CLHS: Issue CLOSED-STREAM-OPERATIONS Writeup

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## Issue CLOSED-STREAM-OPERATIONS Writeup

    Status: Passed Jan 89 X3J13 with amendment to specify that CLOSE on a
    closed stream returns NIL. At Mar 89 X3J13, amendment was withdrawn;
    this version stands.

    Forum:          Cleanup
    Issue:          CLOSED-STREAM-OPERATIONS
    References:     CLOSE (CLtL p 332)
    Category:       CLARIFICATION
    Edit history:   26-Aug-88, Version 1 by Chapman
                     8-Oct-88, Version 2 by Masinter
                    13-Oct-88, Version 3 by van Roggen
                     1-Dec-88, Version 4 by Pitman
                     5-Dec-88, Version 5 by Masinter (separate other issues)
    Related Issues: STREAM-ACCESS, STREAM-INFO, INPUT-STREAM-P-CLOSED,
    			CLOSE-CONSTRUCTED-STREAMS, PATHNAME-STREAM

    Problem Description:

     The description of CLOSE is not completely clear about the functions
     which are allowed to be performed on a closed stream.

     On p332 it says:

      ``The stream is closed. No further Input/output operations may be
        performed on it. However, certain inquiry operations may still
        be performed, ...''

      but the list of inquiry operations is not specified.

    Proposal (CLOSED-STREAM-FUNCTIONS:ALLOW-INQUIRY)

     Clarify the behavior of the following functions on closed streams:

      * STREAMP is unaffected by whether its stream argument is open or closed.

      * If CLOSE is called on a stream which is open, it will return T.
        However, if CLOSE is called on a stream which is closed, it
        will succeed without error but the return value is not specified.

      * PATHNAME is valid on either an open or closed stream. Since some
        implementations cannot provide the truename of a file until the
        file is closed, it would in principle be possible for PATHNAME in
        some implementations to return more specific information after the
        stream is closed. For consistency, however, PATHNAME is prohibited
        from doing this. PATHNAME must return the same pathname after a
        file is closed as it did before. (The passed proposal in issue
        PATHNAME-STREAM still stands.)

      * TRUENAME is valid on either an open or closed stream. Since some
        implementations cannot provide the truename of a file until the
        file is closed, it is permissible TRUENAME to return more specific
        information after the stream is closed.

      * MERGE-PATHNAMES, PATHNAME-HOST, PATHNAME-DEVICE, PATHNAME-DIRECTORY,
        PATHNAME-NAME, PATHNAME-TYPE, PATHNAME-VERSION, NAMESTRING,
        FILE-NAMESTRING, DIRECTORY-NAMESTRING, HOST-NAMESTRING,
        ENOUGH-NAMESTRING, and OPEN are valid on either open or closed streams.
        For any of these operations, using a stream, S, as an argument
        where appropriate is equivalent to using (PATHNAME s). See the
        description of PATHNAME above to understand the consequences of this.

      * PROBE-FILE and DIRECTORY are valid on either open or closed streams.
        For either of these operations, using a stream, S, as an argument
        where appropriate is equivalent to using (PATHNAME s). See the
        description of PATHNAME above to understand the consequences of this.
        In this case of these operators however, closed stream may well be the
        most reliable arguments in some cases, since treatment of open streams
        to the file system may vary considerably between implementations.
        For example, in some operating systems, open files are written under
        temporary names and not renamed until close and/or are held invisible
        until a close is performed. In general, any code with an intent to be
        highly portable should tread lightly when using PROBE-FILE or
        DIRECTORY.

    Rationale:

     One can consider many characteristics of a stream to be independent of
     the ability to do I/O.  Being able to determine a stream's direction and
     its name is often useful for debugging.  A number of the descriptions in
     CLtL imply (weakly) the ability to work on closed streams.  Functions
     such as OPEN and DIRECTORY don't really depend on the stream, but on
     the name of the stream.

    Current Practice:

     At least two implementations differ in which functions are allowed to be
     performed on a closed stream.

    Cost to Implementors:

     Unknown, but likely to be small in most implementations.

     A nontrivial amount of work may be necessary if the pathname information
     is held  externally and is normally deleted when the stream is closed.
     The implementation will have to copy the information at some time for later
     inquiries.

    Cost to Users:

     Likely to be small; users of an implementation forced to change
     by this proposal might have to make some modifications to make their
     programs portable.

    Benefits:

     These clarifications will assist users in writing portable code.

    Aesthetics:

     Most people will probably see these clarifications as an improvement
     in aesthetics.

    Discussion:

     There are some separate, but related, issues regarding what CLOSE
     should do on composite streams or constructed streams such as
     created by MAKE-BROADCAST-STREAM. These issues will be addressed
     in a separate issue (CLOSE-CONSTRUCTED-STREAMS).

     There was some discussion on whether INPUT-STREAM-P and OUTPUT-STREAM-P
     should return "false" on a stream that had been closed. The issue
     STREAM-ACCESS contains a proposal to add a function OPEN-STREAM-P
     which might be useful for the same purpose. This issue was separated
     out into a separate issue (INPUT-STREAM-P-CLOSED).

     The other functions in proposal STREAM-ACCESS:PROVIDE should
     work on closed streams.

     The functions in STREAM-INFO:ONE-DIMENSIONAL-FUNCTIONS should
     not be requred to work on closed streams.

         ----- End Forwarded Messages -----

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