open.2: By contrast with O_RDONLY, no file permissions are required for O_PATH

[thirdparty/man-pages.git] / man2 / open.2

.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
.\" and Copyright (C) 2008 Greg Banks
.\" and Copyright (C) 2006, 2008, 2013, 2014 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
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.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
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.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
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.\"
.\" Modified 1993-07-21 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1994-08-21 by Michael Haardt
.\" Modified 1996-04-13 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 1996-05-13 by Thomas Koenig
.\" Modified 1996-12-20 by Michael Haardt
.\" Modified 1999-02-19 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 1998-11-28 by Joseph S. Myers <jsm28@hermes.cam.ac.uk>
.\" Modified 1999-06-03 by Michael Haardt
.\" Modified 2002-05-07 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" 2004-12-08, mtk, reordered flags list alphabetically
.\" 2004-12-08, Martin Pool <mbp@sourcefrog.net> (& mtk), added O_NOATIME
.\" 2007-09-18, mtk, Added description of O_CLOEXEC + other minor edits
.\" 2008-01-03, mtk, with input from Trond Myklebust
.\"     <trond.myklebust@fys.uio.no> and Timo Sirainen <tss@iki.fi>
.\"     Rewrite description of O_EXCL.
.\" 2008-01-11, Greg Banks <gnb@melbourne.sgi.com>: add more detail
.\"     on O_DIRECT.
.\" 2008-02-26, Michael Haardt: Reorganized text for O_CREAT and mode
.\"
.\" FIXME . Apr 08: The next POSIX revision has O_EXEC, O_SEARCH, and
.\" O_TTYINIT.  Eventually these may need to be documented.  --mtk
.\"
.TH OPEN 2 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
open, openat, creat \- open and possibly create a file
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <sys/stat.h>
.B #include <fcntl.h>
.PP
.BI "int open(const char *" pathname ", int " flags );
.BI "int open(const char *" pathname ", int " flags ", mode_t " mode );
.PP
.BI "int creat(const char *" pathname ", mode_t " mode );
.PP
.BI "int openat(int " dirfd ", const char *" pathname ", int " flags );
.BI "int openat(int " dirfd ", const char *" pathname ", int " flags \
", mode_t " mode );
.fi
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.PP
.BR openat ():
.PD 0
.ad l
.RS 4
.TP 4
Since glibc 2.10:
_POSIX_C_SOURCE\ >=\ 200809L
.TP
Before glibc 2.10:
_ATFILE_SOURCE
.RE
.ad
.PD
.SH DESCRIPTION
Given a
.I pathname
for a file,
.BR open ()
returns a file descriptor, a small, nonnegative integer
for use in subsequent system calls
.RB ( read "(2), " write "(2), " lseek "(2), " fcntl "(2), etc.)."
The file descriptor returned by a successful call will be
the lowest-numbered file descriptor not currently open for the process.
.PP
By default, the new file descriptor is set to remain open across an
.BR execve (2)
(i.e., the
.B FD_CLOEXEC
file descriptor flag described in
.BR fcntl (2)
is initially disabled); the
.B O_CLOEXEC
flag, described below, can be used to change this default.
The file offset is set to the beginning of the file (see
.BR lseek (2)).
.PP
A call to
.BR open ()
creates a new
.IR "open file description" ,
an entry in the system-wide table of open files.
The open file description records the file offset and the file status flags
(see below).
A file descriptor is a reference to an open file description;
this reference is unaffected if
.I pathname
is subsequently removed or modified to refer to a different file.
For further details on open file descriptions, see NOTES.
.PP
The argument
.I flags
must include one of the following
.IR "access modes" :
.BR O_RDONLY ", " O_WRONLY ", or " O_RDWR .
These request opening the file read-only, write-only, or read/write,
respectively.
.PP
In addition, zero or more file creation flags and file status flags
can be
.RI bitwise- or 'd
in
.IR flags .
The
.I file creation flags
are
.BR O_CLOEXEC ,
.BR O_CREAT ,
.BR O_DIRECTORY ,
.BR O_EXCL ,
.BR O_NOCTTY ,
.BR O_NOFOLLOW ,
.BR O_TMPFILE ,
and
.BR O_TRUNC .
The
.I file status flags
are all of the remaining flags listed below.
.\" SUSv4 divides the flags into:
.\" * Access mode
.\" * File creation
.\" * File status
.\" * Other (O_CLOEXEC, O_DIRECTORY, O_NOFOLLOW)
.\" though it's not clear what the difference between "other" and
.\" "File creation" flags is.  I raised an Aardvark to see if this
.\" can be clarified in SUSv4; 10 Oct 2008.
.\" http://thread.gmane.org/gmane.comp.standards.posix.austin.general/64/focus=67
.\" TC1 (balloted in 2013), resolved this, so that those three constants
.\" are also categorized" as file status flags.
.\"
The distinction between these two groups of flags is that
the file creation flags affect the semantics of the open operation itself,
while the file status flags affect the semantics of subsequent I/O operations.
The file status flags can be retrieved and (in some cases)
modified; see
.BR fcntl (2)
for details.
.PP
The full list of file creation flags and file status flags is as follows:
.TP
.B O_APPEND
The file is opened in append mode.
Before each
.BR write (2),
the file offset is positioned at the end of the file,
as if with
.BR lseek (2).
The modification of the file offset and the write operation
are performed as a single atomic step.
.IP
.B O_APPEND
may lead to corrupted files on NFS filesystems if more than one process
appends data to a file at once.
.\" For more background, see
.\" http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=453946
.\" http://nfs.sourceforge.net/
This is because NFS does not support
appending to a file, so the client kernel has to simulate it, which
can't be done without a race condition.
.TP
.B O_ASYNC
Enable signal-driven I/O:
generate a signal
.RB ( SIGIO
by default, but this can be changed via
.BR fcntl (2))
when input or output becomes possible on this file descriptor.
