open

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



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

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

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

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

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

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

       openat():
           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _ATFILE_SOURCE

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

       The return value of open() is a file descriptor, a small, nonnegative
       integer that is used in subsequent system calls (read(2), write(2),
       lseek(2), fcntl(2), etc.) to refer to the open file.  The file
       descriptor returned by a successful call will be the lowest-numbered
       file descriptor not currently open for the process.

       By default, the new file descriptor is set to remain open across an
       execve(2) (i.e., the FD_CLOEXEC file descriptor flag described in
       fcntl(2) is initially disabled); the O_CLOEXEC flag, described below,
       can be used to change this default.  The file offset is set to the
       beginning of the file (see lseek(2)).

       A call to open() creates a new open file description, an entry in the
       system-wide table of open files.  The open file description records the
       file offset and the file status flags (see below).  A file descriptor
       is a reference to an open file description; this reference is
       unaffected if pathname is subsequently removed or modified to refer to
       a different file.  For further details on open file descriptions, see
       NOTES.

       The argument flags must include one of the following access modes:
       O_RDONLY, O_WRONLY, or O_RDWR.  These request opening the file read-
       only, write-only, or read/write, 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_CREAT, O_DIRECTORY, O_EXCL, O_NOCTTY, O_NOFOLLOW, O_TMPFILE, and
       O_TRUNC.  The file status flags are all of the remaining flags listed
       below.  The distinction between these two groups of flags is that the
       file creation flags affect the semantics of the open operation itself,
       while the file status flags affect the semantics of subsequent I/O
       operations.  The file status flags can be retrieved and (in some cases)
       modified; see fcntl(2) for details.

       The full list of file creation flags and file status flags is as
       follows:

       O_APPEND
              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.

       O_ASYNC
              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.)

       O_CREAT
              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 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
              accesses of the newly created file; the open() call that creates
              a read-only file may well return a read/write file descriptor.

              The following symbolic constants are provided for mode:

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

              S_IRUSR  00400 user has read permission

              S_IWUSR  00200 user has write permission

              S_IXUSR  00100 user has execute permission

              S_IRWXG  00070 group has read, write, and execute permission

              S_IRGRP  00040 group has read permission

              S_IWGRP  00020 group has write permission

              S_IXGRP  00010 group has execute permission

              S_IRWXO  00007 others have read, write, and execute permission

              S_IROTH  00004 others have read permission

              S_IWOTH  00002 others have write permission

              S_IXOTH  00001 others have execute permission

              According to POSIX, the effect when other bits are set in mode
              is unspecified.  On Linux, the following bits are also honored
              in mode:

              S_ISUID  0004000 set-user-ID bit

              S_ISGID  0002000 set-group-ID bit (see 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).

       O_DIRECTORY
              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.

       O_DSYNC
              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.

       O_LARGEFILE
              (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
                 file.

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

              This flag is intended for use by indexing or backup programs,
              where its use can significantly reduce the amount of disk
              activity.  This flag may not be effective on all filesystems.
              One example is NFS, where the server maintains the access time.

       O_NOCTTY
              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.

       O_NOFOLLOW
              If 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
              POSIX.1-2008.

              See also O_PATH below.

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

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

              Note that this flag has no effect for regular files and block
              devices; that is, I/O operations will (briefly) block when
              device activity is required, regardless of whether O_NONBLOCK is
              set.  Since O_NONBLOCK semantics might eventually be
              implemented, applications should not depend upon blocking
              behavior when specifying this flag for regular files and block
              devices.

              For the handling of FIFOs (named pipes), see also fifo(7).  For
              a discussion of the effect of O_NONBLOCK in conjunction with
              mandatory file locks and with file leases, see fcntl(2).

       O_PATH (since Linux 2.6.39)
              Obtain a file descriptor that can be used for two purposes: to
              indicate a location in the filesystem tree and to perform
              operations that act purely at the file descriptor level.  The
              file itself is not opened, and other file operations (e.g.,
              read(2), write(2), fchmod(2), fchown(2), fgetxattr(2), ioctl(2),
              mmap(2)) fail with the error EBADF.

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

              *  close(2).

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

              *  fstat(2) (since Linux 3.6).

              *  fstatfs(2) (since Linux 3.12).

              *  Duplicating the file descriptor (dup(2), fcntl(2) F_DUPFD,
                 etc.).

