_open()                 Open a File For Reading or Writing

 #include   <fcntl.h>
 #include   <io.h>

 int        _open(pathname,access);
 const char *pathname;                   Name of file
 int        access;                      Access mode

    _open() opens the file specified by 'pathname', then prepares the
    file for reading and/or writing as defined by 'access'. The value of
    'access' is constructed by combining one of the following constants:

          O_RDONLY          Open for reading only
          O_WRONLY          Open for writing only
          O_RDWR            Open for reading and writing

    with the following additional values (available with dos DOS 3.0 and
    later):

          O_NOINHERIT       File cannot be passed to child programs
          O_DENYALL         Only the current handle has access to the file
          O_DENYWRITE       Any other open is allowed reads only
          O_DENYREAD        Any other open is allowed writes only
          O_DENYNONE        Other shared opens to the file are allowed

    Under DOS 3.0 and later, only one of the O_DENYxxx values can be
    included in a single _open().  These attributes are separate from any
    file-locking operations.

       Returns:     On success, the file handle (a non-negative integer)
                    is returned and the file pointer to the current
                    position is set to the beginning of the file.  On
                    error, -1 is returned and 'errno' (defined in
                    <errno.h>) is set to one of the following:

                    ENOENT    Path or file name not found
                    EMFILE    Too many open files
                    EACCES    Permission denied
                    EINVACC   Invalid access code