OPEN(2)                    Linux Programmer's Manual                   OPEN(2)

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

       #include <sys/types.h>
       #include <sys/stat.h>
       #include <fcntl.h>

       int open(const char *pathname, int flags);
       int open(const char *pathname, int flags, mode_t mode);

       int creat(const char *pathname, mode_t mode);

       int openat(int dirfd, const char *pathname, int flags);
       int openat(int dirfd, const char *pathname, int flags, mode_t mode);

       /* 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  speci-
       fied 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 descrip-
       tor returned by a successful call will be the lowest-numbered file  de-
       scriptor not currently open for the process.

       By default, the new file descriptor is set to remain open across an ex-
       ecve(2) (i.e., the FD_CLOEXEC file descriptor  flag  described  in  fc-
       ntl(2) is initially disabled); the O_CLOEXEC flag, described below, can
       be used to change this default.  The file offset is set to  the  begin-
       ning 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  unaf-
       fected  if  pathname  is subsequently removed or modified to refer to a
       different file.  For further details on  open  file  descriptions,  see

       The  argument  flags  must  include  one of the following access modes:
       O_RDONLY, O_WRONLY, or O_RDWR.  These request opening  the  file  read-
       only, write-only, or read/write, respectively.

       In addition, zero or more file creation flags and file status flags can
       be bitwise-or'd in flags.   The  file  creation  flags  are  O_CLOEXEC,
       O_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 op-
       erations.  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 fol-

              The file is opened in append mode.  Before  each  write(2),  the
              file  offset  is  positioned  at the end of the file, as if with
              lseek(2).  The modification of the file offset and the write op-
              eration 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  be-
              cause  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  be-
              comes  possible on this file descriptor.  This feature is avail-
              able 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 fc-
              ntl(2) F_SETFD operations to set the FD_CLOEXEC flag.

              Note that the use of this  flag  is  essential  in  some  multi-
              threaded programs, because using a separate fcntl(2) F_SETFD op-
              eration to set the FD_CLOEXEC flag does  not  suffice  to  avoid
              race conditions where one thread opens a file descriptor and at-
              tempts 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  de-
              scriptor  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  filesys-
              tems,  the behavior also depends on the bsdgroups and sysvgroups
              mount options described in mount(8).

              The mode argument specifies the file mode bits be applied when a
              new  file  is  created.   This  argument  must  be supplied when
              O_CREAT or O_TMPFILE is specified in flags; if  neither  O_CREAT
              nor O_TMPFILE is specified, then mode is ignored.  The effective
              mode is modified by the process's umask in the usual way: in the
              absence  of  a  default  ACL,  the  mode  of the created file is
              (mode & ~umask).  Note that this mode applies only to future ac-
              cesses of the newly created file; the open() call that creates a
              read-only file may well return a read/write file descriptor.

              The following symbolic constants are provided for mode:

              S_IRWXU  00700 user (file owner) has read,  write,  and  execute

              S_IRUSR  00400 user has read permission

              S_IWUSR  00200 user has write permission

              S_IXUSR  00100 user has execute permission

              S_IRWXG  00070 group has read, write, and execute permission

              S_IRGRP  00040 group has read permission

              S_IWGRP  00020 group has write permission

              S_IXGRP  00010 group has execute permission

              S_IRWXO  00007 others have read, write, and execute permission

              S_IROTH  00004 others have read permission

              S_IWOTH  00002 others have write permission

              S_IXOTH  00001 others have execute permission

              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 guar-
              antee synchronous I/O, O_SYNC must be used in addition to  O_DI-
              RECT.  See NOTES below for further discussion.

              A  semantically similar (but deprecated) interface for block de-
              vices is described in raw(8).

              If pathname is not a directory, cause the open  to  fail.   This
              flag  was  added  in kernel version 2.1.126, to avoid denial-of-
              service problems if opendir(3) is called on a FIFO or  tape  de-

              Write  operations on the file will complete according to the re-
              quirements of synchronized I/O data integrity completion.

