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OPEN(2)			   Linux Programmer's Manual		       OPEN(2)

       open, creat - open and possibly create a	file or	device

       #include	<sys/types.h>
       #include	<sys/stat.h>
       #include	<fcntl.h>

       int open(const char *pathname, int flags);
       int open(const char *pathname, int flags, mode_t	mode);

       int creat(const char *pathname, mode_t mode);

       Given a pathname	for a file, open() returns a file descriptor, a	small,
       nonnegative integer  for	 use  in  subsequent  system  calls  (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  cur-
       rently open for the process.

       By default, the new file	descriptor is set to remain open across	an ex-
       ecve(2) (i.e., the FD_CLOEXEC file descriptor  flag  described  in  fc-
       ntl(2)  is initially disabled; the O_CLOEXEC flag, described below, can
       be used to change this default).	 The file offset is set	to the	begin-
       ning of the file	(see lseek(2)).

       A  call	to open() creates a new	open file description, an entry	in the
       system-wide table of open files.	 This entry records  the  file	offset
       and  the	 file status flags (modifiable via the fcntl(2)	F_SETFL	opera-
       tion).  A file descriptor is a reference	to one of these	entries;  this
       reference is unaffected if pathname is subsequently removed or modified
       to refer	to a different file.  The new open file	 description  is  ini-
       tially  not  shared  with  any other process, but sharing may arise via

       The argument flags must include one  of	the  following	access	modes:
       O_RDONLY,  O_WRONLY,  or	 O_RDWR.  These	request	opening	the file read-
       only, write-only, or read/write,	respectively.

       In addition, zero or more file creation flags and file status flags can
       be  bitwise-or'd	 in  flags.   The  file	 creation flags	are O_CLOEXEC,
       O_TTY_INIT.   The  file	status	flags  are  all	of the remaining flags
       listed below.  The distinction between these two	 groups	 of  flags  is
       that  the  file status flags can	be retrieved and (in some cases) modi-
       fied using fcntl(2).  The full list of file  creation  flags  and  file
       status flags is as follows:

	      The  file	 is  opened in append mode.  Before each write(2), the
	      file offset is positioned	at the end of the  file,  as  if  with
	      lseek(2).	 O_APPEND may lead to corrupted	files on NFS file sys-
	      tems if more than	one process appends data to a  file  at	 once.
	      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.

	      Enable  signal-driven  I/O: generate a signal (SIGIO by default,
	      but this can be changed via fcntl(2)) when input or  output  be-
	      comes  possible on this file descriptor.	This feature is	avail-
	      able only	for terminals, pseudoterminals,	 sockets,  and	(since
	      Linux 2.6) pipes and FIFOs.  See fcntl(2)	for further details.

       O_CLOEXEC (Since	Linux 2.6.23)
	      Enable  the  close-on-exec  flag	for  the  new file descriptor.
	      Specifying this flag permits a program to	avoid  additional  fc-
	      ntl(2) F_SETFD operations	to set the FD_CLOEXEC flag.  Addition-
	      ally, use	of this	flag is	essential in some  multithreaded  pro-
	      grams  since  using a separate fcntl(2) F_SETFD operation	to set
	      the FD_CLOEXEC flag does not suffice to  avoid  race  conditions
	      where one	thread opens a file descriptor at the same time	as an-
	      other thread does	a fork(2) plus execve(2).

	      If the file does not exist it will be created.  The owner	 (user
	      ID)  of the file is set to the effective user ID of the process.
	      The group	ownership (group ID) is	set either  to	the  effective
	      group  ID	of the process or to the group ID of the parent	direc-
	      tory (depending on file system type and mount options,  and  the
	      mode  of	the  parent directory, see the mount options bsdgroups
	      and sysvgroups described in mount(8)).

	      mode specifies the permissions to	use in case a new file is cre-
	      ated.   This argument must be supplied when O_CREAT is specified
	      in flags;	if O_CREAT is not specified,  then  mode  is  ignored.
	      The effective permissions	are modified by	the process's umask in
	      the  usual  way:	The  permissions  of  the  created  file   are
	      (mode _ ~umask).	Note that this mode applies only to future ac-
	      cesses of	the newly created file;	the open() call	that creates a
	      read-only	file may well return a read/write file descriptor.

