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

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

       #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 an index to an entry in the process's table of open file
       descriptors.  The file descriptor 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, 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
       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 condition.

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

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

              Note that the use of this flag is essential in some multithreaded
              programs, because using a separate fcntl(2) F_SETFD operation to
              set the FD_CLOEXEC flag does not suffice to avoid race conditions
              where one thread opens a file descriptor and attempts to set its
              close-on-exec flag using fcntl(2) at the same time as another
              thread does a fork(2) plus execve(2).  Depending on the order of
              execution, the race may lead to the file descriptor returned by
              open() being unintentionally leaked to the program executed by the
              child process created by fork(2).  (This kind of race is in
              principle possible for any system call that creates a file
              descriptor whose close-on-exec flag should be set, and various
              other Linux system calls provide an equivalent of the O_CLOEXEC
              flag to deal with this problem.)

              If 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 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

              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 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 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 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 conditions
              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
              activity.  This flag may not be effective on all filesystems.  One
              example is NFS, where the server maintains the access time.

              If pathname refers to a terminal device—see tty(4)—it will not
              become the process's controlling terminal even if the process does
              not have one.

              If 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 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
              descriptor which is returned will cause the calling process to

              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

              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

              *  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

              *  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

       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, "", 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 O_CREAT.

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

              There are two main use cases for O_TMPFILE:

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

              *  Creating a file that is initially invisible, which is then
                 populated with data and adjusted to have appropriate filesystem
                 attributes (fchown(2), fchmod(2), fsetxattr(2), etc.)  before
                 being atomically linked into the filesystem in a fully formed
                 state (using linkat(2) as described above).

              O_TMPFILE requires support by the underlying filesystem; only a
              subset of Linux filesystems provide that support.  In the initial
              implementation, support was provided in the ext2, ext3, ext4, UDF,
              Minix, and tmpfs 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.

       The dirfd argument is used in conjunction with the pathname argument as

       *  If the pathname given in pathname is absolute, then dirfd is ignored.

       *  If the pathname given in 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 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).  In this case,
          dirfd must be a directory that was opened for reading (O_RDONLY) or
          using the O_PATH flag.

       If the pathname given in pathname is relative, and dirfd is not a valid
       file descriptor, an error (EBADF) results.  (Specifying an invalid file
       descriptor number in dirfd can be used as a means to ensure that pathname
       is absolute.)

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

       On success, open(), openat(), and creat() return the new file descriptor
       (a nonnegative integer).  On error, -1 is returned and errno is set to
       indicate the error.

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

       EACCES The requested access to the file is not allowed, or search
              permission is denied for one of the directories in the path prefix
              of pathname, or the file did not exist yet and write access to the
              parent directory is not allowed.  (See also path_resolution(7).)

       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).

       EBADF  (openat()) pathname is relative but dirfd is neither AT_FDCWD nor
              a valid file descriptor.

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

       EDQUOT Where O_CREAT is specified, the file does not exist, and the
              user's quota of disk blocks or inodes on the filesystem has been

       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 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

              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 dangling
              symbolic link.

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

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

       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

              (openat()) pathname is a relative pathname and dirfd is a file
              descriptor referring to a file other than 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

       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

       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)).

       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

       When a file descriptor is duplicated (using dup(2) or similar), the
       duplicate refers to the same open file description as the original file
       descriptor, and the two file descriptors consequently share the file
       offset and file status flags.  Such sharing can also occur between
       processes: a child process created via fork(2) inherits duplicates of its
       parent's file descriptors, and those duplicates refer to the same open
       file descriptions.

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

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

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

       Linux implements O_SYNC and O_DSYNC, but not O_RSYNC.  Somewhat
       incorrectly, glibc defines O_RSYNC to have the same value as O_SYNC.
       (O_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 O_DSYNC).

       Since Linux 2.6.33, proper O_SYNC support is provided.  However, to
       ensure backward binary compatibility, O_DSYNC was defined with the same
       value as the historical O_SYNC, and O_SYNC was defined as a new (two-bit)
       flag value that includes the O_DSYNC flag value.  This ensures that
       applications compiled against new headers get at least O_DSYNC semantics
       on pre-2.6.33 kernels.

   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

   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)

   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), mknodat(2), mount_setattr(2),
       move_mount(2), name_to_handle_at(2), open_tree(2), openat2(2),
       readlinkat(2), renameat(2), renameat2(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 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 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, the alignment of the user buffer, and
       the file offset must all be multiples of the logical block size of the
       filesystem.  Since Linux 2.6.0, alignment to the logical block size of
       the underlying storage (typically 512 bytes) suffices.  The logical block
       size can be determined using the ioctl(2) BLKSSZGET operation or from the
       shell using the command:

           blockdev --getss

       O_DIRECT I/Os should never be run concurrently with the fork(2) system
       call, if the memory buffer is a private mapping (i.e., any mapping
       created with the mmap(2) MAP_PRIVATE flag; this includes memory allocated
       on the heap and statically allocated buffers).  Any such I/Os, whether
       submitted via an asynchronous I/O interface or from another thread in the
       process, should be completed before fork(2) is called.  Failure to do so
       can result in data corruption and undefined behavior in parent and child
       processes.  This restriction does not apply when the memory buffer for
       the O_DIRECT I/Os was created using shmat(2) or mmap(2) with the
       MAP_SHARED flag.  Nor does this restriction apply when the memory buffer
       has been advised as MADV_DONTFORK with madvise(2), ensuring that it will
       not be available to the child after fork(2).

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

       O_DIRECT support was added under Linux in kernel version 2.4.10.  Older
       Linux kernels simply ignore this flag.  Some filesystems may not
       implement the flag, 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)

       This page is part of release 5.13 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

Linux                              2021-08-27                            OPEN(2)