              By the time write(2) (and similar) return, the output  data  has
              been transferred to the underlying hardware, along with any file
              metadata that would be required to retrieve that data (i.e.,  as
              though  each  write(2)  was followed by a call to fdatasync(2)).
              See NOTES below.

       O_EXCL Ensure that this call creates the file: if this flag  is  speci-
              fied  in  conjunction with O_CREAT, and pathname already exists,
              then open() fails with the error EEXIST.

              When these two flags are specified, symbolic links are not  fol-
              lowed: if pathname is a symbolic link, then open() fails regard-
              less of where the symbolic link points.

              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 in-
              creased to 2, in which case the lock is also successful.

              (LFS) Allow files whose sizes cannot be represented in an  off_t
              (but  can  be  represented  in  an  off64_t)  to be opened.  The
              _LARGEFILE64_SOURCE macro must be defined (before including  any
              header  files)  in order to obtain this definition.  Setting the
              _FILE_OFFSET_BITS feature test macro to 64  (rather  than  using
              O_LARGEFILE) is the preferred method of accessing large files on
              32-bit systems (see feature_test_macros(7)).

       O_NOATIME (since Linux 2.6.8)
              Do not update the file last access time (st_atime in the  inode)
              when the file is read(2).

              This  flag  can  be employed only if one of the following condi-
              tions is true:

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

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

              This  flag  is  intended for use by indexing or backup programs,
              where its use can significantly reduce the amount of disk activ-
              ity.   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 the trailing component (i.e., basename) of pathname is a sym-
              bolic link, then the open fails, with the error ELOOP.  Symbolic
              links in earlier components of the pathname will still  be  fol-
              lowed.   (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 compo-
              nents in the prefix part of the pathname.)

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

              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 de-
              scriptor which is returned will cause  the  calling  process  to

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

              Note that this flag has no effect for regular  files  and  block
              devices;  that  is, I/O operations will (briefly) block when de-
              vice activity is required, regardless of whether  O_NONBLOCK  is
              set.   Since  O_NONBLOCK  semantics  might  eventually be imple-
              mented, 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 opera-
              tions  that  act  purely at the file descriptor level.  The file
              itself is not opened, and other file operations (e.g.,  read(2),
              write(2), fchmod(2), fchown(2), fgetxattr(2), ioctl(2), mmap(2))
              fail with the error EBADF.

              The following operations can be performed on the resulting  file

              *  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,

              *  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 do-
                 main 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 per-
              mission 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  open-
              ing 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 oper-
              ate 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 direc-
              tory 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  permis-
              sion,  and then execute that file, with steps something like the

                  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

       O_SYNC Write  operations on the file will complete according to the re-
              quirements 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, mak-
              ing it permanent, using code like the following:

                  char path[PATH_MAX];
                  fd = open("/path/to/dir", O_TMPFILE | O_RDWR,
                                          S_IRUSR | S_IWUSR);

                  /* File I/O on 'fd'... */

                  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  per-
              mission mode, as with O_CREAT.

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

              There are two main use cases for O_TMPFILE:

              *  Improved tmpfile(3) functionality: race-free creation of tem-
                 porary  files that (1) are automatically deleted when closed;
                 (2) can never be reached via any pathname; (3) are  not  sub-
                 ject to symlink attacks; and (4) do not require the caller to
                 devise unique names.

              *  Creating a file that is initially invisible,  which  is  then
                 populated with data and adjusted to have appropriate filesys-
                 tem attributes  (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 ini-
              tial 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

              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 open()).

       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

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

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

       EACCES The  requested access to the file is not allowed, or search per-
              mission is denied for one of the directories in the path  prefix
              of  pathname,  or the file did not exist yet and write access to
              the parent directory is not  allowed.   (See  also  path_resolu-

       EACCES Where   O_CREAT   is  specified,  the  protected_fifos  or  pro-
              tected_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/pro-
              tected_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

       EEXIST pathname already exists and O_CREAT and O_EXCL were used.

       EFAULT pathname points outside your accessible address space.


