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, mode_t mode);

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, in which case the
                       file is created with mode mode 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_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 operations:
                       if specified together with O_SYNC, each read will wait
                       for the file status to be committed to stable storage.

                       Combining O_RSYNC with O_DSYNC only, or specifying it
                       without any other synchronized I/O integrity completion
                       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 condi-
                       tions 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.

     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:

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

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

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

     [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.

     [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.

     [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.

     [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.

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

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

     [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() call requests write
                        access.

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

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

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

NetBSD 5.0_STABLE              October 23, 2006              NetBSD 5.0_STABLE