              *  Getting and setting file descriptor flags (fcntl(2) F_GETFD
                 and F_SETFD).

              *  Retrieving open file status flags using the fcntl(2) F_GETFL
                 operation: the returned flags will include the bit O_PATH.

              *  Passing the file descriptor as the dirfd argument of openat()
                 and the other "*at()" system calls.  This includes linkat(2)
                 with AT_EMPTY_PATH (or via procfs using AT_SYMLINK_FOLLOW)
                 even if the file is not a directory.

              *  Passing the file descriptor to another process via a UNIX
                 domain socket (see SCM_RIGHTS in unix(7)).

              When O_PATH is specified in flags, flag bits other than
              O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW are ignored.

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

              If pathname is a symbolic link and the O_NOFOLLOW flag is also
              specified, then the call returns a file descriptor referring to
              the symbolic link.  This file descriptor can be used as the
              dirfd argument in calls to fchownat(2), fstatat(2), linkat(2),
              and readlinkat(2) with an empty pathname to have the calls
              operate on the symbolic link.

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

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

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

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

       O_SYNC Write operations on the file will complete according to the
              requirements of synchronized I/O file integrity completion (by
              contrast with the synchronized I/O data integrity completion
              provided by O_DSYNC.)

              By the time write(2) (or similar) returns, the output data and
              associated file metadata have been transferred to the underlying
              hardware (i.e., as though each write(2) was followed by a call
              to fsync(2)).  See NOTES below.

       O_TMPFILE (since Linux 3.11)
              Create an unnamed temporary regular file.  The pathname argument
              specifies a directory; an unnamed inode will be created in that
              directory's filesystem.  Anything written to the resulting file
              will be lost when the last file descriptor is closed, unless the
              file is given a name.

              O_TMPFILE must be specified with one of O_RDWR or O_WRONLY and,
              optionally, O_EXCL.  If O_EXCL is not specified, then linkat(2)
              can be used to link the temporary file into the filesystem,
              making it permanent, using code like the following:

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

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

                  snprintf(path, PATH_MAX,  "/proc/self/fd/%d", fd);
                  linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",
                                          AT_SYMLINK_FOLLOW);

              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 shmem filesystems.  Support for other
              filesystems has subsequently been added as follows: XFS (Linux
              3.15); Btrfs (Linux 3.16); F2FS (Linux 3.16); and ubifs (Linux
              4.9)

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

   creat()
       A call to creat() is equivalent to calling open() with flags equal to
       O_CREAT|O_WRONLY|O_TRUNC.

   openat()
       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.

RETURN VALUE
       open(), openat(), and creat() return the new file descriptor, or -1 if
       an error occurred (in which case, errno is set appropriately).

ERRORS
       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).)

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

       EEXIST pathname already exists and O_CREAT and O_EXCL were used.

       EFAULT pathname points outside your accessible address space.

       EFBIG  See EOVERFLOW.

       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).

       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
              getrlimit(2)).

       ENAMETOOLONG
              pathname was too long.

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

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

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

       ENOENT A directory component in pathname does not exist or is a
              dangling symbolic link.

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

       ENOTDIR
              A component used as a directory in pathname is not, in fact, a
              directory, or O_DIRECTORY was specified and pathname was not a
              directory.

       ENXIO  O_NONBLOCK | O_WRONLY is set, the named file is a FIFO, and no
              process has the FIFO open for reading.

       ENXIO  The file is a device special file and no corresponding device
              exists.

       ENXIO  The file is a UNIX domain socket.

       EOPNOTSUPP
              The filesystem containing pathname does not support O_TMPFILE.

       EOVERFLOW
              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.

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

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

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

       EWOULDBLOCK
              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.

       ENOTDIR
              pathname is a relative pathname and dirfd is a file descriptor
              referring to a file other than a directory.

VERSIONS
       openat() was added to Linux in kernel 2.6.16; library support was added
       to glibc in version 2.4.

CONFORMING TO
       open(), creat() SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.

       openat(): POSIX.1-2008.

       The O_DIRECT, O_NOATIME, O_PATH, and O_TMPFILE flags are Linux-
       specific.  One must define _GNU_SOURCE to obtain their definitions.

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

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

NOTES
       Under Linux, the O_NONBLOCK flag is sometimes used in cases where one
       wants to open but does not necessarily have the intention to read or
       write.  For example, this may be used to open a device in order to get
       a file descriptor for use with ioctl(2).

