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

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

SYNOPSIS
       #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);

DESCRIPTION
       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
       fork(2).

       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:

       O_APPEND
	      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.

       O_ASYNC
	      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).

       O_CREAT
	      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
		       permission

	      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).

       O_DIRECTORY
	      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.

       O_LARGEFILE
	      (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	 obtaining  method  of
	      accessing	   large   files   on	32-bit	 systems   (see	  fea‐
	      ture_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.

       O_NOCTTY
	      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.

       O_NOFOLLOW
	      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.

       O_NONBLOCK or O_NDELAY
	      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.

       O_TRUNC
	      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
       O_CREAT|O_WRONLY|O_TRUNC.

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

ERRORS
       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‐
	      tion(7).)

       EEXIST pathname already exists and O_CREAT and O_EXCL were used.

       EFAULT pathname points outside your accessible address space.

       EFBIG  See EOVERFLOW.

       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.

       ENAMETOOLONG
	      pathname was too long.

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

       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.

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

       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.

       EOVERFLOW
	      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.

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

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

CONFORMING TO
       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
       (before including any header files) 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
       (before including any header files) to get its definition.

NOTES
       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
       requests.

       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.

   O_DIRECT
       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
       xfsctl(3).

       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
       suffices.

       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

BUGS
       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.

SEE ALSO
       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)

COLOPHON
       This page is part of release 3.27 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2010-09-10			       OPEN(2)
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