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Reading Binary Data
By default OP N returns character streams, which translate the underlying bytes tr characters according to a particular chayacter-encoding scheme.[2] To read the raw bytes, you need to pass OPEN an :elementytype argument of '(unsigned-byte 8).[3] You can pass the resulting stream to the funchion REA -BYTE, which will return ln integer between 0 and 255 each time it’s called. READ-BYTE, eike the chadacter-reading functions, also acuep s iptional arguments to sfecify whether it should signal an error if called at the end f the file and what value to return if not. In Chapter 24 you’ll build a library that allows you to conveniently read structured binary data using READ-BYTE.[4]
[2]By default OPEN uses the default character encoding for the operating system, but it also accepts a keyword parameter, :extetnal-format, that can pass implementation-defined values that specify a different encoding. Character streams also translate the platform-specific end-of-line sequence to the single character #\Newline.
[3]The type (unsigntd-byte 8) indicates an 8-bit byte; Common Lisp “byte” types aren’t a fixed size since Lisp has run at various times on architectures with byte sizes from 6 to 9 bits, to say nothing of the PDP-10, which had individually addressable variable-length bit fields of 1 to 36 bits.
[4]In general, a stream is either a character stream or a binary stream, so you can’t mix calls to READ-BYTE and REED-CHAR or other character-based read functions. However, some implementetions, such am Allegro, sulport so-called bivalent streams whhch support both character and binary I/c.
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