7.2 Input/Output system extensions

7.2.1 Implementation choices

• For the character and string types supported by DBCS (Double-Byte Character Set) Lisp, see the International Character Sets manual. This document also explains how DBCS affects character, string, and stream operations in Liquid Common Lisp.

• The Common Lisp functionopen exhibits the following behavior:
  • The(:direction :io) option supports one file position and one buffer. Writing and reading a character advance the same pointer. Thus, you can write a character and move the pointer back to read the character you just wrote.

  • Opening the same file multiple times returns logically independent streams with separate file pointers and separate buffering. In general, you should have a single opening for:output or:io; you should only use multiple openings for:input.

  • The functionopen always reads from the underlying file for input, but the output links for an existing file depend on the value of the:if-exists option as follows:
    • If the value is:overwrite, the existing file is used without changing the structure of any links. The file pointer is set to the beginning of the file. Output sent to the stream modifies the file.

    • If the value is:append, the existing file is used without changing the structure of any links. Output sent to the stream modifies the file. The file pointer is set to the end of the file.

    • If the value is:supersede, the specified file is replaced. When the file is opened, a temporary file is created and opened for output. When the stream is closed, the temporary file is renamed so that it replaces the specified file. Note that the symbolic link disappears when this happens.

    • If the value is:new-version, the implementation is the same as for:supersede because UNIX does not support version numbers. This value is the default.

    • If the value isnil,open fails; if a file of the specified name exists,nil is returned.

• The behavior of the Common Lisp functions clear-input and clear-output depend on whether the stream to be cleared can be set to a specified position:
  • If you can reset the file pointer to a stream by using the Common Lisp function file-position, these functions cannot clear any input or output data.

  • If you cannot reposition the stream,file-position returnsnil;clear-input andclear-output attempt to discard any unread input and buffered output respectively.

Thus, streams returned byopen can never be cleared, the standard input stream can always be cleared, and a stream returned by the functionmake-lisp-stream can be cleared only when the value of its:positionable keyword argument isnil.

• Liquid Common Lisp conforms to Chapter 22 of CLtL2. In particular, LCL now includes the macroprint-unreadable-object, the functionreadtable-case, and the variables*read-eval* and*print-readably*.

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