568 lines
15 KiB
Groff
568 lines
15 KiB
Groff
.\" Copyright (c) 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)mmap.2 8.4 (Berkeley) 5/11/95
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.\" $FreeBSD$
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.\"
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.Dd August 14, 2023
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.Dt MMAP 2
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.Os
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.Sh NAME
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.Nm mmap
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.Nd allocate memory, or map files or devices into memory
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/mman.h
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.Ft void *
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.Fn mmap "void *addr" "size_t len" "int prot" "int flags" "int fd" "off_t offset"
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.Sh DESCRIPTION
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The
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.Fn mmap
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system call causes the pages starting at
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.Fa addr
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and continuing for at most
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.Fa len
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bytes to be mapped from the object described by
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.Fa fd ,
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starting at byte offset
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.Fa offset .
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If
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.Fa len
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is not a multiple of the page size, the mapped region may extend past the
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specified range.
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Any such extension beyond the end of the mapped object will be zero-filled.
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.Pp
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If
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.Fa fd
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references a regular file or a shared memory object, the range of
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bytes starting at
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.Fa offset
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and continuing for
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.Fa len
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bytes must be legitimate for the possible (not necessarily
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current) offsets in the object.
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In particular, the
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.Fa offset
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value cannot be negative.
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If the object is truncated and the process later accesses a page that
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is wholly within the truncated region, the access is aborted and a
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.Dv SIGBUS
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signal is delivered to the process.
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.Pp
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If
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.Fa fd
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references a device file, the interpretation of the
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.Fa offset
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value is device specific and defined by the device driver.
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The virtual memory subsystem does not impose any restrictions on the
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.Fa offset
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value in this case, passing it unchanged to the driver.
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.Pp
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If
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.Fa addr
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is non-zero, it is used as a hint to the system.
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(As a convenience to the system, the actual address of the region may differ
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from the address supplied.)
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If
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.Fa addr
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is zero, an address will be selected by the system.
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The actual starting address of the region is returned.
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A successful
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.Fa mmap
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deletes any previous mapping in the allocated address range.
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.Pp
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The protections (region accessibility) are specified in the
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.Fa prot
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argument by
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.Em or Ns 'ing
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the following values:
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.Pp
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.Bl -tag -width PROT_WRITE -compact
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.It Dv PROT_NONE
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Pages may not be accessed.
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.It Dv PROT_READ
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Pages may be read.
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.It Dv PROT_WRITE
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Pages may be written.
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.It Dv PROT_EXEC
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Pages may be executed.
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.El
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.Pp
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In addition to these protection flags,
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.Fx
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provides the ability to set the maximum protection of a region allocated by
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.Nm
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and later altered by
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.Xr mprotect 2 .
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This is accomplished by
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.Em or Ns 'ing
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one or more
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.Dv PROT_
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values wrapped in the
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.Dv PROT_MAX()
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macro into the
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.Fa prot
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argument.
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.Pp
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The
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.Fa flags
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argument specifies the type of the mapped object, mapping options and
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whether modifications made to the mapped copy of the page are private
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to the process or are to be shared with other references.
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Sharing, mapping type and options are specified in the
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.Fa flags
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argument by
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.Em or Ns 'ing
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the following values:
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.Bl -tag -width MAP_PREFAULT_READ
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.It Dv MAP_32BIT
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Request a region in the first 2GB of the current process's address space.
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If a suitable region cannot be found,
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.Fn mmap
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will fail.
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.It Dv MAP_ALIGNED Ns Pq Fa n
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Align the region on a requested boundary.
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If a suitable region cannot be found,
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.Fn mmap
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will fail.
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The
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.Fa n
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argument specifies the binary logarithm of the desired alignment.
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.It Dv MAP_ALIGNED_SUPER
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Align the region to maximize the potential use of large
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.Pq Dq super
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pages.
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If a suitable region cannot be found,
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.Fn mmap
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will fail.
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The system will choose a suitable page size based on the size of
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mapping.
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The page size used as well as the alignment of the region may both be
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affected by properties of the file being mapped.
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In particular,
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the physical address of existing pages of a file may require a specific
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alignment.