       EINTR  While  blocked  waiting  to  complete  an  open of a slow device
              (e.g., a FIFO; see fifo(7)), the call was interrupted by a  sig-
              nal handler; see signal(7).

       EINVAL The  filesystem  does  not support the O_DIRECT flag.  See NOTES
              for more information.

       EINVAL Invalid value in flags.

       EINVAL O_TMPFILE was specified  in  flags,  but  neither  O_WRONLY  nor
              O_RDWR was specified.

       EINVAL O_CREAT  was  specified in flags and the final component ("base-
              name") 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  filesys-

       EISDIR pathname refers to a directory and the access requested involved
              writing (that is, O_WRONLY or O_RDWR is set).

       EISDIR pathname refers to an existing directory, O_TMPFILE and  one  of
              O_WRONLY or O_RDWR were specified in flags, but this kernel ver-
              sion does not provide the O_TMPFILE functionality.

       ELOOP  Too many symbolic links were encountered in resolving pathname.

       ELOOP  pathname was a symbolic link, and flags specified O_NOFOLLOW but
              not O_PATH.

       EMFILE The per-process limit on the number of open file descriptors has
              been reached (see the  description  of  RLIMIT_NOFILE  in  getr-

              pathname was too long.

       ENFILE The system-wide limit on the total number of open files has been

       ENODEV pathname refers to a device special file  and  no  corresponding
              device  exists.   (This is a Linux kernel bug; in this situation
              ENXIO must be returned.)

       ENOENT O_CREAT is not set and the named file does not exist.

       ENOENT A directory component in pathname does not exist or  is  a  dan-
              gling symbolic link.

       ENOENT pathname refers to a nonexistent directory, O_TMPFILE and one of
              O_WRONLY or O_RDWR were specified in flags, but this kernel ver-
              sion does not provide the O_TMPFILE functionality.

       ENOMEM The  named  file is a FIFO, but memory for the FIFO buffer can't
              be allocated because the per-user hard limit on  memory  alloca-
              tion  for  pipes  has  been reached and the caller is not privi-
              leged; 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

       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

       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

              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-spe-
       cific.  One must define _GNU_SOURCE to obtain their definitions.

       The  O_CLOEXEC,  O_DIRECTORY, and O_NOFOLLOW flags are not specified in
       POSIX.1-2001, but are specified in POSIX.1-2008.  Since glibc 2.12, one
       can  obtain their definitions by defining either _POSIX_C_SOURCE with a
       value greater than or equal to 200809L or _XOPEN_SOURCE  with  a  value
       greater  than  or equal to 700.  In glibc 2.11 and earlier, one obtains
       the definitions by defining _GNU_SOURCE.

       As  noted  in  feature_test_macros(7),  feature  test  macros  such  as
       _POSIX_C_SOURCE,  _XOPEN_SOURCE, and _GNU_SOURCE must be defined before
       including any header files.

       Under Linux, the O_NONBLOCK flag 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 implementa-
       tions.  On many systems the file is actually truncated.

       Note that open() can open device special files, but creat() cannot cre-
       ate them; use mknod(2) instead.

       If  the  file is newly created, its st_atime, st_ctime, st_mtime fields
       (respectively, time of last access, time of  last  status  change,  and
       time  of  last  modification; see stat(2)) are set to the current time,
       and so are the st_ctime and st_mtime fields of  the  parent  directory.
       Otherwise,  if  the  file  is modified because of the O_TRUNC flag, its
       st_ctime and st_mtime fields are set to the current time.

       The files in the /proc/[pid]/fd directory show the open  file  descrip-
       tors of the process with the PID pid.  The files in the /proc/[pid]/fd-
       info 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 du-
       plicate  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 pro-
       cesses: a child process created via fork(2) inherits duplicates of  its
       parent's  file descriptors, and those duplicates refer to the same open
       file descriptions.

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

       On  Linux,  one can use the kcmp(2) KCMP_FILE operation to test whether
       two file descriptors (in the same process  or  in  two  different  pro-
       cesses) refer to the same open file description.