This feature is available only for terminals, pseudoterminals,
sockets, and (since Linux 2.6) pipes and FIFOs.
See
.BR fcntl (2)
for further details.
See also BUGS, below.
.TP
.BR O_CLOEXEC " (since Linux 2.6.23)"
.\" NOTE! several other man pages refer to this text
Enable the close-on-exec flag for the new file descriptor.
.\" FIXME . for later review when Issue 8 is one day released...
.\" POSIX proposes to fix many APIs that provide hidden FDs
.\" http://austingroupbugs.net/tag_view_page.php?tag_id=8
.\" http://austingroupbugs.net/view.php?id=368
Specifying this flag permits a program to avoid additional
.BR fcntl (2)
.B F_SETFD
operations to set the
.B FD_CLOEXEC
flag.
.IP
Note that the use of this flag is essential in some multithreaded programs,
because using a separate
.BR fcntl (2)
.B F_SETFD
operation to set the
.B FD_CLOEXEC
flag does not suffice to avoid race conditions
where one thread opens a file descriptor and
attempts to set its close-on-exec flag using
.BR fcntl (2)
at the same time as another thread does a
.BR fork (2)
plus
.BR execve (2).
Depending on the order of execution,
the race may lead to the file descriptor returned by
.BR open ()
being unintentionally leaked to the program executed by the child process
created by
.BR fork (2).
(This kind of race is in principle possible for any system call
that creates a file descriptor whose close-on-exec flag should be set,
and various other Linux system calls provide an equivalent of the
.BR O_CLOEXEC
flag to deal with this problem.)
.\" This flag fixes only one form of the race condition;
.\" The race can also occur with, for example, file descriptors
.\" returned by accept(), pipe(), etc.
.TP
.B O_CREAT
If the file does not exist, it will be created.
.IP
The owner (user ID) of the new file is set to the effective user ID
of the process.
.IP
The group ownership (group ID) of the new file is set either to
the effective group ID of the process (System V semantics)
or to the group ID of the parent directory (BSD semantics).
On Linux, the behavior depends on whether the
set-group-ID mode bit is set on the parent directory:
if that bit is set, then BSD semantics apply;
otherwise, System V semantics apply.
For some filesystems, the behavior also depends on the
.I bsdgroups
and
.I sysvgroups
mount options described in
.BR mount (8)).
.\" As at 2.6.25, bsdgroups is supported by ext2, ext3, ext4, and
.\" XFS (since 2.6.14).
.RS
.PP
The
.I mode
argument specifies the file mode bits be applied when a new file is created.
This argument must be supplied when
.B O_CREAT
or
.B O_TMPFILE
is specified in
.IR flags ;
if neither
.B O_CREAT
nor
.B O_TMPFILE
is specified, then
.I mode
is ignored.
The effective mode is modified by the process's
.I umask
in the usual way: in the absence of a default ACL, the mode of the
created file is
.IR "(mode\ &\ ~umask)" .
Note that this mode applies only to future accesses of the
newly created file; the
.BR open ()
call that creates a read-only file may well return a read/write
file descriptor.
.PP
The following symbolic constants are provided for
.IR mode :
.TP 9
.B S_IRWXU
00700 user (file owner) has read, write, and execute permission
.TP
.B S_IRUSR
00400 user has read permission
.TP
.B S_IWUSR
00200 user has write permission
.TP
.B S_IXUSR
00100 user has execute permission
.TP
.B S_IRWXG
00070 group has read, write, and execute permission
.TP
.B S_IRGRP
00040 group has read permission
.TP
.B S_IWGRP
00020 group has write permission
.TP
.B S_IXGRP
00010 group has execute permission
.TP
.B S_IRWXO
00007 others have read, write, and execute permission
.TP
.B S_IROTH
00004 others have read permission
.TP
.B S_IWOTH
00002 others have write permission
.TP
.B S_IXOTH
00001 others have execute permission
.RE
.IP
According to POSIX, the effect when other bits are set in
.I mode
is unspecified.
On Linux, the following bits are also honored in
.IR mode :
.RS
.TP 9
.B S_ISUID
0004000 set-user-ID bit
.TP
.B S_ISGID
0002000 set-group-ID bit (see
.BR inode (7)).
.TP
.B S_ISVTX
0001000 sticky bit (see
.BR inode (7)).
.RE
.TP
.BR O_DIRECT " (since Linux 2.4.10)"
Try to minimize cache effects of the I/O to and from this file.
In general this will degrade performance, but it is useful in
special situations, such as when applications do their own caching.
File I/O is done directly to/from user-space buffers.
The
.B O_DIRECT
flag on its own makes an effort to transfer data synchronously,
but does not give the guarantees of the
.B O_SYNC
flag that data and necessary metadata are transferred.
To guarantee synchronous I/O,
.B O_SYNC
must be used in addition to
.BR O_DIRECT .
See NOTES below for further discussion.
.IP
A semantically similar (but deprecated) interface for block devices
is described in
.BR raw (8).
.TP
.B O_DIRECTORY
If \fIpathname\fP is not a directory, cause the open to fail.
.\" But see the following and its replies:
.\" http://marc.theaimsgroup.com/?t=112748702800001&r=1&w=2
.\" [PATCH] open: O_DIRECTORY and O_CREAT together should fail
.\" O_DIRECTORY | O_CREAT causes O_DIRECTORY to be ignored.
This flag was added in kernel version 2.1.126, to
avoid denial-of-service problems if
.BR opendir (3)
is called on a
FIFO or tape device.
.TP
.B O_DSYNC
Write operations on the file will complete according to the requirements of
synchronized I/O
.I data
integrity completion.
.IP
By the time
.BR write (2)
(and similar)
return, the output data
has been transferred to the underlying hardware,
along with any file metadata that would be required to retrieve that data
(i.e., as though each
.BR write (2)
was followed by a call to
.BR fdatasync (2)).
