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

       open, openat, creat - open and possibly create a file

       #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);

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

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

	   Since glibc 2.10:
	       _POSIX_C_SOURCE >= 200809L
	   Before glibc 2.10:

       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 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.	 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	 unaf-
       fected  if  pathname  is subsequently removed or modified to refer to a
       different file.	For further details on	open  file  descriptions,  see

       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_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 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 fcntl(2) for details.

       The  full  list of file creation flags and file status flags is as fol-

	      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).	 The modification of the file  offset  and  the	 write
	      operation are performed as a single atomic step.

	      O_APPEND	may lead to corrupted files on NFS filesystems 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

	      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
	      available only  for  terminals,  pseudoterminals,	 sockets,  and
	      (since  Linux  2.6)  pipes  and FIFOs.  See fcntl(2) for further
	      details.	See also BUGS, below.

       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.

	      Note that the use of this	 flag  is  essential  in  some	multi-
	      threaded	programs,  because  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 and
	      attempts to set its close-on-exec flag  using  fcntl(2)  at  the
	      same  time  as  another  thread  does  a fork(2) plus execve(2).
	      Depending on the order of execution, the race may	 lead  to  the
	      file  descriptor returned by open() being unintentionally leaked
	      to the program executed by the child process created by 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 equiva-
	      lent of the O_CLOEXEC flag to deal with this problem.)

	      If the file does not exist, it will be created.

	      The owner (user ID) of the new file is set to the effective user
	      ID of the process.

	      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  filesys-
	      tems,  the behavior also depends on the bsdgroups and sysvgroups
	      mount options described in mount(8)).

	      The mode argument specifies the file mode bits be applied when a
	      new  file	 is  created.	This  argument	must  be supplied when
	      O_CREAT or O_TMPFILE is specified in flags; if  neither  O_CREAT
	      nor O_TMPFILE is specified, then mode is ignored.	 The effective
	      mode is modified by the process's umask in the usual way: in the
	      absence  of  a  default  ACL,  the  mode	of the created file is
	      (mode & ~umask).	Note that this mode  applies  only  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

	      According	 to  POSIX, the effect when other bits are set in mode
	      is unspecified.  On Linux, the following bits are	 also  honored
	      in mode:

	      S_ISUID  0004000 set-user-ID bit

	      S_ISGID  0002000 set-group-ID bit (see stat(2))

	      S_ISVTX  0001000 sticky bit (see stat(2))

       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_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  was	 added	in kernel version 2.1.126, to avoid denial-of-
	      service problems if opendir(3) is	 called	 on  a	FIFO  or  tape

	      Write  operations	 on  the  file	will complete according to the
	      requirements of synchronized I/O data integrity completion.

	      By the time 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  write(2)  was followed by a call to fdatasync(2)).
	      See NOTES below.

       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 filesystem (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 (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  can  be employed only if one of the following condi-
	      tions is true:

	      *	 The effective UID of the process matches the owner UID of the

	      *	 The calling process has the CAP_FOWNER capability in its user
		 namespace and the owner UID of the file has a mapping in  the

	      This  flag  is  intended for use by indexing or backup programs,
	      where its use can significantly reduce the amount of disk activ-
	      ity.   This  flag	 may not be effective on all filesystems.  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,  with  the
	      error  ELOOP.  Symbolic links in earlier components of the path-
	      name will still be followed.  (Note that the  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.)

	      This  flag  is  a FreeBSD extension, which was added to Linux in
	      version 2.1.126,	and  has  subsequently	been  standardized  in

	      See also O_PATH 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.

	      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 O_NONBLOCK is
	      set.  Since O_NONBLOCK  semantics	 might	eventually  be	imple-
	      mented,  applications  should  not depend upon blocking behavior
	      when specifying this flag for regular files and block devices.

	      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 filesystem tree and to perform opera-
	      tions 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), mmap(2)) fail with
	      the error 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 openat()
		 and  the other "*at()" system calls.  This includes linkat(2)
		 with AT_EMPTY_PATH (or via  procfs  using  AT_SYMLINK_FOLLOW)
		 even if the file is not a directory.

