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OPEN(2)                   FreeBSD 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 argument may indicate the file is to be cre-
     ated if it does not exist (by specifying the O_CREAT flag).  In this case
     open() requires a third argument mode_t mode, and the file is created
     with mode mode as described in chmod(2) and modified by the process'
     umask value (see umask(2)).

     The flags specified are formed by or'ing the following values

           O_RDONLY        open for reading only
           O_WRONLY        open for writing only
           O_RDWR          open for reading and writing
           O_NONBLOCK      do not block on open
           O_APPEND        append on each write
           O_CREAT         create file if it does not exist
           O_TRUNC         truncate size to 0
           O_EXCL          error if create and file exists
           O_SHLOCK        atomically obtain a shared lock
           O_EXLOCK        atomically obtain an exclusive lock
           O_DIRECT        eliminate or reduce cache effects
           O_FSYNC         synchronous writes
           O_NOFOLLOW      do not follow symlinks

     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 imple-
     ment a simple exclusive access locking 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 and the open() system call would result in
     the process being blocked for some reason (e.g., waiting for carrier on a
     dialup line), open() returns immediately.  The descriptor remains in non-
     blocking mode for subsequent operations.

     If O_FSYNC is used in the mask, all writes will immediately be written to
     disk, the kernel will not cache written data and all writes on the
     descriptor will not return until the data to be written completes.

     If O_NOFOLLOW is used in the mask and the target file passed to open() is
     a symbolic link then the open() will fail.

     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 file system supports locking).

     O_DIRECT may be used to minimize or eliminate the cache effects of read-
     ing and writing.  The system will attempt to avoid caching the data you
     read or write.  If it cannot avoid caching the data, it will minimize the
     impact the data has on the cache.  Use of this flag can drastically
     reduce performance if not used with care.

     If successful, open() returns a non-negative integer, termed a file
     descriptor.  It returns -1 on failure.  The file pointer used to mark the
     current position 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.  The getdtablesize(2) system call returns the
     current system limit.

RETURN VALUES
     If successful, open() returns a non-negative integer, termed a file
     descriptor.  It returns -1 on failure, and sets errno to indicate the
     error.

ERRORS
     The named file is opened unless:

     [ENOTDIR]          A component of the path prefix is not a directory.

     [ENAMETOOLONG]     A component of a pathname exceeded 255 characters, or
                        an entire path name exceeded 1023 characters.

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

     [ENOENT]           A component of the path name that must exist does not
                        exist.

     [EACCES]           Search permission is denied for a component of the
                        path prefix.

     [EACCES]           The required permissions (for reading and/or writing)
                        are denied for the given flags.

     [EACCES]           O_CREAT is specified, the file does not exist, and the
                        directory in which it is to be created does not permit
                        writing.

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

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

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

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

     [ENFILE]           The system file table is full.

     [EMLINK]           O_NOFOLLOW was specified and the target is a symbolic
                        link.

     [ENXIO]            The named file is a character special or block special
                        file, and the device associated with this special file
                        does not exist.

     [ENXIO]            The named file is a fifo, no process has it open for
                        reading, and the arguments specify it is to be opened
                        for writing.

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

     [EOPNOTSUPP]       O_SHLOCK or O_EXLOCK is specified but the underlying
                        file system does not support locking.

     [EOPNOTSUPP]       The named file is a special file mounted through a
                        file system that does not support access to it (e.g.
                        NFS).

     [EWOULDBLOCK]      O_NONBLOCK and one of O_SHLOCK or O_EXLOCK is speci-
                        fied and the file is locked.

     [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.

     [ENOSPC]           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.

     [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.

     [EDQUOT]           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.

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

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

     [EFAULT]           The path argument points outside the process's allo-
                        cated address space.

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

     [EOPNOTSUPP]       An attempt was made to open a socket (not currently
                        implemented).

     [EINVAL]           An attempt was made to open a descriptor with an ille-
                        gal combination of O_RDONLY, O_WRONLY, and O_RDWR.

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

HISTORY
     The open() function appeared in Version 6 AT&T UNIX.

FreeBSD 6.2                    November 16, 1993                   FreeBSD 6.2

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY

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