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       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 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,

       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:

              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.

              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.)

              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
              directory  (depending  on  filesystem  type  and mount options, and the mode of the
              parent directory; see the mount  options  bsdgroups  and  sysvgroups  described  in

              mode  specifies  the mode to use in case 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

              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.

              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

              (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
              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.  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.

              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

              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",

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

              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 (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  initial  implementation,  support  was
              provided  in  the ext2, 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 O_CREAT|O_WRONLY|O_TRUNC.

       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

       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 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

       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.


       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

       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)).

              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

       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 directory.

       ENXIO  O_NONBLOCK | O_WRONLY is set, the named file is a FIFO, and no process has the FIFO
              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 (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 (CAP_FOWNER).

       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-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.


       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.

   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

       Each  open(2)  of  a file creates a new open file description; thus, there may be multiple
       open file descriptions corresponding to a file inode.

   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

       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 requests.

       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 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) 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.)

       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

       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


       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).


       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)


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