	      The following symbolic constants are provided for	mode:

	      S_IRWXU  00700  user  (file  owner)  has read, write and execute

	      S_IRUSR  00400 user has read permission

	      S_IWUSR  00200 user has write permission

	      S_IXUSR  00100 user has execute permission

	      S_IRWXG  00070 group has read, write and execute permission

	      S_IRGRP  00040 group has read permission

	      S_IWGRP  00020 group has write permission

	      S_IXGRP  00010 group has execute permission

	      S_IRWXO  00007 others have read, write and execute permission

	      S_IROTH  00004 others have read permission

	      S_IWOTH  00002 others have write permission

	      S_IXOTH  00001 others have execute permission

       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 O_DIRECT flag	on its own makes an effort  to	transfer  data
	      synchronously,  but  does	 not give the guarantees of the	O_SYNC
	      flag that	data and necessary metadata are	transferred.  To guar-
	      antee  synchronous I/O, O_SYNC must be used in addition to O_DI-
	      RECT.  See NOTES below for further discussion.

	      A	semantically similar (but deprecated) interface	for block  de-
	      vices is described in raw(8).

	      If  pathname  is	not a directory, cause the open	to fail.  This
	      flag is Linux-specific, and was added in kernel version 2.1.126,
	      to avoid denial-of-service problems if opendir(3)	is called on a
	      FIFO or tape device.

       O_EXCL Ensure that this call creates the	file: if this flag  is	speci-
	      fied  in	conjunction with O_CREAT, and pathname already exists,
	      then open() will fail.

	      When these two flags are specified, symbolic links are not  fol-
	      lowed: if	pathname is a symbolic link, then open() fails regard-
	      less of where the	symbolic link points to.

	      In general, the behavior of O_EXCL is undefined if  it  is  used
	      without  O_CREAT.	  There	 is  one  exception:  on Linux 2.6 and
	      later, O_EXCL can	be used	without	O_CREAT	if pathname refers  to
	      a	 block	device.	  If  the block	device is in use by the	system
	      (e.g., mounted), open() fails with the error EBUSY.

	      On NFS, O_EXCL is	supported only when using NFSv3	 or  later  on
	      kernel  2.6  or later.  In NFS environments where	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 O_EXCL, can create a	unique
	      file on the same file system (e.g., incorporating	 hostname  and
	      PID),  and  use  link(2)	to  make  a  link to the lockfile.  If
	      link(2) returns 0,  the  lock  is	 successful.   Otherwise,  use
	      stat(2)  on  the	unique file to check if	its link count has in-
	      creased to 2, in which case the lock is also successful.

	      (LFS) Allow files	whose sizes cannot be represented in an	 off_t
	      (but  can	 be  represented  in  an  off64_t)  to be opened.  The
	      _LARGEFILE64_SOURCE macro	must be	defined	(before	including  any
	      header  files)  in order to obtain this definition.  Setting the
	      _FILE_OFFSET_BITS	feature	test macro to 64  (rather  than	 using
	      O_LARGEFILE) is the preferred method of accessing	large files on
	      32-bit systems (see feature_test_macros(7)).

       O_NOATIME (Since	Linux 2.6.8)
	      Do not update the	file last access time (st_atime	in the	inode)
	      when  the	file is	read(2).  This flag is intended	for use	by in-
	      dexing or	backup programs, where its use can  significantly  re-
	      duce  the	 amount	of disk	activity.  This	flag may not be	effec-
	      tive on all file systems.	 One example is	NFS, where the	server
	      maintains	the access time.

	      If pathname refers to a terminal device--see tty(4)--it will not
	      become the process's controlling terminal	even  if  the  process
	      does not have one.

	      If  pathname is a	symbolic link, then the	open fails.  This is a
	      FreeBSD extension, which was added to Linux in version  2.1.126.
	      Symbolic	links in earlier components of the pathname will still
	      be followed.  See	also O_NOPATH below.

