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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);

       /* Documented separately, in openat2(2): */
       int openat2(int dirfd, const char *pathname,
                   const struct open_how *how, size_t size);

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

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


       The  open()  system call opens the file specified by pathname.  If the specified file does
       not exist, it may optionally (if O_CREAT is specified in flags) be created by open().

       The return value of open() is a file descriptor, a small, nonnegative integer that is used
       in  subsequent  system calls (read(2), write(2), lseek(2), fcntl(2), etc.) to refer to the
       open file.  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 creation
       flags affect the semantics of the open operation  itself,  while  the  file  status  flags
       affect the semantics of subsequent I/O operations.  The file status flags can be retrieved
       and (in some cases) modified; see fcntl(2) for details.

       The full list of file creation flags and file status flags is as 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).  The modification of the
              file offset and the write operation are performed as a single atomic step.

              O_APPEND may lead to corrupted files on NFS filesystems if more  than  one  process
              appends  data to a file at once.  This is because NFS does not support appending to
              a file, so the client kernel has to simulate it, which can't be done without a race

              Enable  signal-driven  I/O:  generate  a  signal (SIGIO by default, but this can be
              changed  via  fcntl(2))  when  input  or  output  becomes  possible  on  this  file
              descriptor.   This  feature  is  available  only  for  terminals,  pseudoterminals,
              sockets, and (since Linux 2.6) pipes and FIFOs.  See fcntl(2) for further  details.
              See also BUGS, below.

       O_CLOEXEC (since Linux 2.6.23)
              Enable  the  close-on-exec  flag for the new file descriptor.  Specifying this flag
              permits a program to avoid  additional  fcntl(2)  F_SETFD  operations  to  set  the
              FD_CLOEXEC flag.

              Note that the use of this flag is essential in some 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 pathname does not exist, create it as a regular file.

              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 to be applied when a new file is
              created.  If neither O_CREAT nor O_TMPFILE is specified  in  flags,  then  mode  is
              ignored  (and  can  thus  be specified as 0, or simply omitted).  The mode argument
              must be supplied if O_CREAT or O_TMPFILE is  specified  in  flags;  if  it  is  not
              supplied, some arbitrary bytes from the stack will be applied as the file mode.

              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 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

              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 inode(7)).

              S_ISVTX  0001000 sticky bit (see inode(7)).

       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() fails with the error EEXIST.

              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.

              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

              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

              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 the trailing component (i.e., basename) of pathname is a symbolic link, then the
              open  fails,  with  the  error  ELOOP.  Symbolic links in earlier components of the
              pathname will still be followed.  (Note that the ELOOP error that can occur in this
              case  is  indistinguishable from the case where an open fails because there are too
              many symbolic links found while resolving components in  the  prefix  part  of  the

              This  flag is a FreeBSD extension, which was added to Linux in version 2.1.126, and
              has subsequently been standardized in POSIX.1-2008.

              See also O_PATH below.

              When possible, the file is opened in nonblocking mode.  Neither the open() nor  any
              subsequent  I/O  operations on the file descriptor which is returned will cause the
              calling process to wait.

              Note that the setting of this flag has no  effect  on  the  operation  of  poll(2),
              select(2),  epoll(7),  and similar, since those interfaces merely inform the caller
              about whether a file descriptor is "ready", meaning that an I/O operation performed
              on the file descriptor with the O_NONBLOCK flag clear would not block.

              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), ioctl(2), mmap(2)) fail with
              the error EBADF.

              The following operations can be performed on the resulting file descriptor:

              *  close(2).

              *  fchdir(2), if the file descriptor refers to a directory (since Linux 3.5).

              *  fstat(2) (since Linux 3.6).

              *  fstatfs(2) (since Linux 3.12).

              *  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()  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.

              Opening a file or directory with the O_PATH flag requires  no  permissions  on  the
              object  itself  (but does require execute permission on the directories in the path
              prefix).  Depending  on  the  subsequent  operation,  a  check  for  suitable  file
              permissions  may  be  performed (e.g., fchdir(2) requires execute permission on the
              directory referred to by its file descriptor argument).  By contrast,  obtaining  a
              reference to a filesystem object by opening it with the O_RDONLY flag requires that
              the caller have read permission on the object, even when the  subsequent  operation
              (e.g., fchdir(2), fstat(2)) does not require read permission on the object.

              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.

              If  pathname  refers  to  an automount point that has not yet been triggered, so no
              other filesystem is mounted  on  it,  then  the  call  returns  a  file  descriptor
              referring  to  the  automount directory without triggering a mount.  fstatfs(2) can
              then be used to determine if  it  is,  in  fact,  an  untriggered  automount  point
              (.f_type == AUTOFS_SUPER_MAGIC).

              One  use  of  O_PATH  for  regular  files is to provide the equivalent of POSIX.1's
              O_EXEC functionality.  This permits us to open a file for  which  we  have  execute
              permission  but  not  read  permission,  and  then  execute  that  file, with steps
              something like the following:

                  char buf[PATH_MAX];
                  fd = open("some_prog", O_PATH);
                  snprintf(buf, PATH_MAX, "/proc/self/fd/%d", fd);
                  execl(buf, "some_prog", (char *) NULL);

              An O_PATH file descriptor can also be passed as the argument of fexecve(3).