.IR "See NOTES below" .
.TP
.B O_EXCL
Ensure that this call creates the file:
if this flag is specified in conjunction with
.BR O_CREAT ,
and
.I pathname
already exists, then
.BR open ()
will fail.
.IP
When these two flags are specified, symbolic links are not followed:
.\" POSIX.1-2001 explicitly requires this behavior.
if
.I pathname
is a symbolic link, then
.BR open ()
fails regardless of where the symbolic link points to.
.IP
In general, the behavior of
.B O_EXCL
is undefined if it is used without
.BR O_CREAT .
There is one exception: on Linux 2.6 and later,
.B O_EXCL
can be used without
.B O_CREAT
if
.I pathname
refers to a block device.
If the block device is in use by the system (e.g., mounted),
.BR open ()
fails with the error
.BR EBUSY .
.IP
On NFS,
.B O_EXCL
is supported only when using NFSv3 or later on kernel 2.6 or later.
In NFS environments where
.B O_EXCL
support is not provided, programs that rely on it
for performing locking tasks will contain a race condition.
Portable programs that want to perform atomic file locking using a lockfile,
and need to avoid reliance on NFS support for
.BR O_EXCL ,
can create a unique file on
the same filesystem (e.g., incorporating hostname and PID), and use
.BR link (2)
to make a link to the lockfile.
If
.BR link (2)
returns 0, the lock is successful.
Otherwise, use
.BR stat (2)
on the unique file to check if its link count has increased to 2,
in which case the lock is also successful.
.TP
.B O_LARGEFILE
(LFS)
Allow files whose sizes cannot be represented in an
.I off_t
(but can be represented in an
.IR off64_t )
to be opened.
The
.B _LARGEFILE64_SOURCE
macro must be defined
(before including
.I any
header files)
in order to obtain this definition.
Setting the
.B _FILE_OFFSET_BITS
feature test macro to 64 (rather than using
.BR O_LARGEFILE )
is the preferred
method of accessing large files on 32-bit systems (see
.BR feature_test_macros (7)).
.TP
.BR O_NOATIME " (since Linux 2.6.8)"
Do not update the file last access time
.RI ( st_atime
in the inode)
when the file is
.BR read (2).
.IP
This flag can be employed only if one of the following conditions is true:
.RS
.IP * 3
The effective UID of the process
.\" Strictly speaking: the filesystem UID
matches the owner UID of the file.
.IP *
The calling process has the
.BR CAP_FOWNER
capability in its user namespace and
the owner UID of the file has a mapping in the namespace.
.RE
.IP
This flag is intended for use by indexing or backup programs,
where its use can significantly reduce the amount of disk activity.
This flag may not be effective on all filesystems.
One example is NFS, where the server maintains the access time.
.\" The O_NOATIME flag also affects the treatment of st_atime
.\" by mmap() and readdir(2), MTK, Dec 04.
.TP
.B O_NOCTTY
If
.I pathname
refers to a terminal device\(emsee
.BR tty (4)\(emit
will not become the process's controlling terminal even if the
process does not have one.
.TP
.B O_NOFOLLOW
If \fIpathname\fP is a symbolic link, then the open fails, with the error
.BR ELOOP .
Symbolic links in earlier components of the pathname will still be
followed.
(Note that the
.B ELOOP
error that can occur in this case is indistinguishable from the case where
an open fails because there are too many symbolic links found
while resolving components in the prefix part of the pathname.)
.IP
This flag is a FreeBSD extension, which was added to Linux in version 2.1.126,
and has subsequently been standardized in POSIX.1-2008.
.IP
See also
.BR O_PATH
below.
.\" The headers from glibc 2.0.100 and later include a
.\" definition of this flag; \fIkernels before 2.1.126 will ignore it if
.\" used\fP.
.TP
.BR O_NONBLOCK " or " O_NDELAY
When possible, the file is opened in nonblocking mode.
Neither the
.BR open ()
nor any subsequent operations on the file descriptor which is
returned will cause the calling process to wait.
.IP
Note that this flag has no effect for regular files and block devices;
that is, I/O operations will (briefly) block when device activity
is required, regardless of whether
.B O_NONBLOCK
is set.
Since
.B O_NONBLOCK
semantics might eventually be implemented,
applications should not depend upon blocking behavior
when specifying this flag for regular files and block devices.
.IP
For the handling of FIFOs (named pipes), see also
.BR fifo (7).
For a discussion of the effect of
.B O_NONBLOCK
in conjunction with mandatory file locks and with file leases, see
.BR fcntl (2).
.TP
.BR O_PATH " (since Linux 2.6.39)"
.\" commit 1abf0c718f15a56a0a435588d1b104c7a37dc9bd
.\" commit 326be7b484843988afe57566b627fb7a70beac56
.\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d
.\"
.\" http://thread.gmane.org/gmane.linux.man/2790/focus=3496
.\"	Subject: Re: [PATCH] open(2): document O_PATH
.\"	Newsgroups: gmane.linux.man, gmane.linux.kernel
.\"
Obtain a file descriptor that can be used for two purposes:
to indicate a location in the filesystem tree and
to perform operations that act purely at the file descriptor level.
The file itself is not opened, and other file operations (e.g.,
.BR read (2),
.BR write (2),
.BR fchmod (2),
.BR fchown (2),
.BR fgetxattr (2),
.BR ioctl (2),
.BR mmap (2))
fail with the error
.BR EBADF .
.IP
The following operations
.I can
be performed on the resulting file descriptor:
.RS
.IP * 3
.BR close (2).
.IP *
.BR fchdir (2),
if the file descriptor refers to a directory
(since Linux 3.5).
.\" commit 332a2e1244bd08b9e3ecd378028513396a004a24
.IP *
.BR fstat (2)
(since Linux 3.6).
