OPEN(2)                   NetBSD System Calls Manual                   OPEN(2)

NAME
     open -- open or create a file for reading or writing

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <fcntl.h>

     int
     open(const char *path, int flags, ...);

DESCRIPTION
     The file name specified by path is opened for reading and/or writing as
     specified by the argument flags and the file descriptor returned to the
     calling process.  The flags are specified by or'ing the values listed
     below.  Applications must specify exactly one of the first three values
     (file access methods):

           O_RDONLY     Open for reading only.

           O_WRONLY     Open for writing only.

           O_RDWR       Open for reading and writing.

     Any combination of the following may be used:

           O_NONBLOCK   Do not block on open or for data to become available.

           O_APPEND     Append to the file on each write.

           O_CREAT      Create the file if it does not exist.  The third argu-
                        ment of type mode_t is used to compute the mode bits
                        of the file as described in chmod(2) and modified by
                        the process' umask value (see umask(2)).

           O_TRUNC      Truncate size to 0.

           O_EXCL       Error if O_CREAT and the file already exists.

           O_SHLOCK     Atomically obtain a shared lock.

           O_EXLOCK     Atomically obtain an exclusive lock.

           O_NOFOLLOW   If last path element is a symlink, don't follow it.
                        This option is provided for compatibility with other
                        operating systems, but its security value is question-
                        able.

           O_CLOEXEC    Set the close(2) on exec(3) flag.

           O_NOSIGPIPE  Return EPIPE instead of raising SIGPIPE.

           O_DSYNC      If set, write operations will be performed according
                        to synchronized I/O data integrity completion: each
                        write will wait for the file data to be committed to
                        stable storage.

           O_SYNC       If set, write operations will be performed according
                        to synchronized I/O file integrity completion: each
                        write will wait for both the file data and file status
                        to be committed to stable storage.

           O_RSYNC      If set, read operations will complete at the same
                        level of integrity which is in effect for write opera-
                        tions: if specified together with O_SYNC, each read
                        will wait for the file status to be committed to sta-
                        ble storage.

                        Combining O_RSYNC with O_DSYNC only, or specifying it
                        without any other synchronized I/O integrity comple-
                        tion flag set, has no further effect.

           O_ALT_IO     Alternate I/O semantics will be used for read and
                        write operations on the file descriptor.  Alternate
                        semantics are defined by the underlying layers and
                        will not have any alternate effect in most cases.

           O_NOCTTY     If the file is a terminal device, the opened device is
                        not made the controlling terminal for the session.
                        This flag has no effect on NetBSD, since the system
                        defaults to the abovementioned behaviour.  The flag is
                        present only for standards conformance.

           O_DIRECT     If set on a regular file, data I/O operations will not
                        buffer the data being transferred in the kernel's
                        cache, but rather transfer the data directly between
                        user memory and the underlying device driver if possi-
                        ble.  This flag is advisory; the request may be per-
                        formed in the normal buffered fashion if certain con-
                        ditions are not met, e.g. if the request is not suffi-
                        ciently aligned or if the file is mapped.

                        To meet the alignment requirements for direct I/O, the
                        file offset, the length of the I/O and the address of
                        the buffer in memory must all be multiples of
                        DEV_BSIZE (512 bytes).  If the I/O request is made
                        using an interface that supports scatter/gather via
                        struct iovec, each element of the request must meet
                        the above alignment constraints.

           O_DIRECTORY  Fail if the file is not a directory.

           O_ASYNC      Enable the SIGIO signal to be sent to the process
                        group when I/O is possible, e.g., upon availability of
                        data to be read.

     Opening a file with O_APPEND set causes each write on the file to be
     appended to the end.  If O_TRUNC is specified and the file exists, the
     file is truncated to zero length.

     If O_EXCL is set with O_CREAT and the file already exists, open() returns
     an error.  This may be used to implement a simple exclusive access lock-
     ing mechanism.  If O_EXCL is set and the last component of the pathname
     is a symbolic link, open() will fail even if the symbolic link points to
     a non-existent name.

     If the O_NONBLOCK flag is specified, do not wait for the device or file
     to be ready or available.  If the open() call would result in the process
     being blocked for some reason (e.g., waiting for carrier on a dialup
     line), open() returns immediately.  This flag also has the effect of mak-
     ing all subsequent I/O on the open file non-blocking.