	      When possible, the file is opened	in nonblocking mode.   Neither
	      the  open() nor any subsequent operations	on the file descriptor
	      which is returned	will cause the calling process to  wait.   For
	      the  handling  of	 FIFOs (named pipes), see also fifo(7).	 For a
	      discussion of the	 effect	 of  O_NONBLOCK	 in  conjunction  with
	      mandatory	file locks and with file leases, see fcntl(2).

       O_PATH (since Linux 2.6.39)
	      Obtain  a	 file descriptor that can be used for two purposes: to
	      indicate a location in the file-system tree and to perform oper-
	      ations  that  act	purely at the file descriptor level.  The file
	      itself is	not opened, and	other file operations (e.g.,  read(2),
	      write(2),	 fchmod(2), fchown(2), fgetxattr(2)) fail with the er-
	      ror EBADF.

	      The following operations can be performed	on the resulting  file

	      *	 close(2);  fchdir(2) (since Linux 3.5); fstat(2) (since Linux

	      *	 Duplicating the file descriptor  (dup(2),  fcntl(2)  F_DUPFD,

	      *	 Getting  and  setting file descriptor flags (fcntl(2) F_GETFD
		 and F_SETFD).

	      *	 Retrieving open file status flags using the fcntl(2)  F_GETFL
		 operation: the	returned flags will include the	bit O_PATH.

	      *	 Passing  the  file  descriptor	 as the	dirfd argument of ope-
		 nat(2)	and the	other "*at()" system calls.

	      *	 Passing the file descriptor to	another	process	via a UNIX do-
		 main socket (see SCM_RIGHTS in	unix(7)).

	      When O_PATH is specified in flags, flag bits other than O_DIREC-
	      TORY and O_NOFOLLOW are ignored.

	      If the O_NOFOLLOW	flag is	also specified,	then the call  returns
	      a	file descriptor	referring to the symbolic link.	 This file de-
	      scriptor can be used as the dirfd	argument in  calls  to	fchow-
	      nat(2),  fstatat(2),  linkat(2), and readlinkat(2) with an empty
	      pathname to have the calls operate on the	symbolic link.

       O_SYNC The file is opened for synchronous I/O.  Any  write(2)s  on  the
	      resulting	 file  descriptor will block the calling process until
	      the data has been	physically written to the underlying hardware.
	      But see NOTES below.

	      If  the  file  already exists and	is a regular file and the open
	      mode allows writing (i.e., is O_RDWR or  O_WRONLY)  it  will  be
	      truncated	to length 0.  If the file is a FIFO or terminal	device
	      file, the	O_TRUNC	flag is	 ignored.   Otherwise  the  effect  of
	      O_TRUNC is unspecified.

       Some  of	 these	optional flags can be altered using fcntl(2) after the
       file has	been opened.

       creat()	 is   equivalent   to	open()	  with	  flags	   equal    to

       open()  and  creat()  return the	new file descriptor, or	-1 if an error
       occurred	(in which case,	errno is set appropriately).

       EACCES The requested access to the file is not allowed, or search  per-
	      mission  is denied for one of the	directories in the path	prefix
	      of pathname, or the file did not exist yet and write  access  to
	      the  parent  directory  is  not allowed.	(See also path_resolu-

       EDQUOT Where O_CREAT is specified, the file does	 not  exist,  and  the
	      user's  quota  of	 disk  blocks or inodes	on the file system has
	      been exhausted.

       EEXIST pathname already exists and O_CREAT and O_EXCL were used.

       EFAULT pathname points outside your accessible address space.


       EINTR  While blocked waiting to complete	 an  open  of  a  slow	device
	      (e.g.,  a	FIFO; see fifo(7)), the	call was interrupted by	a sig-
	      nal handler; see signal(7).

       EISDIR pathname refers to a directory and the access requested involved
	      writing (that is,	O_WRONLY or O_RDWR is set).

