open(2) — Linux manual page


open(2)                    System Calls Manual                   open(2)

NAME         top

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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <fcntl.h>

       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, ...
                  /* 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

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

DESCRIPTION         top

       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 ORed in flags.  The file creation flags are
       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

              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 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

              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

              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).

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

              Write operations on the file will complete according to
              the requirements of synchronized I/O data integrity

              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 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.

              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 in Linux
              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 wait.

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

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

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

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

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

              •  close(2).

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

              •  fstat(2) (since Linux 3.6).

              •  fstatfs(2) (since Linux 3.12).

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

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

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

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

              •  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 ==

              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

                  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

              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 O_CREAT|O_WRONLY|O_TRUNC.

       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 follows:

       •  If the pathname given in pathname is absolute, then dirfd is

       •  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

       •  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

       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 openat2(2).

RETURN VALUE         top

       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.

ERRORS         top

       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_sys_fs(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

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

       EEXIST pathname already exists and O_CREAT and O_EXCL were used.

       EFAULT pathname points outside your accessible address space.


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

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

       EINVAL Invalid value in flags.

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

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

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

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

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

       ELOOP  Too many symbolic links were encountered in resolving

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

       EMFILE The per-process limit on the number of open file
              descriptors has been reached (see the description of
              RLIMIT_NOFILE in getrlimit(2)).

              pathname was too long.

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

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

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

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

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

       ENOMEM The named file is a FIFO, but memory for the FIFO buffer
              can't be allocated because the per-user hard limit on
              memory allocation for pipes has been reached and the
              caller is not privileged; see pipe(7).

       ENOMEM Insufficient kernel memory was available.

       ENOSPC pathname was to be created but the device containing
              pathname has no room for the new file.

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

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

       ENXIO  The file is a UNIX domain socket.

              The filesystem containing pathname does not support

              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; before Linux 2.6.24,
              Linux gave the error EFBIG for this case.

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

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

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

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

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

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

              The O_NONBLOCK flag was specified, and an incompatible
              lease was held on the file (see fcntl(2)).

VERSIONS         top

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

   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 before Linux 2.6.33.

   C library/kernel differences
       Since glibc 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 before glibc

STANDARDS         top


       openat2(2) Linux.

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

       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.

HISTORY         top

              SVr4, 4.3BSD, POSIX.1-2001.

              POSIX.1-2008.  Linux 2.6.16, glibc 2.4.

NOTES         top

       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).

       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.

       There are many infelicities in the protocol underlying NFS,
       affecting amongst others O_SYNC and O_NDELAY.

       On NFS filesystems with UID mapping enabled, open() may return a
       file descriptor but, for example, read(2) requests are denied
       with EACCES.  This is because the client performs open() by
       checking the permissions, but UID mapping is performed by the
       server upon read and write requests.

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

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

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

   Rationale for openat() and other directory file descriptor APIs
       openat() and the other system calls and library functions that
       take a directory file descriptor argument (i.e., execveat(2),
       faccessat(2), fanotify_mark(2), fchmodat(2), fchownat(2),
       fspick(2), fstatat(2), futimesat(2), linkat(2), mkdirat(2),
       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

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

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

       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.  The handling of misaligned
       O_DIRECT I/Os also varies; they can either fail with EINVAL or
       fall back to buffered I/O.

       Since Linux 6.1, O_DIRECT support and alignment restrictions for
       a file can be queried using statx(2), using the STATX_DIOALIGN
       flag.  Support for STATX_DIOALIGN varies by filesystem; see

       Some filesystems provide their own interfaces for querying
       O_DIRECT alignment restrictions, for example the XFS_IOC_DIOINFO
       operation in xfsctl(3).  STATX_DIOALIGN should be used instead
       when it is available.

       If none of the above is available, then direct I/O support and
       alignment restrictions can only be assumed from known
       characteristics of the filesystem, the individual file, the
       underlying storage device(s), and the kernel version.  In Linux
       2.4, most filesystems based on block devices require that the
       file offset and the length and memory address of all I/O segments
       be multiples of the filesystem block size (typically 4096 bytes).
       In Linux 2.6.0, this was relaxed to the logical block size of the
       block device (typically 512 bytes).  A block device's 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 in Linux 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

       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

       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

BUGS         top

       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

       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         top

       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)

Linux man-pages (unreleased)   2024-05-02                        open(2)

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