Adding upstream version 4.22.0.upstream/4.22.0

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+'\" t
+.\" Copyright (C) 1996 Andries Brouwer <aeb@cwi.nl>
+.\" and Copyright (C) 2006, 2007 Michael Kerrisk <mtk.manpages@gmail.com>
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com>
+.\" Modified 2000-03-25 by Jim Van Zandt <jrv@vanzandt.mv.com>
+.\" Modified 2001-10-04 by John Levon <moz@compsoc.man.ac.uk>
+.\" Modified 2003-02-02 by Andi Kleen <ak@muc.de>
+.\" Modified 2003-05-21 by Michael Kerrisk <mtk.manpages@gmail.com>
+.\"	MAP_LOCKED works from Linux 2.5.37
+.\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
+.\" Modified 2004-09-11 by aeb
+.\" Modified 2004-12-08, from Eric Estievenart <eric.estievenart@free.fr>
+.\" Modified 2004-12-08, mtk, formatting tidy-ups
+.\" Modified 2006-12-04, mtk, various parts rewritten
+.\" 2007-07-10, mtk, Added an example program.
+.\" 2008-11-18, mtk, document MAP_STACK
+.\"
+.TH mmap 2 2023-07-20 "Linux man-pages 6.05.01"
+.SH NAME
+mmap, munmap \- map or unmap files or devices into memory
+.SH LIBRARY
+Standard C library
+.RI ( libc ", " \-lc )
+.SH SYNOPSIS
+.nf
+.B #include <sys/mman.h>
+.PP
+.BI "void *mmap(void " addr [. length "], size_t " length \
+", int " prot ", int " flags ,
+.BI "           int " fd ", off_t " offset );
+.BI "int munmap(void " addr [. length "], size_t " length );
+.fi
+.PP
+See NOTES for information on feature test macro requirements.
+.SH DESCRIPTION
+.BR mmap ()
+creates a new mapping in the virtual address space of
+the calling process.
+The starting address for the new mapping is specified in
+.IR addr .
+The
+.I length
+argument specifies the length of the mapping (which must be greater than 0).
+.PP
+If
+.I addr
+is NULL,
+then the kernel chooses the (page-aligned) address
+at which to create the mapping;
+this is the most portable method of creating a new mapping.
+If
+.I addr
+is not NULL,
+then the kernel takes it as a hint about where to place the mapping;
+on Linux, the kernel will pick a nearby page boundary (but always above
+or equal to the value specified by
+.IR /proc/sys/vm/mmap_min_addr )
+and attempt to create the mapping there.
+If another mapping already exists there, the kernel picks a new address that
+may or may not depend on the hint.
+.\" Before Linux 2.6.24, the address was rounded up to the next page
+.\" boundary; since Linux 2.6.24, it is rounded down!
+The address of the new mapping is returned as the result of the call.
+.PP
+The contents of a file mapping (as opposed to an anonymous mapping; see
+.B MAP_ANONYMOUS
+below), are initialized using
+.I length
+bytes starting at offset
+.I offset
+in the file (or other object) referred to by the file descriptor
+.IR fd .
+.I offset
+must be a multiple of the page size as returned by
+.IR sysconf(_SC_PAGE_SIZE) .
+.PP
+After the
+.BR mmap ()
+call has returned, the file descriptor,
+.IR fd ,
+can be closed immediately without invalidating the mapping.
+.PP
+The
+.I prot
+argument describes the desired memory protection of the mapping
+(and must not conflict with the open mode of the file).
+It is either
+.B PROT_NONE
+or the bitwise OR of one or more of the following flags:
+.TP 1.1i
+.B PROT_EXEC
+Pages may be executed.
+.TP
+.B PROT_READ
+Pages may be read.
+.TP
+.B PROT_WRITE
+Pages may be written.
+.TP
+.B PROT_NONE
+Pages may not be accessed.
+.\"
+.SS The flags argument
+The
+.I flags
+argument determines whether updates to the mapping
+are visible to other processes mapping the same region,
+and whether updates are carried through to the underlying file.
+This behavior is determined by including exactly one
+of the following values in
+.IR flags :
+.TP
+.B MAP_SHARED
+Share this mapping.