       ELOOP  Too  many	symbolic links were encountered	in resolving pathname,
	      or O_NOFOLLOW was	specified but pathname was a symbolic link.

       EMFILE The process already has the maximum number of files open.

	      pathname was too long.

       ENFILE The system limit on the total number  of	open  files  has  been

       ENODEV pathname	refers	to  a device special file and no corresponding
	      device exists.  (This is a Linux kernel bug; in  this  situation
	      ENXIO must be returned.)

       ENOENT O_CREAT is not set and the named file does not exist.  Or, a di-
	      rectory component	in pathname does not exist or  is  a  dangling
	      symbolic link.

       ENOMEM Insufficient kernel memory was available.

       ENOSPC pathname	was  to	 be created but	the device containing pathname
	      has no room for the new file.

	      A	component used as a directory in pathname is not, in  fact,  a
	      directory,  or  O_DIRECTORY was specified	and pathname was not a

       ENXIO  O_NONBLOCK | O_WRONLY is set, the	named file is a	 FIFO  and  no
	      process has the file open	for reading.  Or, the file is a	device
	      special file and no corresponding	device exists.

	      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  -D_FILE_OFFSET_BITS=64  tried  to
	      open a file whose	size exceeds (2__31)-1 bits; see also O_LARGE-
	      FILE above.  This	is the error  specified	 by  POSIX.1-2001;  in
	      kernels before 2.6.24, Linux gave	the error EFBIG	for this case.

       EPERM  The  O_NOATIME  flag was specified, but the effective user ID of
	      the caller did not match the owner of the	file  and  the	caller
	      was not privileged (CAP_FOWNER).

       EROFS  pathname	refers	to a file on a read-only file system and write
	      access was requested.

	      pathname refers to an executable image which is currently	 being
	      executed and write access	was requested.

	      The O_NONBLOCK flag was specified, and an	incompatible lease was
	      held on the file (see fcntl(2)).

       SVr4, 4.3BSD, POSIX.1-2001.  The	 O_DIRECTORY,  O_NOATIME,  O_NOFOLLOW,
       and  O_PATH  flags  are	Linux-specific,	 and  one  may	need to	define
       _GNU_SOURCE (before including any header	files) to obtain their defini-

       The  O_CLOEXEC  flag is not specified in	POSIX.1-2001, but is specified
       in POSIX.1-2008.

       O_DIRECT	is not specified in POSIX; one has to define _GNU_SOURCE  (be-
       fore including any header files)	to get its definition.

       Under  Linux,  the O_NONBLOCK flag indicates that one wants to open but
       does not	necessarily have the intention to read or write.  This is typ-
       ically  used  to	open devices in	order to get a file descriptor for use
       with ioctl(2).

       Unlike the other	values that can	be specified in	flags, the access mode
       values  O_RDONLY, O_WRONLY, and O_RDWR, do not specify individual bits.
       Rather, they define the low order two bits of flags,  and  are  defined
       respectively  as	0, 1, and 2.  In other words, the combination O_RDONLY
       | O_WRONLY is a logical error, and certainly does  not  have  the  same
       meaning as O_RDWR.  Linux reserves the special, nonstandard access mode
       3 (binary 11) in	flags to mean: check for read and write	permission  on
       the  file  and  return  a  descriptor that can't	be used	for reading or
       writing.	 This nonstandard access mode is used by some Linux drivers to
       return  a  descriptor  that  is	to  be	used  only for device-specific
       ioctl(2)	operations.

       The (undefined) effect of O_RDONLY | O_TRUNC varies  among  implementa-
       tions.  On many systems the file	is actually truncated.

       There  are  many	infelicities in	the protocol underlying	NFS, affecting
       amongst others O_SYNC and O_NDELAY.

