.\" 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>

.\"

.\" %%%LICENSE_START(VERBATIM)

.\" Permission is granted to make and distribute verbatim copies of this

.\" manual provided the copyright notice and this permission notice are

.\" preserved on all copies.

.\"

.\" Permission is granted to copy and distribute modified versions of this

.\" 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.

.\"

.\" Since the Linux kernel and libraries are constantly changing, this

.\" manual page may be incorrect or out-of-date.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,

.\" which is licensed free of charge, as they might when working

.\" professionally.

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.\" Formatted or processed versions of this manual, if unaccompanied by

.\" the source, must acknowledge the copyright and authors of this work.

.\" %%%LICENSE_END

.\"

.\" 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

The

.BR open ()

system call opens the file specified by

.IR pathname .

If the specified file does not exist,

it may optionally (if

.B O_CREAT

is specified in

.IR flags )

be created by

.BR open ().

.PP

The return value of

.BR open ()

is a file descriptor, a small, nonnegative integer that is used

in subsequent system calls

.RB ( read "(2), " write "(2), " lseek "(2), " fcntl (2),

etc.) to refer to the open file.

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

.I pathname

does not exist, create it as a regular file.

.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 ()

fails with the error

.BR EEXIST .

.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.

.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)

(or similar)

returns, 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 regular 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