+Updates to the mapping are visible to other processes mapping the same region,
+and (in the case of file-backed mappings)
+are carried through to the underlying file.
+(To precisely control when updates are carried through
+to the underlying file requires the use of
+.BR msync (2).)
+.TP
+.BR MAP_SHARED_VALIDATE " (since Linux 4.15)"
+This flag provides the same behavior as
+.B MAP_SHARED
+except that
+.B MAP_SHARED
+mappings ignore unknown flags in
+.IR flags .
+By contrast, when creating a mapping using
+.BR MAP_SHARED_VALIDATE ,
+the kernel verifies all passed flags are known and fails the
+mapping with the error
+.B EOPNOTSUPP
+for unknown flags.
+This mapping type is also required to be able to use some mapping flags
+(e.g.,
+.BR MAP_SYNC ).
+.TP
+.B MAP_PRIVATE
+Create a private copy-on-write mapping.
+Updates to the mapping are not visible to other processes
+mapping the same file, and are not carried through to
+the underlying file.
+It is unspecified whether changes made to the file after the
+.BR mmap ()
+call are visible in the mapped region.
+.PP
+Both
+.B MAP_SHARED
+and
+.B MAP_PRIVATE
+are described in POSIX.1-2001 and POSIX.1-2008.
+.B MAP_SHARED_VALIDATE
+is a Linux extension.
+.PP
+In addition, zero or more of the following values can be ORed in
+.IR flags :
+.TP
+.BR MAP_32BIT " (since Linux 2.4.20, 2.6)"
+Put the mapping into the first 2 Gigabytes of the process address space.
+This flag is supported only on x86-64, for 64-bit programs.
+It was added to allow thread stacks to be allocated somewhere
+in the first 2\ GB of memory,
+so as to improve context-switch performance on some early
+64-bit processors.
+.\" See http://lwn.net/Articles/294642 "Tangled up in threads", 19 Aug 08
+Modern x86-64 processors no longer have this performance problem,
+so use of this flag is not required on those systems.
+The
+.B MAP_32BIT
+flag is ignored when
+.B MAP_FIXED
+is set.
+.TP
+.B MAP_ANON
+Synonym for
+.BR MAP_ANONYMOUS ;
+provided for compatibility with other implementations.
+.TP
+.B MAP_ANONYMOUS
+The mapping is not backed by any file;
+its contents are initialized to zero.
+The
+.I fd
+argument is ignored;
+however, some implementations require
+.I fd
+to be \-1 if
+.B MAP_ANONYMOUS
+(or
+.BR MAP_ANON )
+is specified,
+and portable applications should ensure this.
+The
+.I offset
+argument should be zero.
+.\" See the pgoff overflow check in do_mmap().
+.\" See the offset check in sys_mmap in arch/x86/kernel/sys_x86_64.c.
+Support for
+.B MAP_ANONYMOUS
+in conjunction with
+.B MAP_SHARED
+was added in Linux 2.4.
+.TP
+.B MAP_DENYWRITE
+This flag is ignored.
+.\" Introduced in 1.1.36, removed in 1.3.24.
+(Long ago\[em]Linux 2.0 and earlier\[em]it signaled
+that attempts to write to the underlying file should fail with
+.BR ETXTBSY .
+But this was a source of denial-of-service attacks.)
+.TP
+.B MAP_EXECUTABLE
+This flag is ignored.
+.\" Introduced in 1.1.38, removed in 1.3.24. Flag tested in proc_follow_link.
+.\" (Long ago, it signaled that the underlying file is an executable.
+.\" However, that information was not really used anywhere.)
+.\" Linus talked about DOS related to MAP_EXECUTABLE, but he was thinking of
+.\" MAP_DENYWRITE?
+.TP
+.B MAP_FILE
+Compatibility flag.
+Ignored.
+.\" On some systems, this was required as the opposite of
+.\" MAP_ANONYMOUS -- mtk, 1 May 2007
+.TP
+.B MAP_FIXED
+Don't interpret
+.I addr
+as a hint: place the mapping at exactly that address.
+.I addr
+must be suitably aligned: for most architectures a multiple of the page
+size is sufficient; however, some architectures may impose additional
+restrictions.