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

	      When  O_PATH  is	specified  in  flags,  flag  bits  other  than
	      O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW are ignored.

	      If  pathname  is a symbolic link and the 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
	      dirfd argument in calls to fchownat(2),  fstatat(2),  linkat(2),
	      and readlinkat(2) with an empty pathname to have the calls oper-
	      ate on the symbolic link.

       O_SYNC Write operations on the file  will  complete  according  to  the
	      requirements  of	synchronized I/O file integrity completion (by
	      contrast with the synchronized  I/O  data	 integrity  completion
	      provided by O_DSYNC.)

	      By  the  time write(2) (and similar) return, the output data and
	      associated file metadata have been transferred to the underlying
	      hardware	(i.e.,	as though each write(2) was followed by a call
	      to fsync(2)).  See NOTES below.

       O_TMPFILE (since Linux 3.11)
	      Create an unnamed temporary file.	 The pathname argument	speci-
	      fies  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.

	      O_TMPFILE must be specified with one of O_RDWR or O_WRONLY  and,
	      optionally,  O_EXCL.  If O_EXCL is not specified, then linkat(2)
	      can be used to link the temporary file into the filesystem, mak-
	      ing it permanent, using code like the following:

		  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",

	      In  this case, the open() mode argument determines the file per-
	      mission mode, as with O_CREAT.

	      Specifying O_EXCL in conjunction with O_TMPFILE prevents a  tem-
	      porary  file  from being linked into the filesystem in the above
	      manner.  (Note that the meaning of O_EXCL in this case  is  dif-
	      ferent from the meaning of O_EXCL otherwise.)

	      There are two main use cases for O_TMPFILE:

	      *	 Improved tmpfile(3) functionality: race-free creation of tem-
		 porary files that (1) are automatically deleted when  closed;
		 (2)  can  never be reached via any pathname; (3) are not sub-
		 ject to symlink attacks; and (4) do not require the caller to
		 devise unique names.

	      *	 Creating  a  file  that is initially invisible, which is then
		 populated with data and adjusted to have appropriate filesys-
		 tem  attributes  (fchown(2),  fchmod(2),  fsetxattr(2), etc.)
		 before being atomically linked into the filesystem in a fully
		 formed state (using linkat(2) as described above).

	      O_TMPFILE	 requires support by the underlying filesystem; only a
	      subset of Linux filesystems provide that support.	 In  the  ini-
	      tial  implementation,  support  was  provided in the ext2, ext3,
	      ext4, UDF, Minix, and  shmem  filesystems.   Support  for	 other
	      filesystems  has	subsequently been added as follows: XFS (Linux
	      3.15); Btrfs (Linux 3.16); F2FS (Linux 3.16); and	 ubifs	(Linux

	      If  the file already exists and is a regular file and the access
	      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.

       A  call	to creat() is equivalent to calling open() with flags equal to

       The openat() system call operates in exactly the same  way  as  open(),
       except for the differences described here.

       If  the	pathname given in pathname is relative, then it is interpreted
       relative to the directory referred to  by  the  file  descriptor	 dirfd
       (rather	than  relative to the current working directory of the calling
       process, as is done by open() for a relative pathname).

       If pathname is relative and dirfd is the special value  AT_FDCWD,  then
       pathname	 is  interpreted  relative to the current working directory of
       the calling process (like open()).

       If pathname is absolute, then dirfd is ignored.

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

       open(), openat(), and creat() can fail with the following 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-

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

       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).

       EINVAL The  filesystem  does  not support the O_DIRECT flag.  See NOTES
	      for more information.

       EINVAL Invalid value in flags.

       EINVAL O_TMPFILE was specified  in  flags,  but	neither	 O_WRONLY  nor
	      O_RDWR was specified.