.IP *
.\" fstat(): commit 55815f70147dcfa3ead5738fd56d3574e2e3c1c2
.BR fstatfs (2)
(since Linux 3.12).
.\" fstatfs(): commit 9d05746e7b16d8565dddbe3200faa1e669d23bbf
.IP *
Duplicating the file descriptor
.RB ( dup (2),
.BR fcntl (2)
.BR F_DUPFD ,
etc.).
.IP *
Getting and setting file descriptor flags
.RB ( fcntl (2)
.BR F_GETFD
and
.BR F_SETFD ).
.IP *
Retrieving open file status flags using the
.BR fcntl (2)
.BR F_GETFL
operation: the returned flags will include the bit
.BR O_PATH .
.IP *
Passing the file descriptor as the
.IR dirfd
argument of
.BR openat ()
and the other "*at()" system calls.
This includes
.BR linkat (2)
with
.BR AT_EMPTY_PATH
(or via procfs using
.BR AT_SYMLINK_FOLLOW )
even if the file is not a directory.
.IP *
Passing the file descriptor to another process via a UNIX domain socket
(see
.BR SCM_RIGHTS
in
.BR unix (7)).
.RE
.IP
When
.B O_PATH
is specified in
.IR flags ,
flag bits other than
.BR O_CLOEXEC ,
.BR O_DIRECTORY ,
and
.BR O_NOFOLLOW
are ignored.
.IP
Opening a file or directory with the
.B O_PATH
flag requires no permissions on the object itself
(but does require execute permission on the directories in the path prefix).
Depending on the subsequent operation,
a check for suitable file permissions may be performed (e.g.,
.BR fchdir (2)
requires execute permission on the directory referred to
by its file descriptor argument).
By contrast,
obtaining a reference to a filesystem object by opening it with the
.B O_RDONLY
flag requires that the caller have read permission on the object,
even when the subsequent operation (e.g.,
.BR fchdir (2),
.BR fstat (2))
does not require read permission on the object.
.IP
If
.I pathname
is a symbolic link and the
.BR O_NOFOLLOW
flag is also specified,
then the call returns a file descriptor referring to the symbolic link.
This file descriptor can be used as the
.I dirfd
argument in calls to
.BR fchownat (2),
.BR fstatat (2),
.BR linkat (2),
and
.BR readlinkat (2)
with an empty pathname to have the calls operate on the symbolic link.
.IP
If
.I pathname
refers to an automount point that has not yet been triggered, so no
other filesystem is mounted on it, then the call returns a file
descriptor referring to the automount directory without triggering a mount.
.BR fstatfs (2)
can then be used to determine if it is, in fact, an untriggered
automount point
.RB ( ".f_type == AUTOFS_SUPER_MAGIC" ).
.IP
One use of
.B O_PATH
for regular files is to provide the equivalent of POSIX.1's
.B O_EXEC
functionality.
This permits us to open a file for which we have execute
permission but not read permission, and then execute that file,
with steps something like the following:
.IP
.in +4n
.EX
char buf[PATH_MAX];
fd = open("some_prog", O_PATH);
snprintf(buf, "/proc/self/fd/%d", fd);
execl(buf, "some_prog", (char *) NULL);
.EE
.in
.IP
An
.B O_PATH
file descriptor can also be passed as the argument of
.BR fexecve (3).
.TP
.B O_SYNC
Write operations on the file will complete according to the requirements of
synchronized I/O
.I file
integrity completion
(by contrast with the
synchronized I/O
.I data
integrity completion
provided by
.BR O_DSYNC .)
.IP
By the time
.BR write (2)
(and similar)
return, the output data and associated file metadata
have been transferred to the underlying hardware
(i.e., as though each
.BR write (2)
was followed by a call to
.BR fsync (2)).
.IR "See NOTES below" .
.TP
.BR O_TMPFILE " (since Linux 3.11)"
.\" commit 60545d0d4610b02e55f65d141c95b18ccf855b6e
.\" commit f4e0c30c191f87851c4a53454abb55ee276f4a7e
.\" commit bb458c644a59dbba3a1fe59b27106c5e68e1c4bd
Create an unnamed temporary file.
The
.I pathname
argument specifies a directory;
an unnamed inode will be created in that directory's filesystem.
Anything written to the resulting file will be lost when
the last file descriptor is closed, unless the file is given a name.
.IP
.B O_TMPFILE
must be specified with one of
.B O_RDWR
or
.B O_WRONLY
and, optionally,
.BR O_EXCL .
If
.B O_EXCL
is not specified, then
.BR linkat (2)
can be used to link the temporary file into the filesystem, making it
permanent, using code like the following:
.IP
.in +4n
.EX
char path[PATH_MAX];
fd = open("/path/to/dir", O_TMPFILE | O_RDWR,
                        S_IRUSR | S_IWUSR);

/* File I/O on 'fd'... */

snprintf(path, PATH_MAX,  "/proc/self/fd/%d", fd);
linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",
                        AT_SYMLINK_FOLLOW);
.EE
.in
.IP
In this case,
the
.BR open ()
.I mode
argument determines the file permission mode, as with
.BR O_CREAT .
.IP
Specifying
.B O_EXCL
in conjunction with
.B O_TMPFILE
prevents a temporary file from being linked into the filesystem
in the above manner.
(Note that the meaning of
.B O_EXCL
in this case is different from the meaning of
.B O_EXCL
otherwise.)
.IP
There are two main use cases for
.\" Inspired by http://lwn.net/Articles/559147/
.BR O_TMPFILE :
.RS
.IP * 3
Improved
.BR tmpfile (3)
functionality: race-free creation of temporary files that
(1) are automatically deleted when closed;
(2) can never be reached via any pathname;
(3) are not subject to symlink attacks; and
(4) do not require the caller to devise unique names.