+If the memory region specified by
+.I addr
+and
+.I length
+overlaps pages of any existing mapping(s), then the overlapped
+part of the existing mapping(s) will be discarded.
+If the specified address cannot be used,
+.BR mmap ()
+will fail.
+.IP
+Software that aspires to be portable should use the
+.B MAP_FIXED
+flag with care,
+keeping in mind that the exact layout of a process's memory mappings
+is allowed to change significantly between Linux versions,
+C library versions, and operating system releases.
+.I Carefully read the discussion of this flag in NOTES!
+.TP
+.BR MAP_FIXED_NOREPLACE " (since Linux 4.17)"
+.\" commit a4ff8e8620d3f4f50ac4b41e8067b7d395056843
+This flag provides behavior that is similar to
+.B MAP_FIXED
+with respect to the
+.I addr
+enforcement, but differs in that
+.B MAP_FIXED_NOREPLACE
+never clobbers a preexisting mapped range.
+If the requested range would collide with an existing mapping,
+then this call fails with the error
+.B EEXIST.
+This flag can therefore be used as a way to atomically
+(with respect to other threads) attempt to map an address range:
+one thread will succeed; all others will report failure.
+.IP
+Note that older kernels which do not recognize the
+.B MAP_FIXED_NOREPLACE
+flag will typically (upon detecting a collision with a preexisting mapping)
+fall back to a
+.RB \[lq]non- MAP_FIXED \[rq]
+type of behavior:
+they will return an address that is different from the requested address.
+Therefore, backward-compatible software
+should check the returned address against the requested address.
+.TP
+.B MAP_GROWSDOWN
+This flag is used for stacks.
+It indicates to the kernel virtual memory system that the mapping
+should extend downward in memory.
+The return address is one page lower than the memory area that is
+actually created in the process's virtual address space.
+Touching an address in the "guard" page below the mapping will cause
+the mapping to grow by a page.
+This growth can be repeated until the mapping grows to within a
+page of the high end of the next lower mapping,
+at which point touching the "guard" page will result in a
+.B SIGSEGV
+signal.
+.TP
+.BR MAP_HUGETLB " (since Linux 2.6.32)"
+Allocate the mapping using "huge" pages.
+See the Linux kernel source file
+.I Documentation/admin\-guide/mm/hugetlbpage.rst
+for further information, as well as NOTES, below.
+.TP
+.BR MAP_HUGE_2MB ", " MAP_HUGE_1GB " (since Linux 3.8)"
+.\" See https://lwn.net/Articles/533499/
+Used in conjunction with
+.B MAP_HUGETLB
+to select alternative hugetlb page sizes (respectively, 2\ MB and 1\ GB)
+on systems that support multiple hugetlb page sizes.
+.IP
+More generally, the desired huge page size can be configured by encoding
+the base-2 logarithm of the desired page size in the six bits at the offset
+.BR MAP_HUGE_SHIFT .
+(A value of zero in this bit field provides the default huge page size;
+the default huge page size can be discovered via the
+.I Hugepagesize
+field exposed by
+.IR /proc/meminfo .)
+Thus, the above two constants are defined as:
+.IP
+.in +4n
+.EX
+#define MAP_HUGE_2MB    (21 << MAP_HUGE_SHIFT)
+#define MAP_HUGE_1GB    (30 << MAP_HUGE_SHIFT)
+.EE
+.in
+.IP
+The range of huge page sizes that are supported by the system
+can be discovered by listing the subdirectories in
+.IR /sys/kernel/mm/hugepages .
+.TP
+.BR MAP_LOCKED " (since Linux 2.5.37)"
+Mark the mapped region to be locked in the same way as
+.BR mlock (2).
+This implementation will try to populate (prefault) the whole range but the
+.BR mmap ()
+call doesn't fail with
+.B ENOMEM
+if this fails.
+Therefore major faults might happen later on.
+So the semantic is not as strong as
+.BR mlock (2).
+One should use
+.BR mmap ()
+plus
+.BR mlock (2)
+when major faults are not acceptable after the initialization of the mapping.
+The
+.B MAP_LOCKED
+flag is ignored in older kernels.