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

       EISDIR pathname refers to an existing directory, O_TMPFILE and  one  of
	      O_WRONLY or O_RDWR were specified in flags, but this kernel ver-
	      sion does not provide the O_TMPFILE functionality.

       ELOOP  Too many symbolic links were encountered in resolving pathname.

       ELOOP  pathname was a symbolic link, and flags specified O_NOFOLLOW but
	      not O_PATH.

       EMFILE The per-process limit on the number of open file descriptors has
	      been reached (see the  description  of  RLIMIT_NOFILE  in	 getr-

	      pathname was too long.

       ENFILE The system-wide 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.

       ENOENT pathname refers to a nonexistent directory, O_TMPFILE and one of
	      O_WRONLY or O_RDWR were specified in flags, but this kernel ver-
	      sion does not provide the O_TMPFILE functionality.

       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 alloca-
	      tion for pipes has been reached and the  caller  is  not	privi-
	      leged; see pipe(7).

       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 FIFO open for reading.

       ENXIO  The  file	 is  a device special file and no corresponding device

	      The filesystem containing pathname does not support O_TMPFILE.

	      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  (1<<31)-1  bytes;  see also
	      O_LARGEFILE above.  This is the error specified by  POSIX.1;  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.

       EPERM  The operation was prevented by a file seal; see fcntl(2).

       EROFS  pathname	refers	to  a file on a read-only filesystem 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)).

       The following additional errors can occur for openat():

       EBADF  dirfd is not a valid file descriptor.

	      pathname is a relative pathname and dirfd is a  file  descriptor
	      referring to a file other than a directory.

       openat() was added to Linux in kernel 2.6.16; library support was added
       to glibc in version 2.4.

       open(), creat() SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.

       openat(): POSIX.1-2008.

       The O_DIRECT, O_NOATIME, O_PATH, and  O_TMPFILE	flags  are  Linux-spe-
       cific.  One must define _GNU_SOURCE to obtain their definitions.

       The  O_CLOEXEC,	O_DIRECTORY, and 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 _POSIX_C_SOURCE with a
       value greater than or equal to 200809L or _XOPEN_SOURCE	with  a	 value
       greater	than  or equal to 700.	In glibc 2.11 and earlier, one obtains
       the definitions by defining _GNU_SOURCE.

       As  noted  in  feature_test_macros(7),  feature	test  macros  such  as
       _POSIX_C_SOURCE,	 _XOPEN_SOURCE, and _GNU_SOURCE must be defined before
       including any header files.

       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).

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

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

       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 files in the /proc/[pid]/fd directory show the open	file  descrip-
       tors   of   the	 process   with	  the  PID  pid.   The	files  in  the
       /proc/[pid]/fdinfo directory show even  more  information  about	 these
       files  descriptors.   See  proc(5) for further details of both of these

   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--in kernel-developer parlance--
       a struct file.

       When a file descriptor is duplicated (using  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 pro-
       cesses: a child process created via fork(2) inherits duplicates of  its
       parent's	 file descriptors, and those duplicates refer to the same open
       file descriptions.

       Each open() of a file creates a new open file description; thus,	 there
       may be multiple open file descriptions corresponding to a file inode.

       On  Linux,  one can use the kcmp(2) KCMP_FILE operation to test whether
       two file descriptors (in the same process  or  in  two  different  pro-
       cesses) refer to the same open file description.

   Synchronized I/O
       The POSIX.1-2008 "synchronized I/O" option specifies different variants
       of synchronized I/O, and specifies the open()  flags  O_SYNC,  O_DSYNC,
       and  O_RSYNC  for  controlling  the behavior.  Regardless of whether an
       implementation supports this option, it must at least support  the  use
       of O_SYNC for regular files.

       Linux implements O_SYNC and O_DSYNC, but not O_RSYNC.  (Somewhat incor-
       rectly, glibc defines O_RSYNC to have the same value as O_SYNC.)

