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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:
	       _XOPEN_SOURCE >= 700 || _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.	 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,
       and  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; 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).	  O_APPEND may lead to corrupted files on NFS filesys‐
	      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
	      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  desriptor  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 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 filesystem 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 or O_TMPFILE
	      is specified in flags; if neither O_CREAT nor O_TMPFILE is spec‐
	      ified, then mode is ignored.  The effective permissions are mod‐
	      ified 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 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 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 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 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.  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_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.   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 ope‐
		 nat(2) and the other "*at()" system calls.

	      *	 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_DIREC‐
	      TORY 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 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  (chown(2),  chmod(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  ex2,	 ext3,
	      ext4,  UDF, Minix, and shmem filesystems.	 XFS support was added
	      in Linux 3.15.

	      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.

       creat()	 is   equivalent   to	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  relative  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 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.

       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 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.

	      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 (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 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 relative 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.

   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 the two types of completion,
       consider two pieces of file metadata: the file last modification	 time‐
       stamp (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

   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  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) opera‐

   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.,   faccessat(2),  fan‐
       otify_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().  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 openat().

       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.	Under Linux 2.6, alignment to 512-byte boundaries suf‐

       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  behaviour 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‐

       chmod(2), chown(2),  close(2),  dup(2),	fcntl(2),  link(2),  lseek(2),
       mknod(2),  mmap(2),  mount(2),  open_by_name_at(2), read(2), socket(2),
       stat(2), umask(2), unlink(2), write(2), fopen(3), fifo(7), path_resolu‐
       tion(7), symlink(7)

       This  page  is  part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux				  2014-04-20			       OPEN(2)

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