+.\" If set, the mapped pages will not be swapped out.
+.TP
+.BR MAP_NONBLOCK " (since Linux 2.5.46)"
+This flag is meaningful only in conjunction with
+.BR MAP_POPULATE .
+Don't perform read-ahead:
+create page tables entries only for pages
+that are already present in RAM.
+Since Linux 2.6.23,
+.\" commit 54cb8821de07f2ffcd28c380ce9b93d5784b40d7
+this flag causes
+.B MAP_POPULATE
+to do nothing.
+One day, the combination of
+.B MAP_POPULATE
+and
+.B MAP_NONBLOCK
+may be reimplemented.
+.TP
+.B MAP_NORESERVE
+Do not reserve swap space for this mapping.
+When swap space is reserved, one has the guarantee
+that it is possible to modify the mapping.
+When swap space is not reserved one might get
+.B SIGSEGV
+upon a write
+if no physical memory is available.
+See also the discussion of the file
+.I /proc/sys/vm/overcommit_memory
+in
+.BR proc (5).
+Before Linux 2.6, this flag had effect only for
+private writable mappings.
+.TP
+.BR MAP_POPULATE " (since Linux 2.5.46)"
+Populate (prefault) page tables for a mapping.
+For a file mapping, this causes read-ahead on the file.
+This will help to reduce blocking on page faults later.
+The
+.BR mmap ()
+call doesn't fail if the mapping cannot be populated (for example, due
+to limitations on the number of mapped huge pages when using
+.BR MAP_HUGETLB ).
+Support for
+.B MAP_POPULATE
+in conjunction with private mappings was added in Linux 2.6.23.
+.TP
+.BR MAP_STACK " (since Linux 2.6.27)"
+Allocate the mapping at an address suitable for a process
+or thread stack.
+.IP
+This flag is currently a no-op on Linux.
+However, by employing this flag, applications can ensure that
+they transparently obtain support if the flag
+is implemented in the future.
+Thus, it is used in the glibc threading implementation to allow for
+the fact that some architectures may (later) require special treatment
+for stack allocations.
+.\" See http://lwn.net/Articles/294642 "Tangled up in threads", 19 Aug 08
+.\" commit cd98a04a59e2f94fa64d5bf1e26498d27427d5e7
+.\" http://thread.gmane.org/gmane.linux.kernel/720412
+.\" "pthread_create() slow for many threads; also time to revisit 64b
+.\"  context switch optimization?"
+A further reason to employ this flag is portability:
+.B MAP_STACK
+exists (and has an effect) on some other systems (e.g., some of the BSDs).
+.TP
+.BR MAP_SYNC " (since Linux 4.15)"
+This flag is available only with the
+.B MAP_SHARED_VALIDATE
+mapping type;
+mappings of type
+.B MAP_SHARED
+will silently ignore this flag.
+This flag is supported only for files supporting DAX
+(direct mapping of persistent memory).
+For other files, creating a mapping with this flag results in an
+.B EOPNOTSUPP
+error.
+.IP
+Shared file mappings with this flag provide the guarantee that while
+some memory is mapped writable in the address space of the process,
+it will be visible in the same file at the same offset even after
+the system crashes or is rebooted.
+In conjunction with the use of appropriate CPU instructions,
+this provides users of such mappings with a more efficient way
+of making data modifications persistent.
+.TP
+.BR MAP_UNINITIALIZED " (since Linux 2.6.33)"
+Don't clear anonymous pages.
+This flag is intended to improve performance on embedded devices.
+This flag is honored only if the kernel was configured with the
+.B CONFIG_MMAP_ALLOW_UNINITIALIZED
+option.
+Because of the security implications,
+that option is normally enabled only on embedded devices
+(i.e., devices where one has complete control of the contents of user memory).
+.PP
+Of the above flags, only
+.B MAP_FIXED
+is specified in POSIX.1-2001 and POSIX.1-2008.
+However, most systems also support
+.B MAP_ANONYMOUS
+(or its synonym
+.BR MAP_ANON ).
+.\" FIXME . for later review when Issue 8 is one day released...