       O_SYNC provides synchronized I/O	 file  integrity  completion,  meaning
       write  operations  will	flush  data and all associated metadata to the
       underlying hardware.  O_DSYNC provides synchronized I/O 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.

       To understand the difference between the two types of completion,  con-
       sider two pieces of file metadata: the file last modification timestamp
       (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, O_DSYNC would only  guarantee  to	 flush
       updates	to  the file length metadata (whereas O_SYNC would also always
       flush the last modification timestamp metadata).

       Before Linux 2.6.33, Linux implemented only the O_SYNC flag for open().
       However,	 when  that flag was specified, most filesystems actually pro-
       vided the equivalent of	synchronized  I/O  data	 integrity  completion
       (i.e., O_SYNC was actually implemented as the equivalent of O_DSYNC).

       Since  Linux  2.6.33,  proper  O_SYNC support is provided.  However, to
       ensure backward binary compatibility, O_DSYNC was defined with the same
       value  as  the historical O_SYNC, and O_SYNC was defined as a new (two-
       bit) flag value that includes the O_DSYNC  flag	value.	 This  ensures
       that  applications  compiled  against  new headers get at least O_DSYNC
       semantics on pre-2.6.33 kernels.

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

       On  NFS	filesystems 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

       Opening	the  read or write end of a FIFO blocks until the other end is
       also opened (by another process or thread).  See	 fifo(7)  for  further

   File access mode
       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 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  ioctl(2)

   Rationale for openat() and other directory file descriptor APIs
       openat()	 and  the other system calls and library functions that take a
       directory file descriptor argument  (i.e.,  execveat(2),	 faccessat(2),
       fanotify_mark(2),  fchmodat(2),	fchownat(2), fstatat(2), futimesat(2),
       linkat(2), mkdirat(2), mknodat(2), name_to_handle_at(2), readlinkat(2),
       renameat(2),  symlinkat(2), unlinkat(2), utimensat(2), mkfifoat(3), and
       scandirat(3)) are supported for two reasons.  Here, the explanation  is
       in  terms  of the openat() call, but the rationale is analogous for the
       other interfaces.

       First, openat() allows an application to	 avoid	race  conditions  that
       could  occur  when using 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 open() could
       be changed in parallel with the call to open().	Suppose, for  example,
       that we wish to create the file path/to/xxx.dep if the file path/to/xxx
       exists.	The problem is that between the existence check and  the  file
       creation step, path or to (which might be symbolic links) could be mod-
       ified 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 dirfd argument of (say) fstatat(2) and ope-

       Second,	openat()  allows  the  implementation of a per-thread "current
       working directory", via file descriptor(s) maintained by	 the  applica-
       tion.   (This functionality can also be obtained by tricks based on the
       use of /proc/self/fd/dirfd, but less efficiently.)

       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 filesystem and kernel version and	 might
       be  absent entirely.  However there is currently no filesystem-indepen-
       dent interface for an application to discover these restrictions for  a
       given  file  or	filesystem.  Some filesystems 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 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 ioctl(2) BLKSSZGET operation or
       from the shell using the command:

	   blockdev --getss

       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 filesystems may not imple-
       ment 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 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  mmap(2)
       of files with direct I/O to the same files.

       The  behavior  of 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 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_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	  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.

       One must check for two different error codes, EISDIR and	 ENOENT,  when
       trying  to  determine whether the kernel supports O_TMPFILE functional-

       When both O_CREAT and O_DIRECTORY are specified in flags and  the  file
       specified by pathname does not exist, open() will create a regular file
       (i.e., O_DIRECTORY is ignored).

       chmod(2), chown(2),  close(2),  dup(2),	fcntl(2),  link(2),  lseek(2),
       mknod(2),  mmap(2), mount(2), open_by_handle_at(2), read(2), socket(2),
       stat(2), umask(2),  unlink(2),  write(2),  fopen(3),  acl(5),  fifo(7),
       path_resolution(7), symlink(7)

       This  page  is  part of release 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at

Linux				  2016-12-12			       OPEN(2)