.IP *
Creating a file that is initially invisible, which is then populated
with data and adjusted to have appropriate filesystem attributes
.RB ( fchown (2),
.BR fchmod (2),
.BR fsetxattr (2),
etc.)
before being atomically linked into the filesystem
in a fully formed state (using
.BR linkat (2)
as described above).
.RE
.IP
.B O_TMPFILE
requires support by the underlying filesystem;
only a subset of Linux filesystems provide that support.
In the initial implementation, support was provided in
the ext2, ext3, ext4, UDF, Minix, and shmem filesystems.
.\" To check for support, grep for "tmpfile" in kernel sources
Support for other filesystems has subsequently been added as follows:
XFS (Linux 3.15);
.\" commit 99b6436bc29e4f10e4388c27a3e4810191cc4788
.\" commit ab29743117f9f4c22ac44c13c1647fb24fb2bafe
Btrfs (Linux 3.16);
.\" commit ef3b9af50bfa6a1f02cd7b3f5124b712b1ba3e3c
F2FS (Linux 3.16);
.\" commit 50732df02eefb39ab414ef655979c2c9b64ad21c
and ubifs (Linux 4.9)
.TP
.B O_TRUNC
If the file already exists and is a regular file and the access mode allows
writing (i.e., is
.B O_RDWR
or
.BR O_WRONLY )
it will be truncated to length 0.
If the file is a FIFO or terminal device file, the
.B O_TRUNC
flag is ignored.
Otherwise, the effect of
.B O_TRUNC
is unspecified.
.SS creat()
A call to
.BR creat ()
is equivalent to calling
.BR open ()
with
.I flags
equal to
.BR O_CREAT|O_WRONLY|O_TRUNC .
.SS openat()
The
.BR openat ()
system call operates in exactly the same way as
.BR open (),
except for the differences described here.
.IP
If the pathname given in
.I pathname
is relative, then it is interpreted relative to the directory
referred to by the file descriptor
.I dirfd
(rather than relative to the current working directory of
the calling process, as is done by
.BR open ()
for a relative pathname).
.IP
If
.I pathname
is relative and
.I dirfd
is the special value
.BR AT_FDCWD ,
then
.I pathname
is interpreted relative to the current working
directory of the calling process (like
.BR open ()).
.IP
If
.I pathname
is absolute, then
.I dirfd
is ignored.
.SH RETURN VALUE
.BR open (),
.BR openat (),
and
.BR creat ()
return the new file descriptor, or \-1 if an error occurred
(in which case,
.I errno
is set appropriately).
.SH ERRORS
.BR open (),
.BR openat (),
and
.BR creat ()
can fail with the following errors:
.TP
.B EACCES
The requested access to the file is not allowed, or search permission
is denied for one of the directories in the path prefix of
.IR pathname ,
or the file did not exist yet and write access to the parent directory
is not allowed.
(See also
.BR path_resolution (7).)
.TP
.B EDQUOT
Where
.B O_CREAT
is specified, the file does not exist, and the user's quota of disk
blocks or inodes on the filesystem has been exhausted.
.TP
.B EEXIST
.I pathname
already exists and
.BR O_CREAT " and " O_EXCL
were used.
.TP
.B EFAULT
.I pathname
points outside your accessible address space.
.TP
.B EFBIG
See
.BR EOVERFLOW .
.TP
.B EINTR
While blocked waiting to complete an open of a slow device
(e.g., a FIFO; see
.BR fifo (7)),
the call was interrupted by a signal handler; see
.BR signal (7).
.TP
.B EINVAL
The filesystem does not support the
.BR O_DIRECT
flag.
See
.BR NOTES
for more information.
.TP
.B EINVAL
Invalid value in
.\" In particular, __O_TMPFILE instead of O_TMPFILE
.IR flags .
.TP
.B EINVAL
.B O_TMPFILE
was specified in
.IR flags ,
but neither
.B O_WRONLY
nor
.B O_RDWR
was specified.
.TP
.B EISDIR
.I pathname
refers to a directory and the access requested involved writing
(that is,
.B O_WRONLY
or
.B O_RDWR
is set).
.TP
.B EISDIR
.I pathname
refers to an existing directory,
.B O_TMPFILE
and one of
.B O_WRONLY
or
.B O_RDWR
were specified in
.IR flags ,
but this kernel version does not provide the
.B O_TMPFILE
functionality.
.TP
.B ELOOP
Too many symbolic links were encountered in resolving
.IR pathname .
.TP
.B ELOOP
.I pathname
was a symbolic link, and
.I flags
specified
.BR O_NOFOLLOW
but not
.BR O_PATH .
.TP
.B EMFILE
The per-process limit on the number of open file descriptors has been reached
(see the description of
.BR RLIMIT_NOFILE
in
.BR getrlimit (2)).
.TP
.B ENAMETOOLONG
.I pathname
was too long.
.TP
.B ENFILE
The system-wide limit on the total number of open files has been reached.
.TP
.B ENODEV
.I pathname
refers to a device special file and no corresponding device exists.
(This is a Linux kernel bug; in this situation
.B ENXIO
must be returned.)
.TP
.B ENOENT
.B O_CREAT
is not set and the named file does not exist.
Or, a directory component in
.I pathname
does not exist or is a dangling symbolic link.
.TP
.B ENOENT
.I pathname
refers to a nonexistent directory,
.B O_TMPFILE
and one of
.B O_WRONLY
or
.B O_RDWR
were specified in
.IR flags ,
but this kernel version does not provide the
.B O_TMPFILE
functionality.
.TP
.B ENOMEM
The named file is a FIFO,
but memory for the FIFO buffer can't be allocated because
the per-user hard limit on memory allocation for pipes has been reached
and the caller is not privileged; see
.BR pipe (7).
.TP
.B ENOMEM
Insufficient kernel memory was available.
