creat(2) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

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

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

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

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

       /* Documented separately, in openat2(2): */
       int openat2(int dirfd, const char *pathname,
                   const struct open_how *how, size_t size);

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

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

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

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

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

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

       In addition, zero or more file creation flags and file status flags
       can be bitwise-or'd in flags.  The file creation flags are O_CLOEXEC,
       O_TRUNC.  The file status flags are all of the remaining flags listed
       below.  The distinction between these two groups of flags is that the
       file creation flags affect the semantics of the open operation
       itself, while the file status flags affect the semantics of
       subsequent I/O operations.  The file status flags can be retrieved
       and (in some cases) modified; see fcntl(2) for details.

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

              The file is opened in append mode.  Before each write(2), the
              file offset is positioned at the end of the file, as if with
              lseek(2).  The modification of the file offset and the write
              operation are performed as a single atomic step.

              O_APPEND may lead to corrupted files on NFS filesystems if
              more than one process appends data to a file at once.  This is
              because NFS does not support appending to a file, so the
              client kernel has to simulate it, which can't be done without
              a race condition.

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

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

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

              If pathname does not exist, create it as a regular file.

              The owner (user ID) of the new file is set to the effective
              user ID of the process.

              The group ownership (group ID) of the new file is set either
              to the effective group ID of the process (System V semantics)
              or to the group ID of the parent directory (BSD semantics).
              On Linux, the behavior depends on whether the set-group-ID
              mode bit is set on the parent directory: if that bit is set,
              then BSD semantics apply; otherwise, System V semantics apply.
              For some filesystems, the behavior also depends on the
              bsdgroups and sysvgroups mount options described in mount(8).

              The mode argument specifies the file mode bits to be applied
              when a new file is created.  If neither O_CREAT nor O_TMPFILE
              is specified in flags, then mode is ignored (and can thus be
              specified as 0, or simply omitted).  The mode argument must be
              supplied if O_CREAT or O_TMPFILE is specified in flags; if it
              is not supplied, some arbitrary bytes from the stack will be
              applied as the file mode.

              The effective mode is modified by the process's umask in the
              usual way: in the absence of a default ACL, the mode of the
              created file is (mode & ~umask).

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

              The following symbolic constants are provided for mode:

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

              S_IRUSR  00400 user has read permission

              S_IWUSR  00200 user has write permission

              S_IXUSR  00100 user has execute permission

              S_IRWXG  00070 group has read, write, and execute permission

              S_IRGRP  00040 group has read permission

              S_IWGRP  00020 group has write permission

              S_IXGRP  00010 group has execute permission

              S_IRWXO  00007 others have read, write, and execute permission

              S_IROTH  00004 others have read permission

              S_IWOTH  00002 others have write permission

              S_IXOTH  00001 others have execute permission

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

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

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

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

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

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

              When these two flags are specified, symbolic links are not
              followed: if pathname is a symbolic link, then open() fails
              regardless of where the symbolic link points.

              In general, the behavior of O_EXCL is undefined if it is used
              without O_CREAT.  There is one exception: on Linux 2.6 and
              later, O_EXCL can be used without O_CREAT if pathname refers
              to a block device.  If the block device is in use by the
              system (e.g., mounted), open() fails with the error EBUSY.

              On NFS, O_EXCL is supported only when using NFSv3 or later on
              kernel 2.6 or later.  In NFS environments where O_EXCL support
              is not provided, programs that rely on it for performing
              locking tasks will contain a race condition.  Portable
              programs that want to perform atomic file locking using a
              lockfile, and need to avoid reliance on NFS support for
              O_EXCL, can create a unique file on the same filesystem (e.g.,
              incorporating hostname and PID), and use link(2) to make a
              link to the lockfile.  If link(2) returns 0, the lock is
              successful.  Otherwise, use stat(2) on the unique file to
              check if its link count has increased to 2, in which case the
              lock is also successful.

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

       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

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

              If the trailing component (i.e., basename) of pathname is a
              symbolic link, then the open fails, with the error ELOOP.
              Symbolic links in earlier components of the pathname will
              still be followed.  (Note that the ELOOP error that can occur
              in this case is indistinguishable from the case where an open
              fails because there are too many symbolic links found while
              resolving components in the prefix part of the pathname.)

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

              See also O_PATH below.

