libc/sys/mmap.2

.\"	$OpenBSD: mmap.2,v 1.70 2022/10/07 16:31:36 jmc Exp $
.\"	$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 $Mdocdate: October 7 2022 $
.Dt MMAP 2
.Os
.Sh NAME
.Nm mmap
.Nd map files or devices into memory
.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
function causes the contents of
.Fa fd ,
starting at
.Fa offset ,
to be mapped in memory at the given
.Fa addr .
The mapping will extend at least
.Fa len
bytes, subject to page alignment restrictions.
.Pp
The
.Fa addr
argument describes the address where the system should place the mapping.
If the
.Dv MAP_FIXED
flag is specified, the allocation will happen at the specified address,
replacing any previously established mappings in its range.
Otherwise, the mapping will be placed at the available spot at
.Fa addr ;
failing that it will be placed "close by".
If
.Fa addr
is
.Dv NULL ,
the system can pick any address.
Except for
.Dv MAP_FIXED
mappings, the system will never replace existing mappings.
.Pp
The
.Fa len
argument describes the minimum amount of bytes the mapping will span.
Since
.Fn mmap
maps pages into memory,
.Fa len
may be rounded up to hit a page boundary.
If
.Fa len
is 0, the mapping will fail with
.Er EINVAL .
.Pp
If an
.Fa fd
and
.Fa offset
are specified, the resulting address may end up not on a page boundary,
in order to align the page offset in the
.Fa addr
to the page offset in
.Fa offset .
.Pp
The protections (region accessibility) are specified in the
.Fa prot
argument.
It should either be
.Dv PROT_NONE
.Pq no permissions
or the bitwise OR of one or more of the following values:
.Pp
.Bl -tag -width "PROT_WRITEXX" -offset indent -compact
.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 OR'ing the following values.
Exactly one of the first two values must be specified:
.Bl -tag -width MAP_ANONYMOUS -offset indent
.It Dv MAP_PRIVATE
Modifications are private.
.It Dv MAP_SHARED
Modifications are shared.
.El
.Pp
Any combination of the following flags may additionally be used:
.Bl -tag -width MAP_ANONYMOUS -offset indent
.It Dv MAP_ANON
Map anonymous memory not associated with any specific file.
The file descriptor used for creating
.Dv MAP_ANON
must currently be \-1 indicating no name is associated with the
region.
.It Dv MAP_ANONYMOUS
Synonym for
.Dv MAP_ANON .
.It Dv MAP_FIXED
Demand that the mapping is placed at
.Fa addr ,
rather than having the system select a location.
.Fa addr ,
.Fa len
and
.Fa offset
(in the case of
.Fa fd
mappings)
must be multiples of the page size.
Existing mappings in the address range will be replaced.
Use of this option is discouraged.
.It Dv MAP_STACK
Indicate that the mapping is used as a stack.
This flag must be used in combination with
.Dv MAP_ANON
and
.Dv MAP_PRIVATE .
.It Dv MAP_CONCEAL
Exclude the mapping from core dumps.
.El
.Pp
Finally, the following flags are also provided for
source compatibility with code written for other operating systems,
but are not recommended for use in new
.Ox
code:
.Bl -tag -width MAP_ANONYMOUS -offset indent
.It Dv MAP_COPY
Modifications are private and, unlike
.Dv MAP_PRIVATE ,
modifications made by others are not visible.
On
.Ox
this behaves just like
.Dv MAP_PRIVATE .
.It Dv MAP_FILE
Mapped from a regular file, character special file, or block special file
specified by file descriptor
.Fa fd .
On
.Ox
and all systems conforming to
.St -p1003.1-2008
this is the default mapping type and need not be specified.
.It Dv MAP_HASSEMAPHORE
Notify the kernel that the region may contain semaphores and that special
handling may be necessary.
On
.Ox
this flag is ignored.
.It Dv MAP_INHERIT
Permit regions to be inherited across
.Xr execve 2
system calls.
On
.Ox
this flag is ignored.
.It Dv MAP_TRYFIXED
Attempt to use the hint provided by
.Fa addr .
On
.Ox
this is the default behavior.
.El
.Pp
The
.Xr close 2
function does not unmap pages; see
.Xr munmap 2
for further information.
.Sh RETURN VALUES
The
.Fn mmap
function returns a pointer to the mapped region if successful;
otherwise the value
.Dv MAP_FAILED
is returned and the global variable
.Va errno
is set to indicate the error.
A successful return from
.Fn mmap
will never 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_PRIVATE
and
.Dv MAP_SHARED
were both specified.
.It Bq Er EINVAL
.Dv MAP_FIXED
was specified and the
.Fa addr
parameter was not page aligned.
.It Bq Er EINVAL
.Fa addr
and
.Fa len
specified a region that would extend beyond the end of the address space.
.It Bq Er EINVAL
.Fa fd
did not specify a regular, character special, or block special file.
.It Bq Er EINVAL
.Fa fd
specified a character special or block special file and the underlying
device does not support memory mapping.
.It Bq Er EINVAL
The allocation
.Fa len
was 0.
.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.
.It Bq Er ENOTSUP
The accesses requested in the
.Fa prot
argument are not allowed.
In particular,
.Dv PROT_WRITE | PROT_EXEC
mappings are not permitted unless the filesystem is mounted
.Cm wxallowed
and the process is link-time tagged with
.Cm wxneeded .
(See also
.Dv kern.wxabort
in
.Xr sysctl 2
for a method to diagnose failure).
.It Bq Er EPERM
The
.Fa addr
and
.Fa len
parameters
specify a region which contains at least one page marked immutable.
.El
.Sh SEE ALSO
.Xr madvise 2 ,
.Xr mimmutable 2 ,
.Xr mlock 2 ,
.Xr mprotect 2 ,
.Xr mquery 2 ,
.Xr msync 2 ,
.Xr munmap 2 ,
.Xr getpagesize 3 ,
.Xr core 5
.Sh STANDARDS
The
.Fn mmap
function conforms to
.St -p1003.1-2008 .
.Sh HISTORY
.\" A mmap() system call stub appeared in 4.1c BSD,
.\" but the functionality wasn't implemented.
A fully functional
.Fn mmap
system call first appeared in SunOS 4.0
and has been available since
.Bx 4.3 Net/2 .
.Sh CAVEATS
.St -p1003.1-2008
specifies that references to pages beyond the end of a mapped object
shall generate a
.Dv SIGBUS
signal; however, on some architectures
.Ox
generates a
.Dv SIGSEGV
signal in this case instead.
.Pp
Additionally,
.St -p1003.1-2008
specifies that
.Fn mmap
shall fail with
.Er EINVAL
if neither
.Dv MAP_PRIVATE
nor
.Dv MAP_SHARED
is specified by
.Fa flags ;
however, for compatibility with old programs,
.Ox
instead defaults to
.Dv MAP_SHARED
for mappings of character special files, and to
.Dv MAP_PRIVATE
for all other mappings.
New programs should not rely on this behavior.
.Sh BUGS
Due to a limitation of the current vm system (see
.Xr uvm_init 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 ,
which requires both read and write permissions on the underlying
object.