.TP
.B ENOSPC
.I pathname
was to be created but the device containing
.I pathname
has no room for the new file.
.TP
.B ENOTDIR
A component used as a directory in
.I pathname
is not, in fact, a directory, or \fBO_DIRECTORY\fP was specified and
.I pathname
was not a directory.
.TP
.B ENXIO
.BR O_NONBLOCK " | " O_WRONLY
is set, the named file is a FIFO, and
no process has the FIFO open for reading.
.TP
.B ENXIO
The file is a device special file and no corresponding device exists.
.TP
.BR EOPNOTSUPP
The filesystem containing
.I pathname
does not support
.BR O_TMPFILE .
.TP
.B EOVERFLOW
.I pathname
refers to a regular file that is too large to be opened.
The usual scenario here is that an application compiled
on a 32-bit platform without
.I -D_FILE_OFFSET_BITS=64
tried to open a file whose size exceeds
.I (1<<31)-1
bytes;
see also
.B O_LARGEFILE
above.
This is the error specified by POSIX.1;
in kernels before 2.6.24, Linux gave the error
.B EFBIG
for this case.
.\" See http://bugzilla.kernel.org/show_bug.cgi?id=7253
.\" "Open of a large file on 32-bit fails with EFBIG, should be EOVERFLOW"
.\" Reported 2006-10-03
.TP
.B EPERM
The
.B O_NOATIME
flag was specified, but the effective user ID of the caller
.\" Strictly speaking, it's the filesystem UID... (MTK)
did not match the owner of the file and the caller was not privileged.
.TP
.B EPERM
The operation was prevented by a file seal; see
.BR fcntl (2).
.TP
.B EROFS
.I pathname
refers to a file on a read-only filesystem and write access was
requested.
.TP
.B ETXTBSY
.I pathname
refers to an executable image which is currently being executed and
write access was requested.
.TP
.B EWOULDBLOCK
The
.B O_NONBLOCK
flag was specified, and an incompatible lease was held on the file
(see
.BR fcntl (2)).
.PP
The following additional errors can occur for
.BR openat ():
.TP
.B EBADF
.I dirfd
is not a valid file descriptor.
.TP
.B ENOTDIR
.I pathname
is a relative pathname and
.I dirfd
is a file descriptor referring to a file other than a directory.
.SH VERSIONS
.BR openat ()
was added to Linux in kernel 2.6.16;
library support was added to glibc in version 2.4.
.SH CONFORMING TO
.BR open (),
.BR creat ()
SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
.PP
.BR openat ():
POSIX.1-2008.
.PP
The
.BR O_DIRECT ,
.BR O_NOATIME ,
.BR O_PATH ,
and
.BR O_TMPFILE
flags are Linux-specific.
One must define
.B _GNU_SOURCE
to obtain their definitions.
.PP
The
.BR O_CLOEXEC ,
.BR O_DIRECTORY ,
and
.BR O_NOFOLLOW
flags are not specified in POSIX.1-2001,
but are specified in POSIX.1-2008.
Since glibc 2.12, one can obtain their definitions by defining either
.B _POSIX_C_SOURCE
with a value greater than or equal to 200809L or
.BR _XOPEN_SOURCE
with a value greater than or equal to 700.
In glibc 2.11 and earlier, one obtains the definitions by defining
.BR _GNU_SOURCE .
.PP
As noted in
.BR feature_test_macros (7),
feature test macros such as
.BR _POSIX_C_SOURCE ,
.BR _XOPEN_SOURCE ,
and
.B _GNU_SOURCE
must be defined before including
.I any
header files.
.SH NOTES
Under Linux, the
.B O_NONBLOCK
flag indicates that one wants to open
but does not necessarily have the intention to read or write.
This is typically used to open devices in order to get a file descriptor
for use with
.BR ioctl (2).
.PP
The (undefined) effect of
.B O_RDONLY | O_TRUNC
varies among implementations.
On many systems the file is actually truncated.
.\" Linux 2.0, 2.5: truncate
.\" Solaris 5.7, 5.8: truncate
.\" Irix 6.5: truncate
.\" Tru64 5.1B: truncate
.\" HP-UX 11.22: truncate
.\" FreeBSD 4.7: truncate
.PP
Note that
.BR open ()
can open device special files, but
.BR creat ()
cannot create them; use
.BR mknod (2)
instead.
.PP
If the file is newly created, its
.IR st_atime ,
.IR st_ctime ,
.I st_mtime
fields
(respectively, time of last access, time of last status change, and
time of last modification; see
.BR stat (2))
are set
to the current time, and so are the
.I st_ctime
and
.I st_mtime
fields of the
parent directory.
Otherwise, if the file is modified because of the
.B O_TRUNC
flag, its
.I st_ctime
and
.I st_mtime
fields are set to the current time.
.PP
The files in the
.I /proc/[pid]/fd
directory show the open file descriptors of the process with the PID
.IR pid .
The files in the
.I /proc/[pid]/fdinfo
directory show even more information about these files descriptors.
See
.BR proc (5)
for further details of both of these directories.
.\"
.\"
.SS Open file descriptions
The term open file description is the one used by POSIX to refer to the
entries in the system-wide table of open files.
In other contexts, this object is
variously also called an "open file object",
a "file handle", an "open file table entry",
or\(emin kernel-developer parlance\(ema
.IR "struct file" .
.PP
When a file descriptor is duplicated (using
.BR dup (2)
or similar),
the duplicate refers to the same open file description
as the original file descriptor,
and the two file descriptors consequently share
the file offset and file status flags.
Such sharing can also occur between processes:
a child process created via
.BR fork (2)
inherits duplicates of its parent's file descriptors,
and those duplicates refer to the same open file descriptions.
.PP
Each
.BR open ()
of a file creates a new open file description;
thus, there may be multiple open file descriptions
corresponding to a file inode.