       The (undefined) effect of O_RDONLY | O_TRUNC varies among
       implementations.  On many systems the file is actually truncated.

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

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

       The files in the /proc/[pid]/fd directory show the open file
       descriptors of the process with the PID pid.  The files in the
       /proc/[pid]/fdinfo directory show even more information about these
       file descriptors.  See proc(5) for further details of both of these
       directories.

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

   Open file descriptions
       The term open file description is the one used by POSIX to refer to the
       entries in the system-wide table of open files.  In other contexts,
       this object is variously also called an "open file object", a "file
       handle", an "open file table entry", or—in kernel-developer parlance—a
       struct file.

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

   NFS
       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.

   FIFOs
       Opening the read or write end of a FIFO blocks until the other end is
       also opened (by another process or thread).  See fifo(7) for further
       details.

   File access mode
       Unlike the other values that can be specified in flags, the access mode
       values O_RDONLY, O_WRONLY, and O_RDWR do not specify individual bits.
       Rather, they define the low order two bits of flags, and are defined
       respectively as 0, 1, and 2.  In other words, the combination O_RDONLY
       | O_WRONLY is a logical error, and certainly does not have the same
       meaning as O_RDWR.

       Linux reserves the special, nonstandard access mode 3 (binary 11) in
       flags to mean: check for read and write permission on the file and
       return a file descriptor that can't be used for reading or writing.
       This nonstandard access mode is used by some Linux drivers to return a
       file descriptor that is to be used only for device-specific ioctl(2)
       operations.

   Rationale for openat() and other directory file descriptor APIs
       openat() and the other system calls and library functions that take a
       directory file descriptor argument (i.e., execveat(2), faccessat(2),
       fanotify_mark(2), fchmodat(2), fchownat(2), fstatat(2), futimesat(2),
       linkat(2), mkdirat(2), mknodat(2), name_to_handle_at(2), readlinkat(2),
       renameat(2), statx(2), symlinkat(2), unlinkat(2), utimensat(2),
       mkfifoat(3), and scandirat(3)) address two problems with the older
       interfaces that preceded them.  Here, the explanation is in terms of
       the openat() call, but the rationale is analogous for the other
       interfaces.

       First, openat() allows an application to avoid race conditions that
       could occur when using open() to open files in directories other than
       the current working directory.  These race conditions result from the
       fact that some component of the directory prefix given to open() could
       be changed in parallel with the call to open().  Suppose, for example,
       that we wish to create the file dir1/dir2/xxx.dep if the file
       dir1/dir2/xxx exists.  The problem is that between the existence check
       and the file-creation step, dir1 or dir2 (which might be symbolic
       links) could be modified to point to a different location.  Such races
       can be avoided by opening a file descriptor for the target directory,
       and then specifying that file descriptor as the dirfd argument of (say)
       fstatat(2) and openat().  The use of the dirfd file descriptor also has
       other benefits:

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

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

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

   O_DIRECT
       The O_DIRECT flag may impose alignment restrictions on the length and
       address of user-space buffers and the file offset of I/Os.  In Linux
       alignment restrictions vary by filesystem and kernel version and might
       be absent entirely.  However there is currently no
       filesystem-independent interface for an application to discover these
       restrictions for a given file or filesystem.  Some filesystems provide
       their own interfaces for doing so, for example the XFS_IOC_DIOINFO
       operation in xfsctl(3).

       Under Linux 2.4, transfer sizes, and the alignment of the user buffer
       and the file offset must all be multiples of the logical block size of
       the filesystem.  Since Linux 2.6.0, alignment to the logical block size
       of the underlying storage (typically 512 bytes) suffices.  The logical
       block size can be determined using the ioctl(2) BLKSSZGET operation or
       from the shell using the command:

           blockdev --getss

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

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

BUGS
       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).

SEE ALSO
       chmod(2), chown(2), close(2), dup(2), fcntl(2), link(2), lseek(2),
       mknod(2), mmap(2), mount(2), open_by_handle_at(2), read(2), socket(2),
       stat(2), umask(2), unlink(2), write(2), fopen(3), acl(5), fifo(7),
       inode(7), path_resolution(7), symlink(7)

COLOPHON
       This page is part of release 5.01 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
       https://www.kernel.org/doc/man-pages/.



Linux                             2018-04-30                           OPEN(2)