specified and the
.Fa fd
parameter 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
.Fa Offset
was not page-aligned.
(See
.Sx BUGS
below.)
.It Bq Er ENOMEM
.Dv MAP_FIXED
was specified and the
.Fa addr
parameter wasn't 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 mlock 2 ,
.Xr mprotect 2 ,
.Xr msync 2 ,
.Xr munlock 2 ,
.Xr munmap 2 ,
.Xr getpagesize 3
.Sh BUGS
.Fa len
is limited to 2GB.  Mmapping slightly more than 2GB doesn't 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
.Dx
not wanting to use 64 bit offsets in the VM system due to
the extreme performance penalty.
So
.Dx
uses 32bit page indexes and
this gives
.Dx
a maximum of 8TB filesizes.
It's actually bugs in
the filesystem 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 filesystem metadata can
reside at negative offsets.