.PP
On Linux, one can use the
.BR kcmp (2)
.B KCMP_FILE
operation to test whether two file descriptors
(in the same process or in two different processes)
refer to the same open file description.
.\"
.\"
.SS Synchronized I/O
The POSIX.1-2008 "synchronized I/O" option
specifies different variants of synchronized I/O,
and specifies the
.BR open ()
flags
.BR O_SYNC ,
.BR O_DSYNC ,
and
.BR O_RSYNC
for controlling the behavior.
Regardless of whether an implementation supports this option,
it must at least support the use of
.BR O_SYNC
for regular files.
.PP
Linux implements
.BR O_SYNC
and
.BR O_DSYNC ,
but not
.BR O_RSYNC .
(Somewhat incorrectly, glibc defines
.BR O_RSYNC
to have the same value as
.BR O_SYNC .)
.PP
.BR O_SYNC
provides synchronized I/O
.I file
integrity completion,
meaning write operations will flush data and all associated metadata
to the underlying hardware.
.BR O_DSYNC
provides synchronized I/O
.I data
integrity completion,
meaning write operations will flush data
to the underlying hardware,
but will only flush metadata updates that are required
to allow a subsequent read operation to complete successfully.
Data integrity completion can reduce the number of disk operations
that are required for applications that don't need the guarantees
of file integrity completion.
.PP
To understand the difference between the two types of completion,
consider two pieces of file metadata:
the file last modification timestamp
.RI ( st_mtime )
and the file length.
All write operations will update the last file modification timestamp,
but only writes that add data to the end of the
file will change the file length.
The last modification timestamp is not needed to ensure that
a read completes successfully, but the file length is.
Thus,
.BR O_DSYNC
would only guarantee to flush updates to the file length metadata
(whereas
.BR O_SYNC
would also always flush the last modification timestamp metadata).
.PP
Before Linux 2.6.33, Linux implemented only the
.BR O_SYNC
flag for
.BR open ().
However, when that flag was specified,
most filesystems actually provided the equivalent of synchronized I/O
.I data
integrity completion (i.e.,
.BR O_SYNC
was actually implemented as the equivalent of
.BR O_DSYNC ).
.PP
Since Linux 2.6.33, proper
.BR O_SYNC
support is provided.
However, to ensure backward binary compatibility,
.BR O_DSYNC
was defined with the same value as the historical
.BR O_SYNC ,
and
.BR O_SYNC
was defined as a new (two-bit) flag value that includes the
.BR O_DSYNC
flag value.
This ensures that applications compiled against
new headers get at least
.BR O_DSYNC
semantics on pre-2.6.33 kernels.
.\"
.\"
.SS NFS
There are many infelicities in the protocol underlying NFS, affecting
amongst others
.BR O_SYNC " and " O_NDELAY .
.PP
On NFS filesystems with UID mapping enabled,
.BR open ()
may
return a file descriptor but, for example,
.BR read (2)
requests are denied
with \fBEACCES\fP.
This is because the client performs
.BR open ()
by checking the
permissions, but UID mapping is performed by the server upon
read and write requests.
.\"
.\"
.SS FIFOs
Opening the read or write end of a FIFO blocks until the other
end is also opened (by another process or thread).
See
.BR fifo (7)
for further details.
.\"
.\"
.SS File access mode
Unlike the other values that can be specified in
.IR flags ,
the
.I "access mode"
values
.BR O_RDONLY ", " O_WRONLY ", and " O_RDWR
do not specify individual bits.
Rather, they define the low order two bits of
.IR flags ,
and are defined respectively as 0, 1, and 2.
In other words, the combination
.B "O_RDONLY | O_WRONLY"
is a logical error, and certainly does not have the same meaning as
.BR O_RDWR .
.PP
Linux reserves the special, nonstandard access mode 3 (binary 11) in
.I flags
to mean:
check for read and write permission on the file and return a file descriptor
that can't be used for reading or writing.
This nonstandard access mode is used by some Linux drivers to return a
file descriptor that is to be used only for device-specific
.BR ioctl (2)
operations.
.\" See for example util-linux's disk-utils/setfdprm.c
.\" For some background on access mode 3, see
.\" http://thread.gmane.org/gmane.linux.kernel/653123
.\" "[RFC] correct flags to f_mode conversion in __dentry_open"
.\" LKML, 12 Mar 2008
.\"
.\"
.SS Rationale for openat() and other "directory file descriptor" APIs
.BR openat ()
and the other system calls and library functions that take
a directory file descriptor argument
(i.e.,
.BR execveat (2),
.BR faccessat (2),
.BR fanotify_mark (2),
.BR fchmodat (2),
.BR fchownat (2),
.BR fstatat (2),
.BR futimesat (2),
.BR linkat (2),
.BR mkdirat (2),
.BR mknodat (2),
.BR name_to_handle_at (2),
.BR readlinkat (2),
.BR renameat (2),
.BR statx (2),
.BR symlinkat (2),
.BR unlinkat (2),
.BR utimensat (2),
.BR mkfifoat (3),
and
.BR scandirat (3))
address two problems with the older interfaces that preceded them.
Here, the explanation is in terms of the
.BR openat ()
call, but the rationale is analogous for the other interfaces.
.PP
First,
.BR openat ()
allows an application to avoid race conditions that could
occur when using
.BR open ()
to open files in directories other than the current working directory.
These race conditions result from the fact that some component
of the directory prefix given to
.BR open ()
could be changed in parallel with the call to
.BR open ().
Suppose, for example, that we wish to create the file
.I dir1/dir2/xxx.dep
if the file
.I dir1/dir2/xxx
exists.
The problem is that between the existence check and the file creation step,
.I dir1
or
.I dir2
(which might be symbolic links)
could be modified to point to a different location.
