freebsd-skq/lib/libc/sys/mmap.2

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.\"	@(#)mmap.2	8.4 (Berkeley) 5/11/95
.\"
.Dd "May 11, 1995"
.Dt MMAP 2
.Os BSD 4
.Sh NAME
.Nm mmap
.Nd map files or devices into memory
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/mman.h>
.Ft caddr_t
.Fn mmap "caddr_t 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 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 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_FIXEDX
.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 cannot be used,
.Fn 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_INHERIT
Permit regions to be inherited across
.Xr execve 2
system calls.
.It Dv MAP_PRIVATE
Modifications are private.
.It Dv MAP_SHARED
Modifications are shared.
.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 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
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 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.
.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
.Ar 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.

The limit is imposed for a variety of reasons. Most of them have to do
with FreeBSD not wanting to use 64 bit offsets in the VM system due to
the extreme performance penalty. So FreeBSD uses 32bit page indexes and
this gives FreeBSD 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).

Another reason for the 2GB limit is that filesystem metadata can
reside at negative offsets.

We currently can only deal with page aligned file offsets.