       POSIX provides for three	different variants of synchronized I/O,	corre-
       sponding	  to  the  flags  O_SYNC,  O_DSYNC,  and  O_RSYNC.   Currently
       (2.6.31), Linux implements only O_SYNC,	but  glibc  maps  O_DSYNC  and
       O_RSYNC to the same numerical value as O_SYNC.  Most Linux file systems
       don't actually implement	the POSIX O_SYNC semantics, which require  all
       metadata	 updates  of a write to	be on disk on returning	to user	space,
       but only	the O_DSYNC semantics, which require only actual file data and
       metadata	 necessary to retrieve it to be	on disk	by the time the	system
       call returns.

       Note that open()	can open device	special	files, but creat() cannot cre-
       ate them; use mknod(2) instead.

       On  NFS file systems with UID mapping enabled, open() may return	a file
       descriptor but, for example, read(2) requests are denied	 with  EACCES.
       This is because the client performs open() by checking the permissions,
       but UID mapping is performed by the server  upon	 read  and  write  re-

       If  the	file is	newly created, its st_atime, st_ctime, st_mtime	fields
       (respectively, time of last access, time	of  last  status  change,  and
       time  of	 last  modification; see stat(2)) are set to the current time,
       and so are the st_ctime and st_mtime fields of  the  parent  directory.
       Otherwise,  if  the  file  is modified because of the O_TRUNC flag, its
       st_ctime	and st_mtime fields are	set to the current time.

       The 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 file system and kernel version and might
       be absent entirely.  However there is currently no file system-indepen-
       dent interface for an application to discover these restrictions	for  a
       given  file or file system.  Some file systems provide their own	inter-
       faces for doing so, for example the XFS_IOC_DIOINFO  operation  in  xf-

       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  file  system.   Under  Linux 2.6, alignment	to 512-byte boundaries

       O_DIRECT	I/Os should never be run concurrently with the fork(2)	system
       call, if	the memory buffer is a private mapping (i.e., any mapping cre-
       ated with the mmap(2) 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	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 O_DIRECT I/Os was	created	using shmat(2) or mmap(2) with
       the  MAP_SHARED	flag.  Nor does	this restriction apply when the	memory
       buffer has been advised as MADV_DONTFORK	with madvise(2), ensuring that
       it will not be available	to the child after fork(2).

       The  O_DIRECT  flag  was	introduced in SGI IRIX,	where it has alignment
       restrictions similar to those of	Linux 2.4.  IRIX has also  a  fcntl(2)
       call  to	 query	appropriate alignments,	and sizes.  FreeBSD 4.x	intro-
       duced a flag of the same	name, but without alignment restrictions.

       O_DIRECT	support	was added under	Linux in kernel	version	2.4.10.	 Older
       Linux  kernels  simply ignore this flag.	 Some file systems may not im-
       plement the flag	and open() will	fail with EINVAL if it is used.

       Applications should avoid mixing	O_DIRECT and normal I/O	 to  the  same
       file,  and  especially  to  overlapping	byte regions in	the same file.
       Even when the file system correctly handles  the	 coherency  issues  in
       this  situation,	overall	I/O throughput is likely to be slower than us-
       ing either mode alone.	Likewise,  applications	 should	 avoid	mixing
       mmap(2) of files	with direct I/O	to the same files.

       The behaviour of	O_DIRECT with NFS will differ from local file systems.
       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 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
       O_DIRECT.   Some	servers	will perform poorly under these	circumstances,
       especially if the I/O size is small.  Some servers may also be  config-
       ured  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 O_DIRECT I/O.

       In summary, O_DIRECT is a potentially powerful tool that	should be used
       with  caution.	It is recommended that applications treat use of O_DI-
       RECT as a performance option which is disabled by default.

	      "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	  sub-

       Currently, it is	not possible to	enable signal-driven I/O by specifying
       O_ASYNC when calling open(); use	fcntl(2) to enable this	flag.

       chmod(2), chown(2),  close(2),  dup(2),	fcntl(2),  link(2),  lseek(2),
       mknod(2),  mmap(2),  mount(2),  openat(2), read(2), socket(2), stat(2),
       umask(2), unlink(2), write(2), fopen(3),	 fifo(7),  path_resolution(7),

       This  page  is  part of release 3.53 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found	at

Linux				  2013-07-21			       OPEN(2)


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