   Synchronized I/O
       The POSIX.1-2008 "synchronized I/O" option specifies different variants
       of synchronized I/O, and specifies the open()  flags  O_SYNC,  O_DSYNC,
       and O_RSYNC for controlling the behavior.  Regardless of whether an im-
       plementation supports this option, it must at least support the use  of
       O_SYNC for regular files.

       Linux  implements O_SYNC and O_DSYNC, but not O_RSYNC.  Somewhat incor-
       rectly, glibc defines  O_RSYNC  to  have  the  same  value  as  O_SYNC.
       (O_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 un-
       derlying 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 al-
       low  a subsequent read operation to complete successfully.  Data integ-
       rity completion can reduce the number of disk operations that  are  re-
       quired  for  applications that don't need the guarantees of file integ-
       rity completion.

       To understand the difference between the two types of completion,  con-
       sider 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 pro-
       vided the equivalent of  synchronized  I/O  data  integrity  completion
       (i.e., O_SYNC was actually implemented as the equivalent of O_DSYNC).

       Since Linux 2.6.33, proper O_SYNC support is provided.  However, to en-
       sure 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 se-
       mantics 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  re-

       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

   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 re-
       turn 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)  opera-

   Rationale for openat() and other directory file descriptor APIs
       openat()  and  the other system calls and library functions that take a
       directory file descriptor argument  (i.e.,  execveat(2),  faccessat(2),
       fanotify_mark(2),  fchmodat(2), fchownat(2), fspick(2), fstatat(2), fu-
       timesat(2),   linkat(2),   mkdirat(2),    move_mount(2),    mknodat(2),
       name_to_handle_at(2),   open_tree(2),  openat2(2),  readlinkat(2),  re-
       nameat(2), statx(2),  symlinkat(2),  unlinkat(2),  utimensat(2),  mkfi-
       foat(3),  and  scandirat(3)) address two problems with the older inter-
       faces 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 be-
          ing  dismounted, just as when a process has a current working direc-
          tory on a filesystem.

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

       The  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 apply-
       ing dirfd(3) to a directory stream created using opendir(3).

       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 was 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-indepen-
       dent  interface for an application to discover these restrictions for a
       given file or filesystem.  Some filesystems provide  their  own  inter-
       faces  for  doing  so, for example the XFS_IOC_DIOINFO operation in xf-

       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 cre-
       ated  with the mmap(2) MAP_PRIVATE flag; this includes memory allocated
       on the heap and statically allocated buffers).  Any such I/Os,  whether
       submitted  via  an asynchronous I/O interface or from another thread in
       the process, should be completed before fork(2) is called.  Failure  to
       do  so  can  result in data corruption and undefined behavior in parent
       and child processes.  This restriction does not apply when  the  memory
       buffer for the O_DIRECT I/Os was created using shmat(2) or mmap(2) with
       the MAP_SHARED flag.  Nor does this restriction apply when  the  memory
       buffer has been advised as MADV_DONTFORK with madvise(2), ensuring that
       it will not be available to the child after fork(2).

       The O_DIRECT flag was introduced in SGI IRIX, where  it  has  alignment
       restrictions  similar  to those of Linux 2.4.  IRIX has also a fcntl(2)
       call to query appropriate alignments, and sizes.   FreeBSD  4.x  intro-
       duced a flag of the same name, but without alignment restrictions.

       O_DIRECT support was added under Linux in kernel version 2.4.10.  Older
       Linux kernels simply ignore this flag.  Some filesystems may not imple-
       ment  the  flag, 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 ei-
       ther 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  config-
       ured  to  lie  to  clients about the I/O having reached stable storage;
       this will avoid the performance penalty at some risk to data  integrity
       in  the  event of server power failure.  The Linux NFS client places no
       alignment restrictions on O_DIRECT I/O.

       In summary, O_DIRECT is a potentially powerful tool that should be used
       with  caution.   It is recommended that applications treat use of O_DI-
       RECT 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  functional-

       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)

       This page is part of release 5.07 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at

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