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From: sjc@arthur.cs.purdue.edu (Steve Chapin)
Newsgroups: sci.lang,comp.lang.misc
Subject: Re: Using C to describe itself
Message-ID: <1286@arthur.cs.purdue.edu>
Date: Fri, 24-Apr-87 14:55:18 EDT
Article-I.D.: arthur.1286
Posted: Fri Apr 24 14:55:18 1987
Date-Received: Sun, 26-Apr-87 20:18:07 EDT
References: <437@haddock.UUCP} <1829@enea.UUCP> <440@haddock.UUCP> <7360@boring.mcvax.cwi.nl>
Organization: Department of Computer Science, Purdue University
Lines: 37
Summary: Wrong answer...
Xref: mnetor sci.lang:536 comp.lang.misc:364

In article <7360@boring.mcvax.cwi.nl}, steven@mcvax.cwi.nl (Steven Pemberton) writes:
} In article <1403@uwmacc.UUCP> edwards@unix.macc.wisc.edu.UUCP (mark edwards) writes:
} >   Yes indeed C can describe itself.
} 
} Actually, I'm not completely convinced of this. The question of whether C
} can be used to describe itself or not seems to be the same as whether the
} sentence "This sentence is true" is true or false: it depends on your
} assumptions.
} 
} Evidence of this is in Dennis Ritchie's Turing award lecture: he had built a
} trojan horse into the C compiler. The compiler looked identical to its
} previous self, but actually compiled a (slightly) different language. This
} suggests to me that you can't look at the compiler source to determine what
} a certain construct in C means, even if you know the meaning of the language
} it compiles to.
} 
} Steven Pemberton, CWI, Amsterdam; steven@cwi.nl or try steven@mcvax.uucp

If I recall correctly, he first wrote a compiler that on a special
string input, output a binary that did NOT correctly match the
source code.  Then he fed it a "clean" C compiler, and it output
that C compiler with the additional property that it also contained
the trojan horse.  Thus, as long as you kept compiling the C
compiler with the C compiler, you would get a trojan horse C,
which would do a couple of interesting things when fed the right
input file.

But, the point is, that the original compiler and all its
descendants were NOT (!!!!!) actual C compilers, since the
did NOT correctly transform all proper inputs into equivalent
outputs.  So, this example is not relevant.

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