     When opening a file, a lock with flock(2) semantics can be obtained by
     setting O_SHLOCK for a shared lock, or O_EXLOCK for an exclusive lock.
     If creating a file with O_CREAT, the request for the lock will never fail
     (provided that the underlying filesystem supports locking).

     If open() is successful, the file pointer used to mark the current posi-
     tion within the file is set to the beginning of the file.

     When a new file is created it is given the group of the directory which
     contains it.

     The new descriptor is set to remain open across execve(2) system calls;
     see close(2) and fcntl(2).

     The system imposes a limit on the number of file descriptors open simul-
     taneously by one process.  Calling getdtablesize(3) returns the current
     system limit.

RETURN VALUES
     If successful, open() returns a non-negative integer, termed a file
     descriptor.  Otherwise, a value of -1 is returned and errno is set to
     indicate the error.

ERRORS
     The named file is opened unless:

     [EACCES]           Search permission is denied for a component of the
                        path prefix, the required permissions (for reading
                        and/or writing) are denied for the given flags, or
                        O_CREAT is specified, the file does not exist, and the
                        directory in which it is to be created does not permit
                        writing.

     [EDQUOT]           O_CREAT is specified, the file does not exist, and the
                        directory in which the entry for the new file is being
                        placed cannot be extended because the user's quota of
                        disk blocks on the file system containing the direc-
                        tory has been exhausted; or O_CREAT is specified, the
                        file does not exist, and the user's quota of inodes on
                        the file system on which the file is being created has
                        been exhausted.

     [EEXIST]           O_CREAT and O_EXCL were specified and the file exists.

     [EFAULT]           path points outside the process's allocated address
                        space.

     [EFTYPE]           O_NOFOLLOW was specified, but the last path component
                        is a symlink.  Note: IEEE Std 1003.1-2008
                        (``POSIX.1'') specifies returning [ELOOP] for this
                        case.

     [EINTR]            The open() operation was interrupted by a signal.

     [EIO]              An I/O error occurred while making the directory entry
                        or allocating the inode for O_CREAT.

     [EISDIR]           The named file is a directory, and the arguments spec-
                        ify it is to be opened for writing.

     [ELOOP]            Too many symbolic links were encountered in translat-
                        ing the pathname.

     [EMFILE]           The process has already reached its limit for open
                        file descriptors.

     [ENAMETOOLONG]     A component of a pathname exceeded {NAME_MAX} charac-
                        ters, or an entire path name exceeded {PATH_MAX} char-
                        acters.

     [ENFILE]           The system file table is full.

     [ENOENT]           O_CREAT is not set and the named file does not exist,
                        or a component of the path name that must exist does
                        not exist.

     [ENOSPC]           O_CREAT is specified, the file does not exist, and the
                        directory in which the entry for the new file is being
                        placed cannot be extended because there is no space
                        left on the file system containing the directory; or
                        O_CREAT is specified, the file does not exist, and
                        there are no free inodes on the file system on which
                        the file is being created.

     [ENOTDIR]          A component of the path prefix is not a directory; or
                        O_DIRECTORY is specified and the last path component
                        is not a directory.

     [ENXIO]            The named file is a character special or block special
                        file, and the device associated with this special file
                        does not exist, or the named file is a FIFO,
                        O_NONBLOCK and O_WRONLY is set and no process has the
                        file open for reading.

     [EOPNOTSUPP]       O_SHLOCK or O_EXLOCK is specified but the underlying
                        filesystem does not support locking; or an attempt was
                        made to open a socket (not currently implemented).

     [EPERM]            The file's flags (see chflags(2)) don't allow the file
                        to be opened.

     [EROFS]            The named file resides on a read-only file system, and
                        the file is to be modified.

     [ETXTBSY]          The file is a pure procedure (shared text) file that
                        is being executed and the open() call requests write
                        access.

SEE ALSO
     chmod(2), close(2), dup(2), lseek(2), read(2), umask(2), write(2),
     getdtablesize(3)

STANDARDS
     The open() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1'').  The
     flags values O_DSYNC, O_SYNC and O_RSYNC are extensions defined in IEEE
     Std 1003.1b-1993 (``POSIX.1'').

     The O_SHLOCK and O_EXLOCK flags are non-standard extensions and should
     not be used if portability is of concern.

HISTORY
     An open() function call appeared in Version 2 AT&T UNIX.

NetBSD 6.0                     January 23, 2012                     NetBSD 6.0

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