+.\" POSIX may add MAP_ANON in the future
+.\" http://austingroupbugs.net/tag_view_page.php?tag_id=8
+.\" http://austingroupbugs.net/view.php?id=850
+.SS munmap()
+The
+.BR munmap ()
+system call deletes the mappings for the specified address range, and
+causes further references to addresses within the range to generate
+invalid memory references.
+The region is also automatically unmapped
+when the process is terminated.
+On the other hand, closing the file
+descriptor does not unmap the region.
+.PP
+The address
+.I addr
+must be a multiple of the page size (but
+.I length
+need not be).
+All pages containing a part
+of the indicated range are unmapped, and subsequent references
+to these pages will generate
+.BR SIGSEGV .
+It is not an error if the
+indicated range does not contain any mapped pages.
+.SH RETURN VALUE
+On success,
+.BR mmap ()
+returns a pointer to the mapped area.
+On error, the value
+.B MAP_FAILED
+(that is,
+.IR "(void\ *)\ \-1" )
+is returned, and
+.I errno
+is set to indicate the error.
+.PP
+On success,
+.BR munmap ()
+returns 0.
+On failure, it returns \-1, and
+.I errno
+is set to indicate the error (probably to
+.BR EINVAL ).
+.SH ERRORS
+.TP
+.B EACCES
+A file descriptor refers to a non-regular file.
+Or a file mapping was requested, but
+.I fd
+is not open for reading.
+Or
+.B MAP_SHARED
+was requested and
+.B PROT_WRITE
+is set, but
+.I fd
+is not open in read/write
+.RB ( O_RDWR )
+mode.
+Or
+.B PROT_WRITE
+is set, but the file is append-only.
+.TP
+.B EAGAIN
+The file has been locked, or too much memory has been locked (see
+.BR setrlimit (2)).
+.TP
+.B EBADF
+.I fd
+is not a valid file descriptor (and
+.B MAP_ANONYMOUS
+was not set).
+.TP
+.B EEXIST
+.B MAP_FIXED_NOREPLACE
+was specified in
+.IR flags ,
+and the range covered by
+.I addr
+and
+.I length
+clashes with an existing mapping.
+.TP
+.B EINVAL
+We don't like
+.IR addr ,
+.IR length ,
+or
+.I offset
+(e.g., they are too large, or not aligned on a page boundary).
+.TP
+.B EINVAL
+(since Linux 2.6.12)
+.I length
+was 0.
+.TP
+.B EINVAL
+.I flags
+contained none of
+.BR MAP_PRIVATE ,
+.BR MAP_SHARED ,
+or
+.BR MAP_SHARED_VALIDATE .
+.TP
+.B ENFILE
+.\" This is for shared anonymous segments
+.\" [2.6.7] shmem_zero_setup()-->shmem_file_setup()-->get_empty_filp()
+The system-wide limit on the total number of open files has been reached.
+.\" .TP
+.\" .B ENOEXEC
+.\" A file could not be mapped for reading.
+.TP
+.B ENODEV
+The underlying filesystem of the specified file does not support
+memory mapping.
+.TP
+.B ENOMEM
+No memory is available.
+.TP
+.B ENOMEM
+The process's maximum number of mappings would have been exceeded.
+This error can also occur for
+.BR munmap (),
+when unmapping a region in the middle of an existing mapping,
+since this results in two smaller mappings on either side of
+the region being unmapped.
+.TP
+.B ENOMEM
+(since Linux 4.7)
+The process's
+.B RLIMIT_DATA
+limit, described in
+.BR getrlimit (2),
+would have been exceeded.
+.TP
+.B ENOMEM
+We don't like
+.IR addr ,
+because it exceeds the virtual address space of the CPU.
+.TP
+.B EOVERFLOW
+On 32-bit architecture together with the large file extension
+(i.e., using 64-bit
+.IR off_t ):
+the number of pages used for
+.I length
+plus number of pages used for
+.I offset
+would overflow
+.I "unsigned long"
+(32 bits).
+.TP
+.B EPERM
+The
+.I prot
+argument asks for
+.B PROT_EXEC
+but the mapped area belongs to a file on a filesystem that
+was mounted no-exec.
+.\" (Since Linux 2.4.25 / Linux 2.6.0.)