Such races can be avoided by
opening a file descriptor for the target directory,
and then specifying that file descriptor as the
.I dirfd
argument of (say)
.BR fstatat (2)
and
.BR openat ().
The use of the
.I dirfd
file descriptor also has other benefits:
.IP * 3
the file descriptor is a stable reference to the directory,
even if the directory is renamed; and
.IP *
the open file descriptor prevents the underlying filesystem from
being dismounted,
just as when a process has a current working directory on a filesystem.
.PP
Second,
.BR openat ()
allows the implementation of a per-thread "current working
directory", via file descriptor(s) maintained by the application.
(This functionality can also be obtained by tricks based
on the use of
.IR /proc/self/fd/ dirfd,
but less efficiently.)
.\"
.\"
.SS O_DIRECT
.PP
The
.B O_DIRECT
flag may impose alignment restrictions on the length and address
of user-space buffers and the file offset of I/Os.
In Linux alignment
restrictions vary by filesystem and kernel version and might be
absent entirely.
However there is currently no filesystem\-independent
interface for an application to discover these restrictions for a given
file or filesystem.
Some filesystems provide their own interfaces
for doing so, for example the
.B XFS_IOC_DIOINFO
operation in
.BR xfsctl (3).
.PP
Under Linux 2.4, transfer sizes, and the alignment of the user buffer
and the file offset must all be multiples of the logical block size
of the filesystem.
Since Linux 2.6.0, alignment to the logical block size of the
underlying storage (typically 512 bytes) suffices.
The logical block size can be determined using the
.BR ioctl (2)
.B BLKSSZGET
operation or from the shell using the command:
.PP
.EX
    blockdev \-\-getss
.EE
.PP
.B O_DIRECT
I/Os should never be run concurrently with the
.BR fork (2)
system call,
if the memory buffer is a private mapping
(i.e., any mapping created with the
.BR mmap (2)
.BR MAP_PRIVATE
flag;
this includes memory allocated on the heap and statically allocated buffers).
Any such I/Os, whether submitted via an asynchronous I/O interface or from
another thread in the process,
should be completed before
.BR fork (2)
is called.
Failure to do so can result in data corruption and undefined behavior in
parent and child processes.
This restriction does not apply when the memory buffer for the
.B O_DIRECT
I/Os was created using
.BR shmat (2)
or
.BR mmap (2)
with the
.B MAP_SHARED
flag.
Nor does this restriction apply when the memory buffer has been advised as
.B MADV_DONTFORK
with
.BR madvise (2),
ensuring that it will not be available
to the child after
.BR fork (2).
.PP
The
.B O_DIRECT
flag was introduced in SGI IRIX, where it has alignment
restrictions similar to those of Linux 2.4.
IRIX has also a
.BR fcntl (2)
call to query appropriate alignments, and sizes.
FreeBSD 4.x introduced
a flag of the same name, but without alignment restrictions.
.PP
.B O_DIRECT
support was added under Linux in kernel version 2.4.10.
Older Linux kernels simply ignore this flag.
Some filesystems may not implement the flag and
.BR open ()
will fail with
.B EINVAL
if it is used.
.PP
Applications should avoid mixing
.B O_DIRECT
and normal I/O to the same file,
and especially to overlapping byte regions in the same file.
Even when the filesystem correctly handles the coherency issues in
this situation, overall I/O throughput is likely to be slower than
using either mode alone.
Likewise, applications should avoid mixing
.BR mmap (2)
of files with direct I/O to the same files.
.PP
The behavior of
.B O_DIRECT
with NFS will differ from local filesystems.
Older kernels, or
kernels configured in certain ways, may not support this combination.
The NFS protocol does not support passing the flag to the server, so
.B O_DIRECT
I/O will bypass the page cache only on the client; the server may
still cache the I/O.
The client asks the server to make the I/O
synchronous to preserve the synchronous semantics of
.BR O_DIRECT .
Some servers will perform poorly under these circumstances, especially
if the I/O size is small.
Some servers may also be configured to
lie to clients about the I/O having reached stable storage; this
will avoid the performance penalty at some risk to data integrity
in the event of server power failure.
The Linux NFS client places no alignment restrictions on
.B O_DIRECT
I/O.
.PP
In summary,
.B O_DIRECT
is a potentially powerful tool that should be used with caution.
It is recommended that applications treat use of
.B O_DIRECT
as a performance option which is disabled by default.
.PP
.RS
"The thing that has always disturbed me about O_DIRECT is that the whole
interface is just stupid, and was probably designed by a deranged monkey
on some serious mind-controlling substances."\(emLinus
.RE
.SH BUGS
Currently, it is not possible to enable signal-driven
I/O by specifying
.B O_ASYNC
when calling
.BR open ();
use
.BR fcntl (2)
to enable this flag.
.\" FIXME . Check bugzilla report on open(O_ASYNC)
.\" See http://bugzilla.kernel.org/show_bug.cgi?id=5993
.PP
One must check for two different error codes,
.B EISDIR
and
.BR ENOENT ,
when trying to determine whether the kernel supports
.B O_TMPFILE
functionality.
.PP
When both
.B O_CREAT
and
.B O_DIRECTORY
are specified in
.IR flags
and the file specified by
.I pathname
does not exist,
.BR open ()
will create a regular file (i.e.,
.B O_DIRECTORY
is ignored).
.SH SEE ALSO
.BR chmod (2),
.BR chown (2),
.BR close (2),
.BR dup (2),
.BR fcntl (2),
.BR link (2),
.BR lseek (2),
.BR mknod (2),
.BR mmap (2),
.BR mount (2),
.BR open_by_handle_at (2),
.BR read (2),
.BR socket (2),
.BR stat (2),
.BR umask (2),
.BR unlink (2),
.BR write (2),
.BR fopen (3),
.BR acl (5),
.BR fifo (7),
.BR inode (7),
.BR path_resolution (7),
.BR symlink (7)