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The region is not guaranteed to be aligned on any specific boundary.
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.It Dv MAP_ANON
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Map anonymous memory not associated with any specific file.
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The file descriptor used for creating
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.Dv MAP_ANON
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must be \-1.
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The
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.Fa offset
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argument must be 0.
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.\".It Dv MAP_FILE
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.\"Mapped from a regular file or character-special device memory.
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.It Dv MAP_ANONYMOUS
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This flag is identical to
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.Dv MAP_ANON
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and is provided for compatibility.
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.It Dv MAP_EXCL
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This flag can only be used in combination with
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.Dv MAP_FIXED .
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Please see the definition of
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.Dv MAP_FIXED
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for the description of its effect.
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.It Dv MAP_FIXED
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Do not permit the system to select a different address than the one
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specified.
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If the specified address cannot be used,
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.Fn mmap
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will fail.
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If
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.Dv MAP_FIXED
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is specified,
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.Fa addr
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must be a multiple of the page size.
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If
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.Dv MAP_EXCL
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is not specified, a successful
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.Dv MAP_FIXED
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request replaces any previous mappings for the process'
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pages in the range from
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.Fa addr
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to
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.Fa addr
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+
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.Fa len .
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In contrast, if
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.Dv MAP_EXCL
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is specified, the request will fail if a mapping
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already exists within the range.
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.It Dv MAP_GUARD
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Instead of a mapping, create a guard of the specified size.
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Guards allow a process to create reservations in its address space,
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which can later be replaced by actual mappings.
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.Pp
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.Fa mmap
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will not create mappings in the address range of a guard unless
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the request specifies
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.Dv MAP_FIXED .
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Guards can be destroyed with
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.Xr munmap 2 .
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Any memory access by a thread to the guarded range results
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in the delivery of a
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.Dv SIGSEGV
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signal to that thread.
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.It Dv MAP_NOCORE
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Region is not included in a core file.
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.It Dv MAP_NOSYNC
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Causes data dirtied via this VM map to be flushed to physical media
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only when necessary (usually by the pager) rather than gratuitously.
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Typically this prevents the update daemons from flushing pages dirtied
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through such maps and thus allows efficient sharing of memory across
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unassociated processes using a file-backed shared memory map.
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Without
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this option any VM pages you dirty may be flushed to disk every so often
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(every 30-60 seconds usually) which can create performance problems if you
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do not need that to occur (such as when you are using shared file-backed
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mmap regions for IPC purposes).
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Dirty data will be flushed automatically when all mappings of an object are
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removed and all descriptors referencing the object are closed.
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Note that VM/file system coherency is
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maintained whether you use
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.Dv MAP_NOSYNC
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or not.
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This option is not portable
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across
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.Ux
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platforms (yet), though some may implement the same behavior
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by default.
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.Pp
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.Em WARNING !
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Extending a file with
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.Xr ftruncate 2 ,
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thus creating a big hole, and then filling the hole by modifying a shared
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.Fn mmap
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can lead to severe file fragmentation.
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In order to avoid such fragmentation you should always pre-allocate the
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file's backing store by
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.Fn write Ns ing
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zero's into the newly extended area prior to modifying the area via your
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.Fn mmap .
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The fragmentation problem is especially sensitive to
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.Dv MAP_NOSYNC
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pages, because pages may be flushed to disk in a totally random order.
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.Pp
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The same applies when using
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.Dv MAP_NOSYNC
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to implement a file-based shared memory store.
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It is recommended that you create the backing store by
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.Fn write Ns ing
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zero's to the backing file rather than
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.Fn ftruncate Ns ing
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it.
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You can test file fragmentation by observing the KB/t (kilobytes per
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transfer) results from an
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.Dq Li iostat 1
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while reading a large file sequentially, e.g.,\& using
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.Dq Li dd if=filename of=/dev/null bs=32k .
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.Pp
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The
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.Xr fsync 2
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system call will flush all dirty data and metadata associated with a file,
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including dirty NOSYNC VM data, to physical media.
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The
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.Xr sync 8
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command and
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.Xr sync 2
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system call generally do not flush dirty NOSYNC VM data.
