openat(2)


NAME

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

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);

   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)):

   openat():
       Since glibc 2.10:
           _POSIX_C_SOURCE >= 200809L
       Before glibc 2.10:
           _ATFILE_SOURCE

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
   currently 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
   unaffected  if pathname is subsequently removed or modified to refer to
   a different file.  For further details on open file  descriptions,  see
   NOTES.

   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_CREAT,  O_DIRECTORY,  O_EXCL,  O_NOCTTY,  O_NOFOLLOW,  O_TMPFILE, 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)  modified;  see
   fcntl(2) for details.

   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
          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 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
          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
          multithreaded  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
          equivalent of the O_CLOEXEC flag to deal with this problem.)

   O_CREAT
          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
          filesystems, 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
                   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

          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
          guarantee  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).

   O_DIRECTORY
          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
          device.

   O_DSYNC
          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
          specified in conjunction  with  O_CREAT,  and  pathname  already
          exists, then open() will fail.

          When  these  two  flags  are  specified,  symbolic links are not
          followed: if pathname is a  symbolic  link,  then  open()  fails
          regardless 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.

   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 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
          conditions is true:

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

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

          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 effective  on  all  filesystems.
          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.  See also O_PATH below.

   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.

          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
          implemented,  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
          operations 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
          descriptor:

          *  close(2); fchdir(2) (since Linux 3.5); fstat(2) (since  Linux
             3.6).

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

          *  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(2)  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
          operate 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
          specifies  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,
          making 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",
                                      AT_SYMLINK_FOLLOW);

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

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

          There are two main use cases for O_TMPFILE:

          *  Improved  tmpfile(3)  functionality:  race-free  creation  of
             temporary files  that  (1)  are  automatically  deleted  when
             closed;  (2)  can  never be reached via any pathname; (3) are
             not subject 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
             filesystem  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
          initial  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
          4.9)

   O_TRUNC
          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()
   A  call  to creat() is equivalent to calling open() with flags equal to
   O_CREAT|O_WRONLY|O_TRUNC.

   openat()
   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.

RETURN VALUE

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

ERRORS

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

   EACCES The  requested  access  to  the  file  is not allowed, or search
          permission 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_resolution(7).)

   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
          exhausted.

   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
          signal 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
          version 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
          getrlimit(2)).

   ENAMETOOLONG
          pathname was too long.

   ENFILE The system-wide 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.

   ENOENT pathname refers to a nonexistent directory, O_TMPFILE and one of
          O_WRONLY  or  O_RDWR  were  specified  in flags, but this kernel
          version 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
          allocation for pipes has been reached  and  the  caller  is  not
          privileged; 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.

   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 FIFO open for reading.

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

   EOPNOTSUPP
          The filesystem containing pathname does not support O_TMPFILE.

   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  (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.

   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)).

   The following additional errors can occur for openat():

   EBADF  dirfd is not a valid file descriptor.

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

VERSIONS

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

CONFORMING TO

   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-
   specific.  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.

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
   typically 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
   implementations.  On many systems the file is actually truncated.

   Note that open() can open device  special  files,  but  creat()  cannot
   create 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
   descriptors  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
   directories.

   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
   processes: 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
   processes) 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
   incorrectly, 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,
   consider  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
   provided 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.

   NFS
   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
   requests.

   FIFOs
   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
   details.

   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)
   operations.

   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
   modified 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 openat().

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

   O_DIRECT
   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-independent interface for an application to  discover  these
   restrictions  for a given file or filesystem.  Some filesystems provide
   their own interfaces 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 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
   created  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
   introduced 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
   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 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
   configured 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.

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

   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).

SEE ALSO

   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)

COLOPHON

   This page is part of release 4.09 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
   https://www.kernel.org/doc/man-pages/.


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