NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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);

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

       openat():
           Since glibc 2.10:
               _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _ATFILE_SOURCE

DESCRIPTION         top

       Given a pathname for a file, open() returns a file descriptor, a
       small, nonnegative integer for use in subsequent system calls
       (read(2), write(2), lseek(2), fcntl(2), etc.).  The file descriptor
       returned by a successful call will be the lowest-numbered file
       descriptor not currently open for the process.

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

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

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

       In addition, zero or more file creation flags and file status flags
       can be bitwise-or'd in flags.  The file creation flags are O_CLOEXEC,
       O_CREAT, O_DIRECTORY, O_EXCL, O_NOCTTY, O_NOFOLLOW, O_TMPFILE,
       O_TRUNC, and O_TTY_INIT.  The file status flags are all of the
       remaining flags listed below.  The distinction between these two
       groups of flags is that the file status flags can be retrieved and
       (in some cases) modified; see fcntl(2) for details.

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

       O_APPEND
              The file is opened in append mode.  Before each write(2), the
              file offset is positioned at the end of the file, as if with
              lseek(2).  O_APPEND may lead to corrupted files on NFS
              filesystems if more than one process appends data to a file at
              once.  This is because NFS does not support appending to a
              file, so the client kernel has to simulate it, which can't be
              done without a race condition.

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

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

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

       O_CREAT
              If the file does not exist, it will be created.  The owner
              (user ID) of the file is set to the effective user ID of the
              process.  The group ownership (group ID) is set either to the
              effective group ID of the process or to the group ID of the
              parent directory (depending on filesystem type and mount
              options, and the mode of the parent directory; see the mount
              options bsdgroups and sysvgroups described in mount(8)).

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

              The following symbolic constants are provided for mode:

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

              S_IRUSR  00400 user has read permission

              S_IWUSR  00200 user has write permission

              S_IXUSR  00100 user has execute permission

              S_IRWXG  00070 group has read, write and execute permission

              S_IRGRP  00040 group has read permission

              S_IWGRP  00020 group has write permission

              S_IXGRP  00010 group has execute permission

              S_IRWXO  00007 others have read, write and execute permission

              S_IROTH  00004 others have read permission

              S_IWOTH  00002 others have write permission

              S_IXOTH  00001 others have execute permission

       O_DIRECT (since Linux 2.4.10)
              Try to minimize cache effects of the I/O to and from this
              file.  In general this will degrade performance, but it is
              useful in special situations, such as when applications do
              their own caching.  File I/O is done directly to/from user-
              space buffers.  The O_DIRECT flag on its own makes an effort
              to transfer data synchronously, but does not give the
              guarantees of the O_SYNC flag that data and necessary metadata
              are transferred.  To guarantee synchronous I/O, O_SYNC must be
              used in addition to O_DIRECT.  See NOTES below for further
              discussion.

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

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

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

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

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

              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 to.

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

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

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

       O_NOATIME (since Linux 2.6.8)
              Do not update the file last access time (st_atime in the
              inode) when the file is read(2).  This flag is intended for
              use by indexing or backup programs, where its use can
              significantly reduce the amount of disk activity.  This flag
              may not be effective on all filesystems.  One example is NFS,
              where the server maintains the access time.

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

       O_NOFOLLOW
              If pathname is a symbolic link, then the open fails.  This is
              a FreeBSD extension, which was added to Linux in version
              2.1.126.  Symbolic links in earlier components of the pathname
              will still be followed.  See also O_PATH below.

       O_NONBLOCK or O_NDELAY
              When possible, the file is opened in nonblocking mode.
              Neither the open() nor any subsequent operations on the file
              descriptor which is returned will cause the calling process to
              wait.  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),
              mmap(2)) fail with the error EBADF.

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

              *  close(2); fchdir(2) (since Linux 3.5); fstat(2) (since
                 Linux 3.6).

              *  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(2) 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.

              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.

       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) (and similar) return, 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 file.  The pathname argument
              specifies a directory; an unnamed inode will be created in
              that directory's filesystem.  Anything written to the
              resulting file will be lost when the last file descriptor is
              closed, unless the file is given a name.

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

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

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

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

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

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

              There are two main use cases for O_TMPFILE:

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

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

              O_TMPFILE requires support by the underlying filesystem; only
              a subset of Linux filesystems provide that support.  In the
              initial implementation, support was provided in the ext2,
              ext3, ext4, UDF, Minix, and shmem filesystems.  XFS support
              was added in Linux 3.15.

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

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

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

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

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

       If pathname is absolute, then dirfd is ignored.

RETURN VALUE         top

       open(), openat(), and creat() return the new file descriptor, 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
              path_resolution(7).)

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

       EEXIST pathname already exists and O_CREAT and O_EXCL were used.

       EFAULT pathname points outside your accessible address space.

       EFBIG  See EOVERFLOW.

