Re: keyword pairs and OPEN

[HEDRICK at RUTGERS (Mgr DEC-20s/Dir LCSR Comp Facility)]

Here is an attempt to flesh out the meanings of the flags you mentioned.

    :ACCESS
	This flag indicates what kind of I/O you intend to do with the
	file.  Unless otherwise specified, I/O operations will begin
	at the beginning of the file.
	  input - you are only going to read from it.
	  output - you want to start with an empty file, and write new
		data to it.  (Some operating systems allow two modes of
		writing to a file, one of which clears the file beforehand
		and one of which does not.  Typically "output" means that
		you want a file which is clear.  Indeed the alternative
		may be so bizaare that we don't even want to worry about
		it.)
	  write-only - you are only going to write to it, but any existing
		data should not be cleared.  (This is the one that is so
		bizaare you may choose to ignore it.  Probably it would be
		enough to have update mode.)
	  append - like write-only, but if the file already exists,
		writing will start from the end of the existing data.
		(Logically, this is not required, as it is equivalent to
		a write-only open followed by random access to the end
		of file. However a number of operating systems make
		special provisions for append access to files where you
		would not normally be able to open for output and then
		position to the end of file.  Thus it is a useful
		distinction.) 
	  update - you are going to do both input and output.

    :RECOG
	This flag indicates attributes of the file that may help in
	finding it.
	  old - file must exist, the existing version is used
	  new - file must not exist, it is created
	  old-version - file need not exist.  If it does, the existing
		version is used.  If it does not, the file is created.
	  new-version - file need not exist.  If it does, a new version
		is created.  If it does not, the file is created.
	Lest you think I have designed this specifically for Tops-20,
	which allows multiple versions, these distinctions are also
	meaningful on Tops-10, which does not.  On Tops-10, if the file
	exists, old-version would update the existing file and
	new-version would supercede it with a new one.

	Defaults are:
	  input - old
	  output - new-version
	  write-only - old-version
	  append - old-version
	  update - old-version

	Certain combinations are meaningless.  Others may not be allowed
	by the operating system.  Exactly which combinations are allowed
	is implementation-dependent.  However every implementation is
	required to support the following combinations
	  input, old
	  output, new-version
	and all implementations are strongly urged to implement all 5
	default combinations, plus append/old and update/old.

    :RANDOM-ACCESS 
	This flag indicates that the file will be accessed in some
	manner other than sequentially. I don't need it for Tops-20, but
	I think some operating systems may.  In my opinion we should 
	include it if any system of interest requires it.  Does any?

    :IMAGE
	Most runtime systems end up doing various character processing
	such as removing null characters, turning CRLF into LF, removing
	the parity bit, turning control characters into representations
	with ^, turning two-character CDC "ASCII" into actual ASCII (if
	anyone is going to do a CDC implementation). This turns all of
	that off and give you characters unchanged.  No doubt
	implementors will choose their own extensions to control
	behavior appropriate for the operating system, but this one
	option seems generally useful enough to put in the language.

INPUT, NEW - this is generally meaningless.  Some implementations
would probably create a zero-length file and give immediate end of
file.  Others might call it an error.  (Do we want to try to define
exactly what happens?  Does anyone understand I/O well enough to try
to make a model at that level of detail?)

OUTPUT, OLD - this one supercedes an existing file.  It guarantees
that the file must exist.  It is possible that some implementations
would not allow it, but I would think it would be meaningful for most.

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