Re: Reading from binary input streams??

[Bruno Haible <haible@ilog.fr>]

Bruce L. Lambert writes:
> but when I say
> 
> (with-open-file (istream "filename" :direction :input :element-type
>   '(signed-byte 4)) (read-byte istream))
> 
> My system (CLISP) says "filename is not an integer file."

Well. When CLISP opens a file with element-type (signed-byte 4), it does
not know whether the number of items in the file is even or odd - because
the system's notion of file length is measured in bytes, so you can't
decide from the file length whether it contains 2n or 2n-1 elements.

To overcome this, CLISP expects 4-byte header in front of the data, giving
length (in items) of the file.

> What do I do to read I this binary data?

Add a 4 byte header to the file, and put in the number of items ...
[looking deep into the source] in little-endian format.

> "common lisp currently specifies only a very
> simple faciltiy for binary input [output]" known as read-byte or write-byte
> respectively.

Now there is also `read-sequence' in ANSI CL, called `read-byte-sequence'
in CLISP. As with `read-byte' the translation unit can be 4-bit numbers.
if that's the way you opened the stream.


Bruno Haible                                     email: <haible@ilog.fr>
Software Engineer                                phone: +33-1-49083585