Lstreams and Lisp

Why does it say /* #define CHECK_LSTREAM(x) CHECK_RECORD (x, lstream) Lstream pointers should never escape to the Lisp level, so functions should not be doing this. */ in lstream.h? The reason I'm wondering is that I'd like to make my encode-binary and decode-binary functions work with arbitrary output sinks/input sources, so the obvious implementation is to create a suitable Lstream within the Lisp-visible functions. The trouble is that I want the interface to the functions to include the facility to add user-defined conversions, which means that a Lisp function may have to accept an Lstream parameter... so I'm wondering whether there was any reason for this comment ;-) Just in case you're interested, here's the interface I'm proposing (there'll be an additional encode-binary-string function that works in an efficient way). The STREAM parameter could accept any Lisp object for which an Lstream can be created. DEFUN ("encode-binary-stream", Fencode_binary_stream, 3, 3, 0, /* Encode the sequence DATA into a binary STREAM using the specified binary FORMAT vector. Each element of the FORMAT vector should either be a symbol, or a list of the form (SYMBOL PARAMETER...). SYMBOL may be one of binary string bit-vector integer float space vector or alternatively the name of a Lisp function that will be called with the remaining data, the output stream and a list of PARAMETER values as its arguments. i.e. it's declaration should look something like the following (defun my-conversion data stream parameter-list ... ) and it will be called using (my-conversion data stream parameter-list) Such a function should return the remaining data after it has consumed whatever it required. The built-in encodings support the following parameters: Encoding Parameters binary :length string :length :pad :terminator bit-vector :length :direction integer :length :signed :direction float :length :format :direction space :length vector FORMAT-ELT :length :pack where FORMAT-ELT is anything that could be an element of the FORMAT parameter. :length is followed by a length in bytes (or in bits for bit-vector). :pad is followed by a character used to pad the string to the specified length. :terminator is followed by a character used to terminate the string. :direction is followed by one of `big-endian', `little-endian', `host' or `network'. :signed is followed by t or nil. :format is followed by `native'. Conversions to other floating point formats are currently not supported. :pack is followed by an integer specifying the vector stride (e.g. the format [(vector (integer :length 2) :pack 4)] represents an array of 16-bit integers, but with a gap of 2 bytes between successive elements). The function returns a string containing the raw binary data. */ (format, data, stream)) DEFUN ("decode-binary-stream", Fdecode_binary_stream, 2, 2, 0, /* Decode the specified STREAM using the binary FORMAT vector. See `encode-binary-stream' for more information on the FORMAT vector; note however that user-defined conversion functions should be declared as (defun my-conversion stream parameter-list ...) and should return the data they have converted. */ (format, stream)) Kind Regards, Alastair. ____________________________________________________________ Alastair Houghton ajhoughton(a)lineone.net