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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
       execve(2) (i.e.,	the  FD_CLOEXEC	 file  descriptor  flag	 described  in
       fcntl(2)	 is  initially	disabled;  the	Linux-specific O_CLOEXEC flag,
       described below,	can be used to change this default).  The file	offset
       is set to the beginning 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_CREAT, O_EXCL,
       O_NOCTTY, and O_TRUNC.  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) mod-
       ified  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
	      becomes possible on this file descriptor.	 This feature is  only
	      available	 for  terminals, pseudo-terminals, 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
	      fcntl(2) F_SETFD operations to set the FD_CLOEXEC	 flag.	 Addi-
	      tionally,	 use  of  this flag is essential in some multithreaded
	      programs since using a separate fcntl(2)	F_SETFD	 operation  to
	      set  the	FD_CLOEXEC  flag does not suffice to avoid race	condi-
	      tions where one thread opens a file descriptor at	the same  time
	      as another 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  only  applies  to	future
	      accesses 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 at an effort to transfer data
	      synchronously, but does not give the guarantees  of  the	O_SYNC
	      that  data and necessary metadata	are transferred.  To guarantee
	      synchronous I/O the O_SYNC must be used in addition to O_DIRECT.
	      See NOTES	below for further discussion.

	      A	 semantically  similar	(but  deprecated)  interface for block
	      devices 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,  but  should  not	be used	outside	of the
	      implementation of	opendir(3).

       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.  The behavior of O_EXCL is	 undefined  if
	      O_CREAT is not specified.

	      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.

	      On  NFS,	O_EXCL	is only	supported 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
	      increased	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 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	obtaining 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
	      indexing	or  backup  programs,  where its use can significantly
	      reduce 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.

	      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_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-

       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
	      directory	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,	and O_NOFOLLOW
       flags are Linux-specific, and one may need  to  define  _GNU_SOURCE  to
       obtain their definitions.

       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  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	only  to  be  used 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 only implements 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 userspace,
       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

       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  userspace  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

       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

       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
       implement 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
       using  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 only bypass the page cache	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_DIRECT	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 substances."
	      -- Linus

       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),  feature_test_macros(7),
       fifo(7),	path_resolution(7), symlink(7)

       This page is part of release 3.25 of the	Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found	at

Linux				  2010-06-14			       OPEN(2)


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