.\"	$NetBSD: mmap.2,v 1.5 1995/06/24 10:48:59 cgd Exp $
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.\"	@(#)mmap.2	8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt MMAP 2
.Os BSD 4
.Sh NAME
.Nm mmap
.Nd map files or devices into memory
.Sh SYNOPSIS
.Fd #include <sys/mman.h>
.Ft void *
.Fo mmap
.Fa "void *addr"
.Fa "size_t len"
.Fa "int prot"
.Fa "int flags"
.Fa "int fildes"
.Fa "off_t offset"
.Fc
.Sh DESCRIPTION
The
.Nm 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 fildes ,
starting at byte offset
.Fa offset .
If
.Fa offset
or
.Fa len
is not a multiple of the pagesize, the mapped region may extend past the
specified range.
.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 MAP_FIXEDX
.It Dv PROT_EXEC
Pages may be executed.
.It Dv PROT_READ
Pages may be read.
.It Dv PROT_WRITE
Pages may be written.
.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 (copy-on-write)
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:
.Pp
.Bl -tag -width MAP_HASSEMAPHOREX
.It Dv MAP_ANON
Map anonymous memory not associated with any specific file.
Mac OS X specific: the file descriptor used for creating
.Dv MAP_ANON
regions can be used to pass some Mach VM flags, and can 
be specified as \-1 if no such flags are associated with 
the region.  Mach VM flags are defined in 
<mach/vm_statistics.h> and the ones that currently apply 
to 
.Nm mmap
are:
.Pp
VM_FLAGS_PURGABLE	to create Mach purgable (i.e. volatile) memory
VM_MAKE_TAG(tag)	to associate an 8-bit tag with the region
.Pp
<mach/vm_statistics.h> defines some preset tags (with a VM_MEMORY_ prefix).
Users are encouraged to use tags between 240 and 255.
Tags are used by tools such as vmmap(1) to help identify specific memory regions.
.It Dv MAP_FILE
Mapped from a regular file or character-special device memory.  (This is
the default mapping type, and need not be specified.)
.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,
.Nm mmap
will fail.
If 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_PRIVATE
Modifications are private (copy-on-write).
.It Dv MAP_SHARED
Modifications are shared.
.It Dv MAP_NOCACHE
Pages in this mapping are not retained in the kernel's memory cache.
If the system runs low on memory, pages in MAP_NOCACHE mappings will be among
the first to be reclaimed.
This flag is intended for mappings that have little locality and 
provides a hint to the kernel that pages in this mapping are unlikely to be needed
again in the near future.
.El
.Pp
Conforming applications must specify either MAP_PRIVATE or MAP_SHARED.
.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,
.Nm mmap
returns a pointer to the mapped region.
Otherwise, a value of -1 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
.Fa Fildes
is not open for reading.
.\" ===========
.It Bq Er EACCES
The flags
.Dv PROT_WRITE
and
.Dv MAP_SHARED
are specified as part of the
.Fa flags
and
.Fa prot
parameters and
.Fa fildes
is not open for writing.
.\" ===========
.It Bq Er EBADF
.Fa fildes
is not a valid file descriptor for an open file.
.It Bq Er EINVAL
.Dv MAP_FIXED
is specified and the
.I addr
parameter is not page aligned.
.\" ===========
.It Bq Er EINVAL
.Fa fildes
does not reference a regular or character special file.
.\" ===========
.It Bq Er EINVAL
.Fa flags
does not include either MAP_PRIVATE or MAP_SHARED.
.\" ===========
.It Bq Er EINVAL
.Fa len
is not greater than zero.
.\" ===========
.It Bq Er EINVAL
.Fa offset
is not a multiple of the page size,
as returned by
.Xr sysconf 3 .
.\" ===========
.It Bq Er EMFILE
The limit on mapped regions (per process or system) is exceeded.
.\" ===========
.It Bq Er ENODEV
The file type for
.Fa fildes
is not supported for mapping.
.\" ===========
.It Bq Er ENOMEM
.Dv MAP_FIXED
is specified and the address range specified
exceeds the address space limit for the process.
.\" ===========
.It Bq Er ENOMEM
.Dv MAP_FIXED
is specified and the address specified by the
.Fa addr
parameter isn't available.
.\" ===========
.It Bq Er ENOMEM
.Dv MAP_ANON
is specified and insufficient memory is available.
.\" ===========
.It Bq Er ENXIO
Addresses in the specified range are invalid for fildes.
.\" ===========
.It Bq Er EOVERFLOW
Addresses in the specified range exceed the maximum offset
set for fildes.
.El
.Sh LEGACY SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/mman.h>
.Pp
The include file
.In sys/types.h
is necessary.
.Sh COMPATIBILITY
.Fn mmap
now returns with
.Va errno
set to EINVAL in places that historically succeeded.
The rules have changed as follows:
.Bl -bullet
.It
The
.Fa flags
parameter must specify either MAP_PRIVATE or MAP_SHARED.
.It
The
.Fa size
parameter must not be 0.
.It
The
.Fa off
parameter must be a multiple of pagesize,
as returned by
.Fn sysconf .
.El
.Sh "SEE ALSO"
.Xr getpagesize 2 ,
.Xr madvise 2 ,
.Xr mincore 2 ,
.Xr mlock 2 ,
.Xr mprotect 2 ,
.Xr msync 2 ,
.Xr munmap 2 ,
.Xr sysconf 3 ,
.Xr compat 5