Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!mips!news.cs.indiana.edu!att!att!dptg!ulysses!andante!alice!ark
From: ark@alice.att.com (Andrew Koenig)
Newsgroups: comp.lang.c
Subject: Re: Is this a valid ANSI program?
Message-ID: <20461@alice.att.com>
Date: 26 Jun 91 18:41:19 GMT
References: <609@mtndew.Tustin.CA.US>
Reply-To: ark@alice.UUCP ()
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 37

In article <609@mtndew.Tustin.CA.US> friedl@mtndew.Tustin.CA.US (Stephen J. Friedl) writes:

> 	void foo(const char **xxx) { }

> 	main()
> 	{
> 	char	**p = 0;

> 		foo(p);
> 	}

> The compiler claims that the argument /p/ to the function foo()
> is incompatible with the prototype, and I just don't believe it.

The compiler is correct: you cannot convert char ** to const char ** .
To see why, consider the following program fragment:

	main()
	{
		const char victim = '?';
		char *accomplice;
		const char **sneak = &accomplice;	/* */
		*sneak = &victim;
		*accomplice = '!';
	}

If char ** could be converted to const char **, the line with the
comment would be legal.  None of the other lines is the least bit
controversial; the declarations are surely beyond reproach,
the assignment to *sneak places the address of a "const char"
into a "const char *", and the assignment to *accomplice
assigns an "int" to a "char".  Thus allowing the commented line
would open a hole in the type system by allowing you to modify
a "const char" object without an explicit cast.
-- 
				--Andrew Koenig
				  ark@europa.att.com