       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) (or similar) returns, 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  regular  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'... */

                  linkat(fd, NULL, AT_FDCWD, "/path/for/file", AT_EMPTY_PATH);

                  /* If the caller doesn't have the CAP_DAC_READ_SEARCH
                     capability (needed to use AT_EMPTY_PATH with linkat(2)),
                     and there is a proc(5) filesystem mounted, then the
                     linkat(2) call above can be replaced with:

                  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  (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)

              If the file already exists and is a regular file and the access mode allows writing
              (i.e.,  is  O_RDWR or O_WRONLY) it will be truncated to length 0.  If the file is a
              FIFO or terminal device file, the O_TRUNC flag is ignored.  Otherwise,  the  effect
              of O_TRUNC is unspecified.

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

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

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

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

       If pathname is absolute, then dirfd is ignored.

       The  openat2(2)  system  call  is an extension of openat(), and provides a superset of the
       features of openat().  It is documented separately, in openat2(2).


       open(), openat(), and creat() return the new file descriptor (a nonnegative  integer),  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

       EACCES Where O_CREAT is specified, the  protected_fifos  or  protected_regular  sysctl  is
              enabled,  the  file  already exists and is a FIFO or regular file, the owner of the
              file is neither the current user nor the owner of the containing directory, and the
              containing directory is both world- or group-writable and sticky.  For details, see
              the descriptions of /proc/sys/fs/protected_fifos and /proc/sys/fs/protected_regular
              in proc(5).

       EBUSY  O_EXCL  was specified in flags and pathname refers to a block device that is in use
              by the system (e.g., it is mounted).

       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.

       EINVAL O_CREAT was specified in flags and the final  component  ("basename")  of  the  new
              file's  pathname  is  invalid  (e.g.,  it  contains characters not permitted by the
              underlying filesystem).

       EINVAL The final  component  ("basename")  of  pathname  is  invalid  (e.g.,  it  contains
              characters not permitted by the underlying filesystem).

       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.

       ENOENT 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 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.

              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.

       ENXIO  The file is a UNIX domain socket.

              The filesystem containing pathname does not support O_TMPFILE.

              pathname  refers  to  a  regular  file  that  is too large to be opened.  The usual
              scenario here is  that  an  application  compiled  on  a  32-bit  platform  without
              -D_FILE_OFFSET_BITS=64 tried to open a file whose size exceeds (1<<31)-1 bytes; see
              also O_LARGEFILE above.  This is the error specified by POSIX.1; in kernels  before
              2.6.24, Linux gave the error EFBIG for this case.

       EPERM  The  O_NOATIME  flag was specified, but the effective user ID of the caller did not
              match the owner of the file and the caller was not privileged.

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

       EROFS  pathname refers to a file on a read-only filesystem and write access was requested.

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

              pathname  refers to a file that is currently in use as a swap file, and the O_TRUNC
              flag was specified.

              pathname refers to a file that is currently being read by  the  kernel  (e.g.,  for
              module/firmware loading), 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.

       openat2(2) is Linux-specific.

       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 is sometimes used in cases where one wants  to  open  but
       does  not  necessarily have the intention to read or write.  For example, this may be used
       to open a device 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 file descriptors.  See proc(5) for  further  details  of  both  of
       these directories.

       The  Linux  header  file  <asm/fcntl.h>  doesn't  define O_ASYNC; the (BSD-derived) FASYNC
       synonym is defined instead.

   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() 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

   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_RSYNC is defined in the Linux header file
       <asm/fcntl.h> on HP PA-RISC, but it is not used.)

       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.

   C library/kernel differences
       Since  version  2.26,  the  glibc  wrapper function for open() employs the openat() system
       call, rather than the kernel's open() system call.  For  certain  architectures,  this  is
       also true in glibc versions before 2.26.

       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 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),  fspick(2),  fstatat(2),  futimesat(2), linkat(2), mkdirat(2), move_mount(2),
       mknodat(2), name_to_handle_at(2), open_tree(2),  openat2(2),  readlinkat(2),  renameat(2),
       statx(2),  symlinkat(2), unlinkat(2), utimensat(2), mkfifoat(3), and scandirat(3)) address
       two problems with the older interfaces that preceded them.  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  dir1/dir2/xxx.dep  if the file dir1/dir2/xxx exists.  The
       problem is that between the existence check and  the  file-creation  step,  dir1  or  dir2
       (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().
       The use of the dirfd file descriptor also has other benefits:

       *  the file descriptor is a stable reference to the directory, even if  the  directory  is
          renamed; and

       *  the open file descriptor prevents the underlying filesystem from being dismounted, just
          as when a process has a current working directory on a filesystem.

       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 dirfd argument for these APIs can be obtained by using open() or openat()  to  open  a
       directory  (with  either  the  O_RDONLY  or  the O_PATH flag).  Alternatively, such a file
       descriptor can be obtained by applying  dirfd(3)  to  a  directory  stream  created  using

       When  these  APIs  are  given  a  dirfd  argument of AT_FDCWD or the specified pathname is
       absolute, then they handle their pathname argument in the same way  as  the  corresponding
       conventional  APIs.  However, in this case, several of the APIs have a flags argument that
       provides access to functionality that is not available with the corresponding conventional

       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, in which case
       open() fails with the error 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.


       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),   openat2(2),  read(2),  socket(2),  stat(2),  umask(2),
       unlink(2), write(2), fopen(3), acl(5), fifo(7), inode(7), path_resolution(7), symlink(7)


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