path: root/lib/libc/sys/mmap.2

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.\"	@(#)mmap.2	8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt MMAP 2
.Os
.Sh NAME
.Nm mmap
.Nd map files or devices into memory
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/mman.h>
.Ft void *
.Fn mmap "void *addr" "size_t len" "int prot" "int flags" "int fd" "off_t offset"
.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 fd ,
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
.Tn 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
regions is used only for
naming, and may be specified as \-1 if no name is associated with the
region.
.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
.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
Permit regions to be inherited across
.Xr exec 2
system calls.
.It Dv MAP_PRIVATE
Modifications are private.
.It Dv MAP_SHARED
Modifications are shared.
.It Dv MAP_COPY
Modifications are private and unlike
.Dv MAP_PRIVATE
you don't see modifications made by others.
This option is deprecated, shouldn't be used and behaves just like
.Dv MAP_PRIVATE
in the current implementation.
.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,
.Nm 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.
The symbol
.Dv MAP_FAILED
is defined in the header
.Ao Pa sys/mman.h Ac .
No successful return from
.Fn mmap
will return the value
.Dv MAP_FAILED .
.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.
.Fa fd
did not reference a regular or character special file.
.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.
.El
.Sh BUGS
Due to a limitation of the current vm system (see
.Xr uvm 9 ),
mapping descriptors
.Dv PROT_WRITE
without also specifying
.Dv PROT_READ 
is useless 
(results in a segmentation fault when first accessing the mapping).
This means that such descriptors must be opened with
.Dv O_RDWR ,
for which you need both read and write permissions on the underlying
object.
.Sh SEE ALSO
.Xr madvise 2 ,
.Xr mincore 2 ,
.Xr mlock 2 ,
.Xr mprotect 2 ,
.Xr msync 2 ,
.Xr munmap 2 ,
.Xr getpagesize 3