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

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

       EINVAL Invalid value in flags.

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

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

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

       ELOOP  Too many symbolic links were encountered in resolving
              pathname.

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

       EMFILE The process already has the maximum number of files open.

       ENAMETOOLONG
              pathname was too long.

       ENFILE The system 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.  Or, 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 Insufficient kernel memory was available.

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

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

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

       EOPNOTSUPP
              The filesystem containing pathname does not support O_TMPFILE.

       EOVERFLOW
              pathname refers to a regular file that is too large to be
              opened.  The usual scenario here is that an application
              compiled on a 32-bit platform without -D_FILE_OFFSET_BITS=64
              tried to open a file whose size exceeds (2<<31)-1 bits; see
              also O_LARGEFILE above.  This is the error specified by
              POSIX.1-2001; 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 (CAP_FOWNER).

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

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

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

       The following additional errors can occur for openat():

       EBADF  dirfd is not a valid file descriptor.

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

VERSIONS         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.

       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 indicates that one wants to open but
       does not necessarily have the intention to read or write.  This is
       typically used to open devices 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.

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

   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_SYNC provides synchronized I/O file integrity completion, meaning
       write operations will flush data and all associated metadata to the
       underlying hardware.  O_DSYNC provides synchronized I/O data
       integrity completion, meaning write operations will flush data to the
       underlying hardware, but will only flush metadata updates that are
       required to allow a subsequent read operation to complete
       successfully.  Data integrity completion can reduce the number of
       disk operations that are required for applications that don't need
       the guarantees of file integrity completion.

       To understand the difference between the two types of completion,
       consider two pieces of file metadata: the file last modification
       timestamp (st_mtime) and the file length.  All write operations will
       update the last file modification timestamp, but only writes that add
       data to the end of the file will change the file length.  The last
       modification timestamp is not needed to ensure that a read completes
       successfully, but the file length is.  Thus, O_DSYNC would only
       guarantee to flush updates to the file length metadata (whereas
       O_SYNC would also always flush the last modification timestamp
       metadata).

       Before Linux 2.6.33, Linux implemented only the O_SYNC flag for
       open().  However, when that flag was specified, most filesystems
       actually provided the equivalent of synchronized I/O data integrity
       completion (i.e., O_SYNC was actually implemented as the equivalent
       of O_DSYNC).

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

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

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

   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 descriptor that can't be used for reading or writing.  This
       nonstandard access mode is used by some Linux drivers to return a
       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., faccessat(2),
       fanotify_mark(2), fchmodat(2), fchownat(2), fstatat(2), futimesat(2),
       linkat(2), mkdirat(2), mknodat(2), name_to_handle_at(2),
       readlinkat(2), renameat(2), symlinkat(2), unlinkat(2), utimensat(2)
       mkfifoat(3), and scandirat(3)) are supported for two reasons.  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().  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
       openat().

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

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

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

           blockdev --getss

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

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

       O_DIRECT support was added under Linux in kernel version 2.4.10.
       Older Linux kernels simply ignore this flag.  Some filesystems may
       not implement the flag and open() will fail with EINVAL if it is
       used.

       Applications should avoid mixing O_DIRECT and normal I/O to the same
       file, and especially to overlapping byte regions in the same file.
       Even when the filesystem correctly handles the coherency issues in
       this situation, overall I/O throughput is likely to be slower than
       using either mode alone.  Likewise, applications should avoid mixing
       mmap(2) of files with direct I/O to the same files.

       The behavior of O_DIRECT with NFS will differ from local filesystems.
       Older kernels, or kernels configured in certain ways, may not support
       this combination.  The NFS protocol does not support passing the flag
       to the server, so O_DIRECT I/O will bypass the page cache only on the
       client; the server may still cache the I/O.  The client asks the
       server to make the I/O synchronous to preserve the synchronous
       semantics of O_DIRECT.  Some servers will perform poorly under these
       circumstances, especially if the I/O size is small.  Some servers may
       also be configured to lie to clients about the I/O having reached
       stable storage; this will avoid the performance penalty at some risk
       to data integrity in the event of server power failure.  The Linux
       NFS client places no alignment restrictions on O_DIRECT I/O.

       In summary, O_DIRECT is a potentially powerful tool that should be
       used with caution.  It is recommended that applications treat use of
       O_DIRECT as a performance option which is disabled by default.

              "The thing that has always disturbed me about O_DIRECT is that
              the whole interface is just stupid, and was probably designed
              by a deranged monkey on some serious mind-controlling
              substances."—Linus

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
       functionality.

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), read(2),
       socket(2), stat(2), umask(2), unlink(2), write(2), fopen(3), fifo(7),
       path_resolution(7), symlink(7)

COLOPHON         top

       This page is part of release 3.72 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
       http://www.kernel.org/doc/man-pages/.

Linux                            2014-07-08                          OPEN(2)