+.TP
+.B EPERM
+The operation was prevented by a file seal; see
+.BR fcntl (2).
+.TP
+.B EPERM
+The
+.B MAP_HUGETLB
+flag was specified, but the caller was not privileged (did not have the
+.B CAP_IPC_LOCK
+capability)
+and is not a member of the
+.I sysctl_hugetlb_shm_group
+group; see the description of
+.I /proc/sys/vm/sysctl_hugetlb_shm_group
+in
+.TP
+.B ETXTBSY
+.B MAP_DENYWRITE
+was set but the object specified by
+.I fd
+is open for writing.
+.PP
+Use of a mapped region can result in these signals:
+.TP
+.B SIGSEGV
+Attempted write into a region mapped as read-only.
+.TP
+.B SIGBUS
+Attempted access to a page of the buffer that lies beyond the
+end of the mapped file.
+For an explanation of the treatment of the bytes in the page that
+corresponds to the end of a mapped file that is not a multiple
+of the page size, see NOTES.
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lbx lb lb
+l l l.
+Interface	Attribute	Value
+T{
+.na
+.nh
+.BR mmap (),
+.BR munmap ()
+T}	Thread safety	MT-Safe
+.TE
+.sp 1
+.SH VERSIONS
+On some hardware architectures (e.g., i386),
+.B PROT_WRITE
+implies
+.BR PROT_READ .
+It is architecture dependent whether
+.B PROT_READ
+implies
+.B PROT_EXEC
+or not.
+Portable programs should always set
+.B PROT_EXEC
+if they intend to execute code in the new mapping.
+.PP
+The portable way to create a mapping is to specify
+.I addr
+as 0 (NULL), and omit
+.B MAP_FIXED
+from
+.IR flags .
+In this case, the system chooses the address for the mapping;
+the address is chosen so as not to conflict with any existing mapping,
+and will not be 0.
+If the
+.B MAP_FIXED
+flag is specified, and
+.I addr
+is 0 (NULL), then the mapped address will be 0 (NULL).
+.PP
+Certain
+.I flags
+constants are defined only if suitable feature test macros are defined
+(possibly by default):
+.B _DEFAULT_SOURCE
+with glibc 2.19 or later;
+or
+.B _BSD_SOURCE
+or
+.B _SVID_SOURCE
+in glibc 2.19 and earlier.
+(Employing
+.B _GNU_SOURCE
+also suffices,
+and requiring that macro specifically would have been more logical,
+since these flags are all Linux-specific.)
+The relevant flags are:
+.BR MAP_32BIT ,
+.B MAP_ANONYMOUS
+(and the synonym
+.BR MAP_ANON ),
+.BR MAP_DENYWRITE ,
+.BR MAP_EXECUTABLE ,
+.BR MAP_FILE ,
+.BR MAP_GROWSDOWN ,
+.BR MAP_HUGETLB ,
+.BR MAP_LOCKED ,
+.BR MAP_NONBLOCK ,
+.BR MAP_NORESERVE ,
+.BR MAP_POPULATE ,
+and
+.BR MAP_STACK .
+.SS C library/kernel differences
+This page describes the interface provided by the glibc
+.BR mmap ()
+wrapper function.
+Originally, this function invoked a system call of the same name.
+Since Linux 2.4, that system call has been superseded by
+.BR mmap2 (2),
+and nowadays
+.\" Since around glibc 2.1/2.2, depending on the platform.
+the glibc
+.BR mmap ()
+wrapper function invokes
+.BR mmap2 (2)
+with a suitably adjusted value for
+.IR offset .
+.SH STANDARDS
+POSIX.1-2008.
+.SH HISTORY
+POSIX.1-2001, SVr4, 4.4BSD.
+.\" SVr4 documents additional error codes ENXIO and ENODEV.
+.\" SUSv2 documents additional error codes EMFILE and EOVERFLOW.
+.PP
+On POSIX systems on which
+.BR mmap (),
+.BR msync (2),
+and
+.BR munmap ()
+are available,
+.B _POSIX_MAPPED_FILES
+is defined in \fI<unistd.h>\fP to a value greater than 0.
+(See also
+.BR sysconf (3).)
+.\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L.
