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

       open, creat - open and possibly create a file or device

       #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);

       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.  This entry records the file offset
       and the file status flags (modifiable via the fcntl(2) F_SETFL
       operation).  A file descriptor is a reference to one of these entries;
       this reference is unaffected if pathname is subsequently removed or
       modified to refer to a different file.  The new open file description
       is initially not shared with any other process, but sharing may arise
       via fork(2).

       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_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 using fcntl(2).  The full list of file creation flags and file
       status flags is as follows:

              The file is opened in append mode.  Before each write(2), the
              file offset is positioned at the end of the file, as if with
              lseek(2).  O_APPEND may lead to corrupted files on NFS file
              systems 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

       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.
              Additionally, use of this flag is essential in some
              multithreaded programs since 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 at the
              same time as another thread does a fork(2) plus execve(2).

              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 file system 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 is
              specified in flags; if O_CREAT is not 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

              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

       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 is Linux-specific, and 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_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 file system (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 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 file systems.  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 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_NOPATH below.

              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 file-system 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)) fail with
              the error EBADF.

              The following operations can be performed on the resulting file

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

              *  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 O_PATH.

              *  Passing the file descriptor as the dirfd argument of
                 openat(2) and the other "*at()" system calls.

              *  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_DIRECTORY and O_NOFOLLOW are ignored.

              If 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 The file is opened for synchronous I/O.  Any write(2)s on the
              resulting file descriptor will block the calling process until
              the data has been physically written to the underlying hardware.
              But see NOTES below.

              If the file already exists and is a regular file and the open
              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.

       Some of these optional flags can be altered using fcntl(2) after the
       file has been opened.

       creat() is equivalent to open() with flags equal to

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

       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

       EDQUOT Where O_CREAT is specified, the file does not exist, and the
              user's quota of disk blocks or inodes on the file system 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).

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

       ELOOP  Too many symbolic links were encountered in resolving pathname,
              or O_NOFOLLOW was specified but pathname was a symbolic link.

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

              pathname was too long.

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

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

       ENOENT O_CREAT is not set and the named file does not exist.  Or, a
              directory component in pathname does not exist or is a dangling
              symbolic link.

       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 file open for reading.  Or, the file is a device
              special file and no corresponding device exists.

              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

       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 file system and write
              access was requested.

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

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

       and O_PATH flags are Linux-specific, and one may need to define
       _GNU_SOURCE (before including any header files) to obtain their

       The O_CLOEXEC flag is not specified in POSIX.1-2001, but is specified
       in POSIX.1-2008.

       O_DIRECT is not specified in POSIX; one has to define _GNU_SOURCE
       (before including any header files) to get its definition.

       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).

       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.

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

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

       POSIX provides for three different variants of synchronized I/O,
       corresponding to the flags O_SYNC, O_DSYNC, and O_RSYNC.  Currently
       (2.6.31), Linux implements only O_SYNC, but glibc maps O_DSYNC and
       O_RSYNC to the same numerical value as O_SYNC.  Most Linux file systems
       don't actually implement the POSIX O_SYNC semantics, which require all
       metadata updates of a write to be on disk on returning to user space,
       but only the O_DSYNC semantics, which require only actual file data and
       metadata necessary to retrieve it to be on disk by the time the system
       call returns.

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

       On NFS file systems 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

       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 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 file system and kernel version and might
       be absent entirely.  However there is currently no file
       system-independent interface for an application to discover these
       restrictions for a given file or file system.  Some file systems
       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 file systems 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 file system 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 behaviour of O_DIRECT with NFS will differ from local file systems.
       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

       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.

       chmod(2), chown(2), close(2), dup(2), fcntl(2), link(2), lseek(2),
       mknod(2), mmap(2), mount(2), openat(2), read(2), socket(2), stat(2),
       umask(2), unlink(2), write(2), fopen(3), fifo(7), path_resolution(7),
       symlink(7), blockdev(8)

       This page is part of release 3.53 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at

Linux                             2013-07-21                           OPEN(2)