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

              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,

              *  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

              *  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 underly‐
              ing 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 argu‐
              ment specifies a directory; an unnamed inode will be created
              in that directory's filesystem.  Anything written to the re‐
              sulting 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 follow‐

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

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

                  linkat(fd, NULL, AT_FDCWD, "/path/for/file", AT_EMPTY_PATH);

                  /* If the caller doesn't have the CAP_DAC_READ_SEARCH
                     capability (needed to use AT_EMPTY_PATH with linkat(2)),
                     and there is a proc(5) filesystem mounted, then the
                     linkat(2) call above can be replaced with:

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

              In this case, the open() mode argument determines the file
              permission mode, as with 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

              O_TMPFILE requires support by the underlying filesystem; only
              a subset of Linux filesystems provide that support.  In the
              initial implementation, support was provided in the ext2,
              ext3, ext4, UDF, Minix, and shmem filesystems.  Support for
              other filesystems has subsequently been added as follows: XFS
              (Linux 3.15); Btrfs (Linux 3.16); F2FS (Linux 3.16); and ubifs
              (Linux 4.9)

              If the file already exists and is a regular file and the ac‐
              cess 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 ef‐
              fect of O_TRUNC is unspecified.

       A call to creat() is equivalent to calling open() with flags equal to

       The openat() system call operates in exactly the same way as open(),
       except for the differences described here.

       If the pathname given in pathname is relative, then it is interpreted
       relative to the directory referred to by the file descriptor dirfd
       (rather than relative to the current working directory of the calling
       process, as is done by open() for a relative pathname).

       If pathname is relative and dirfd is the special value AT_FDCWD, then
       pathname is interpreted relative to the current working directory of
       the calling process (like open()).

       If pathname is absolute, then dirfd is ignored.

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

RETURN VALUE         top

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

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

       EACCES Where O_CREAT is specified, the protected_fifos or
              protected_regular sysctl is enabled, the file already exists
              and is a FIFO or regular file, the owner of the file is
              neither the current user nor the owner of the containing
              directory, and the containing directory is both world- or
              group-writable and sticky.  For details, see the descriptions
              of /proc/sys/fs/protected_fifos and
              /proc/sys/fs/protected_regular in proc(5).

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

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

       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

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

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

              pathname was too long.

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

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

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

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

       ENXIO  The file is a UNIX domain socket.

              The filesystem containing pathname does not support O_TMPFILE.

              pathname refers to a regular file that is too large to be
              opened.  The usual scenario here is that an application
              compiled on a 32-bit platform without -D_FILE_OFFSET_BITS=64
              tried to open a file whose size exceeds (1<<31)-1 bytes; see
              also O_LARGEFILE above.  This is the error specified by
              POSIX.1; in kernels before 2.6.24, Linux gave the error EFBIG
              for this case.

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

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

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

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

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

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

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

       The following additional errors can occur for openat():

       EBADF  dirfd is not a valid file descriptor.

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

VERSIONS         top

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

CONFORMING TO         top

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

       openat(): POSIX.1-2008.

       openat2(2) is Linux-specific.

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

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

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

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).

       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

       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

       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

       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

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

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

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

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

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

   Rationale for openat() and other directory file descriptor APIs
       openat() and the other system calls and library functions that take a
       directory file descriptor argument (i.e., execveat(2), faccessat(2),
       fanotify_mark(2), fchmodat(2), fchownat(2), fspick(2), fstatat(2),
       futimesat(2), linkat(2), mkdirat(2), move_mount(2), mknodat(2),
       name_to_handle_at(2), open_tree(2), openat2(2), readlinkat(2),
       renameat(2), statx(2), symlinkat(2), unlinkat(2), utimensat(2),
       mkfifoat(3), and scandirat(3)) address two problems with the older
       interfaces that preceded them.  Here, the explanation is in terms of
       the openat() call, but the rationale is analogous for the other

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

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

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

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

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

       When these APIs are given a dirfd argument of AT_FDCWD or the
       specified pathname is absolute, then they handle their pathname
       argument in the same way as the corresponding conventional APIs.
       However, in this case, several of the APIs have a flags argument that
       provides access to functionality that is not available with the
       corresponding conventional 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.  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) sys‐
       tem call, if the memory buffer is a private mapping (i.e., any map‐
       ping 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 an‐
       other 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 restric‐
       tion apply when the memory buffer has been advised as MADV_DONTFORK
       with madvise(2), ensuring that it will not be available to the child
       after fork(2).

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

       O_DIRECT support was added under Linux in kernel version 2.4.10.
       Older Linux kernels simply ignore this flag.  Some filesystems may
       not 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 seman‐
       tics of O_DIRECT.  Some servers will perform poorly under these cir‐
       cumstances, 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         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

       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)

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-11-01                          OPEN(2)

Pages that refer to this page: fcntl(2)fcntl64(2)syscalls(2)signal-safety(7)