+.\" -1: unavailable, 0: ask using sysconf().
+.\" glibc defines it to 1.
+.SH NOTES
+Memory mapped by
+.BR mmap ()
+is preserved across
+.BR fork (2),
+with the same attributes.
+.PP
+A file is mapped in multiples of the page size.
+For a file that is not
+a multiple of the page size,
+the remaining bytes in the partial page at the end of the mapping
+are zeroed when mapped,
+and modifications to that region are not written out to the file.
+The effect of
+changing the size of the underlying file of a mapping on the pages that
+correspond to added or removed regions of the file is unspecified.
+.PP
+An application can determine which pages of a mapping are
+currently resident in the buffer/page cache using
+.BR mincore (2).
+.\"
+.SS Using MAP_FIXED safely
+The only safe use for
+.B MAP_FIXED
+is where the address range specified by
+.I addr
+and
+.I length
+was previously reserved using another mapping;
+otherwise, the use of
+.B MAP_FIXED
+is hazardous because it forcibly removes preexisting mappings,
+making it easy for a multithreaded process to corrupt its own address space.
+.PP
+For example, suppose that thread A looks through
+.IR /proc/ pid /maps
+in order to locate an unused address range that it can map using
+.BR MAP_FIXED ,
+while thread B simultaneously acquires part or all of that same
+address range.
+When thread A subsequently employs
+.BR mmap(MAP_FIXED) ,
+it will effectively clobber the mapping that thread B created.
+In this scenario,
+thread B need not create a mapping directly; simply making a library call
+that, internally, uses
+.BR dlopen (3)
+to load some other shared library, will suffice.
+The
+.BR dlopen (3)
+call will map the library into the process's address space.
+Furthermore, almost any library call may be implemented in a way that
+adds memory mappings to the address space, either with this technique,
+or by simply allocating memory.
+Examples include
+.BR brk (2),
+.BR malloc (3),
+.BR pthread_create (3),
+and the PAM libraries
+.UR http://www.linux-pam.org
+.UE .
+.PP
+Since Linux 4.17, a multithreaded program can use the
+.B MAP_FIXED_NOREPLACE
+flag to avoid the hazard described above
+when attempting to create a mapping at a fixed address
+that has not been reserved by a preexisting mapping.
+.\"
+.SS Timestamps changes for file-backed mappings
+For file-backed mappings, the
+.I st_atime
+field for the mapped file may be updated at any time between the
+.BR mmap ()
+and the corresponding unmapping; the first reference to a mapped
+page will update the field if it has not been already.
+.PP
+The
+.I st_ctime
+and
+.I st_mtime
+field for a file mapped with
+.B PROT_WRITE
+and
+.B MAP_SHARED
+will be updated after
+a write to the mapped region, and before a subsequent
+.BR msync (2)
+with the
+.B MS_SYNC
+or
+.B MS_ASYNC
+flag, if one occurs.
+.\"
+.SS Huge page (Huge TLB) mappings
+For mappings that employ huge pages, the requirements for the arguments of
+.BR mmap ()
+and
+.BR munmap ()
+differ somewhat from the requirements for mappings
+that use the native system page size.
+.PP
+For
+.BR mmap (),
+.I offset
+must be a multiple of the underlying huge page size.
+The system automatically aligns
+.I length
+to be a multiple of the underlying huge page size.
+.PP
+For
+.BR munmap (),
+.IR addr ,
+and
+.I length
+must both be a multiple of the underlying huge page size.
+.\"
+.SH BUGS
+On Linux, there are no guarantees like those suggested above under
+.BR MAP_NORESERVE .
+By default, any process can be killed
+at any moment when the system runs out of memory.
+.PP
+Before Linux 2.6.7, the
+.B MAP_POPULATE
+flag has effect only if
+.I prot
+is specified as
+.BR PROT_NONE .
+.PP
+SUSv3 specifies that
+.BR mmap ()
+should fail if
+.I length
+is 0.
+However, before Linux 2.6.12,
+.BR mmap ()
+succeeded in this case: no mapping was created and the call returned
+.IR addr .
+Since Linux 2.6.12,
+.BR mmap ()
+fails with the error
+.B EINVAL
+for this case.
