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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).

       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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