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The
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.Xr msync 2
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system call is usually not needed since
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.Bx
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implements a coherent file system buffer cache.
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However, it may be
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used to associate dirty VM pages with file system buffers and thus cause
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them to be flushed to physical media sooner rather than later.
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.It Dv MAP_PREFAULT_READ
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Immediately update the calling process's lowest-level virtual address
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translation structures, such as its page table, so that every memory
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resident page within the region is mapped for read access.
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Ordinarily these structures are updated lazily.
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The effect of this option is to eliminate any soft faults that would
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otherwise occur on the initial read accesses to the region.
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Although this option does not preclude
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.Fa prot
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from including
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.Dv PROT_WRITE ,
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it does not eliminate soft faults on the initial write accesses to the
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region.
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.It Dv MAP_PRIVATE
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Modifications are private.
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.It Dv MAP_SHARED
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Modifications are shared.
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.It Dv MAP_STACK
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Creates both a mapped region that grows downward on demand and an
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adjoining guard that both reserves address space for the mapped region
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to grow into and limits the mapped region's growth.
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Together, the mapped region and the guard occupy
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.Fa len
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bytes of the address space.
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The guard starts at the returned address, and the mapped region ends at
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the returned address plus
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.Fa len
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bytes.
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Upon access to the guard, the mapped region automatically grows in size,
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and the guard shrinks by an equal amount.
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Essentially, the boundary between the guard and the mapped region moves
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downward so that the access falls within the enlarged mapped region.
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However, the guard will never shrink to less than the number of pages
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specified by the sysctl
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.Dv security.bsd.stack_guard_page ,
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thereby ensuring that a gap for detecting stack overflow always exists
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between the downward growing mapped region and the closest mapped region
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beneath it.
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.Pp
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.Dv MAP_STACK
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implies
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.Dv MAP_ANON
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and
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.Fa offset
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of 0.
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The
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.Fa fd
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argument
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must be -1 and
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.Fa prot
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must include at least
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.Dv PROT_READ
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and
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.Dv PROT_WRITE .
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The size of the guard, in pages, is specified by sysctl
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.Dv security.bsd.stack_guard_page .
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.El
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.Pp
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The
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.Xr close 2
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system call does not unmap pages, see
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.Xr munmap 2
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for further information.
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.Sh NOTES
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Although this implementation does not impose any alignment restrictions on
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the
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.Fa offset
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argument, a portable program must only use page-aligned values.
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.Pp
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Large page mappings require that the pages backing an object be
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aligned in matching blocks in both the virtual address space and RAM.
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The system will automatically attempt to use large page mappings when
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mapping an object that is already backed by large pages in RAM by
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aligning the mapping request in the virtual address space to match the
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alignment of the large physical pages.
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The system may also use large page mappings when mapping portions of an
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object that are not yet backed by pages in RAM.
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The
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.Dv MAP_ALIGNED_SUPER
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flag is an optimization that will align the mapping request to the
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size of a large page similar to
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.Dv MAP_ALIGNED ,
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except that the system will override this alignment if an object already
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uses large pages so that the mapping will be consistent with the existing
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large pages.
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This flag is mostly useful for maximizing the use of large pages on the
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first mapping of objects that do not yet have pages present in RAM.
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.Sh RETURN VALUES
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Upon successful completion,
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.Fn mmap
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returns a pointer to the mapped region.
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Otherwise, a value of
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.Dv MAP_FAILED
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is returned and
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.Va errno
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is set to indicate the error.
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.Sh ERRORS
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The
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.Fn mmap
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system call
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EACCES
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The flag
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.Dv PROT_READ
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was specified as part of the
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.Fa prot
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argument and
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.Fa fd
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was not open for reading.
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The flags
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.Dv MAP_SHARED
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and
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.Dv PROT_WRITE
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were specified as part of the
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.Fa flags
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and
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.Fa prot
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argument and
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.Fa fd
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was not open for writing.
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.It Bq Er EBADF
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The
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.Fa fd
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argument
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is not a valid open file descriptor.