+.PP
+POSIX specifies that the system shall always
+zero fill any partial page at the end
+of the object and that system will never write any modification of the
+object beyond its end.
+On Linux, when you write data to such partial page after the end
+of the object, the data stays in the page cache even after the file
+is closed and unmapped
+and even though the data is never written to the file itself,
+subsequent mappings may see the modified content.
+In some cases, this could be fixed by calling
+.BR msync (2)
+before the unmap takes place;
+however, this doesn't work on
+.BR tmpfs (5)
+(for example, when using the POSIX shared memory interface documented in
+.BR shm_overview (7)).
+.SH EXAMPLES
+.\" FIXME . Add an example here that uses an anonymous shared region for
+.\" IPC between parent and child.
+The following program prints part of the file specified in
+its first command-line argument to standard output.
+The range of bytes to be printed is specified via offset and length
+values in the second and third command-line arguments.
+The program creates a memory mapping of the required
+pages of the file and then uses
+.BR write (2)
+to output the desired bytes.
+.SS Program source
+.\" SRC BEGIN (mmap.c)
+.EX
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <unistd.h>
+\&
+#define handle_error(msg) \e
+    do { perror(msg); exit(EXIT_FAILURE); } while (0)
+\&
+int
+main(int argc, char *argv[])
+{
+    int          fd;
+    char         *addr;
+    off_t        offset, pa_offset;
+    size_t       length;
+    ssize_t      s;
+    struct stat  sb;
+\&
+    if (argc < 3 || argc > 4) {
+        fprintf(stderr, "%s file offset [length]\en", argv[0]);
+        exit(EXIT_FAILURE);
+    }
+\&
+    fd = open(argv[1], O_RDONLY);
+    if (fd == \-1)
+        handle_error("open");
+\&
+    if (fstat(fd, &sb) == \-1)           /* To obtain file size */
+        handle_error("fstat");
+\&
+    offset = atoi(argv[2]);
+    pa_offset = offset & \[ti](sysconf(_SC_PAGE_SIZE) \- 1);
+        /* offset for mmap() must be page aligned */
+\&
+    if (offset >= sb.st_size) {
+        fprintf(stderr, "offset is past end of file\en");
+        exit(EXIT_FAILURE);
+    }
+\&
+    if (argc == 4) {
+        length = atoi(argv[3]);
+        if (offset + length > sb.st_size)
+            length = sb.st_size \- offset;
+                /* Can\[aq]t display bytes past end of file */
+\&
+    } else {    /* No length arg ==> display to end of file */
+        length = sb.st_size \- offset;
+    }
+\&
+    addr = mmap(NULL, length + offset \- pa_offset, PROT_READ,
+                MAP_PRIVATE, fd, pa_offset);
+    if (addr == MAP_FAILED)
+        handle_error("mmap");
+\&
+    s = write(STDOUT_FILENO, addr + offset \- pa_offset, length);
+    if (s != length) {
+        if (s == \-1)
+            handle_error("write");
+\&
+        fprintf(stderr, "partial write");
+        exit(EXIT_FAILURE);
+    }
+\&
+    munmap(addr, length + offset \- pa_offset);
+    close(fd);
+\&
+    exit(EXIT_SUCCESS);
+}
+.EE
+.\" SRC END
+.SH SEE ALSO
+.BR ftruncate (2),
+.BR getpagesize (2),
+.BR memfd_create (2),
+.BR mincore (2),
+.BR mlock (2),
+.BR mmap2 (2),
+.BR mprotect (2),
+.BR mremap (2),
+.BR msync (2),
+.BR remap_file_pages (2),
+.BR setrlimit (2),
+.BR shmat (2),
+.BR userfaultfd (2),
+.BR shm_open (3),
+.BR shm_overview (7)
+.PP
+The descriptions of the following files in
+.BR proc (5):
+.IR /proc/ pid /maps ,
+.IR /proc/ pid /map_files ,
+and
+.IR /proc/ pid /smaps .
+.PP
+B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128\[en]129 and 389\[en]391.
+.\"
+.\" Repeat after me: private read-only mappings are 100% equivalent to
+.\" shared read-only mappings. No ifs, buts, or maybes. -- Linus