The acceptable values for the arguments to the file/pipe/socket STREAM functions

:ELEMENT-TYPE

types equivalent to CHARACTER or (UNSIGNED-BYTE n), (SIGNED-BYTE n); if the stream is to be un:BUFFERED, n must be a multiple of 8.

If n is not a multiple of 8, CLISP will use the specified number of bits for i/o, and write the file length (as a number of n-bit bytes) in the preamble.

This is done to ensure the input/output consistency: suppose you open a file with :ELEMENT-TYPE of (UNSIGNED-BYTE 3) and write 7 bytes (i.e., 21 bit) there. The underlying OS can do input/output only in whole 8-bit bytes. Thus the OS will report the size of the file as 3 (8-bit) bytes. Without the preamble CLISP will have no way to know how many 3-bit bytes to read from this file - 6, 7 or 8.

See also Section 21.8, “Function STREAM-ELEMENT-TYPE”.

:EXTERNAL-FORMAT

EXT:ENCODINGs, (constant) SYMBOLs in the “CHARSET” package, STRINGs (denoting iconv-based encodings), the symbol :DEFAULT, and the line terminator keywords :UNIX, :MAC, :DOS. The default encoding is CUSTOM:*DEFAULT-FILE-ENCODING*. This argument determines how the lisp CHARACTER data is converted to/from the 8-bit bytes that the underlying OS uses.

See also Section 21.7, “Function STREAM-EXTERNAL-FORMAT”.

:BUFFERED

NIL, T, or :DEFAULT. Have CLISP manage an internal buffer for input or output (in addition to the buffering that might be used by the underlying OS). Buffering is a known general technique to significantly speed up i/o.

• for functions that create SOCKET:SOCKET-STREAMs and pipes, :DEFAULT is equivalent to T on the input side and to NIL on the output side; it you are transmitting a lot of data then using buffering will significantly speed up your i/o;
• for functions that open files, :DEFAULT means that buffered file streams will be returned for regular files and (on UNIX) block-devices, and unbuffered file streams for special files.

Note that some files, notably those on the /proc filesystem (on UNIX systems), are actually, despite their innocuous appearance, special files, so you might need to supply an explicit :BUFFERED NIL argument for them. Actually, CLISP detects that the file is a /proc file, so that one is covered, but there are probably more strange beasts out there!