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.It Bq Er EINVAL
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An invalid (negative) value was passed in the
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.Fa offset
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argument, when
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.Fa fd
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referenced a regular file or shared memory.
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.It Bq Er EINVAL
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An invalid value was passed in the
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.Fa prot
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argument.
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.It Bq Er EINVAL
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An undefined option was set in the
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.Fa flags
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argument.
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.It Bq Er EINVAL
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Both
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.Dv MAP_PRIVATE
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and
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.Dv MAP_SHARED
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were specified.
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.It Bq Er EINVAL
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None of
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.Dv MAP_ANON ,
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.Dv MAP_GUARD ,
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.Dv MAP_PRIVATE ,
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.Dv MAP_SHARED ,
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or
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.Dv MAP_STACK
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was specified.
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At least one of these flags must be included.
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.It Bq Er EINVAL
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.Dv MAP_STACK
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was specified and
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.Va len
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is less than or equal to the guard size.
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.It Bq Er EINVAL
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.Dv MAP_FIXED
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was specified and the
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.Fa addr
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argument was not page aligned, or part of the desired address space
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resides out of the valid address space for a user process.
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.It Bq Er EINVAL
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Both
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.Dv MAP_FIXED
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and
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.Dv MAP_32BIT
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were specified and part of the desired address space resides outside
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of the first 2GB of user address space.
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.It Bq Er EINVAL
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The
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.Fa len
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argument
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was equal to zero.
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.It Bq Er EINVAL
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.Dv MAP_ALIGNED
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was specified and the desired alignment was either larger than the
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virtual address size of the machine or smaller than a page.
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.It Bq Er EINVAL
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.Dv MAP_ANON
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was specified and the
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.Fa fd
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argument was not -1.
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.It Bq Er EINVAL
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.Dv MAP_ANON
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was specified and the
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.Fa offset
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argument was not 0.
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.It Bq Er EINVAL
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Both
|
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.Dv MAP_FIXED
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and
|
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.Dv MAP_EXCL
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were specified, but the requested region is already used by a mapping.
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.It Bq Er EINVAL
|
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.Dv MAP_EXCL
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was specified, but
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.Dv MAP_FIXED
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was not.
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.It Bq Er EINVAL
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.Dv MAP_GUARD
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was specified, but the
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.Fa offset
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argument was not zero, the
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.Fa fd
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argument was not -1, or the
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.Fa prot
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argument was not
|
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.Dv PROT_NONE .
|
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.It Bq Er EINVAL
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.Dv MAP_GUARD
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was specified together with one of the flags
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.Dv MAP_ANON ,
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.Dv MAP_PREFAULT ,
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.Dv MAP_PREFAULT_READ ,
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.Dv MAP_PRIVATE ,
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.Dv MAP_SHARED ,
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.Dv MAP_STACK .
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.It Bq Er ENODEV
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.Dv MAP_ANON
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has not been specified and
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.Fa fd
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did not reference a regular or character special file.
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.It Bq Er ENOMEM
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.Dv MAP_FIXED
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was specified and the
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.Fa addr
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argument was not available.
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.Dv MAP_ANON
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was specified and insufficient memory was available.
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.It Bq Er ENOTSUP
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|
The
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.Fa prot
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argument contains protections which are not a subset of the specified
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maximum protections.
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.El
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.Sh SEE ALSO
|
|
.Xr madvise 2 ,
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.Xr mincore 2 ,
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.Xr minherit 2 ,
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.Xr mlock 2 ,
|
|
.Xr mprotect 2 ,
|
|
.Xr msync 2 ,
|
|
.Xr munlock 2 ,
|
|
.Xr munmap 2 ,
|
|
.Xr getpagesize 3 ,
|
|
.Xr getpagesizes 3
|
|
.Sh HISTORY
|
|
The
|
|
.Nm
|
|
system call was first documented in
|
|
.Bx 4.2
|
|
and implemented in
|
|
.Bx 4.4 .
|
|
.\" XXX: lots of missing history of FreeBSD additions.
|
|
.Pp
|
|
The
|
|
.Dv PROT_MAX
|
|
functionality was introduced in
|
|
.Fx 13 .
|