From:	Eric Backus <ericb AT lsid DOT hp DOT com>
Subject:	Re: Pre-processor bug (?)
To:	djgpp AT sun DOT soe DOT clarkson DOT edu (djgpp)
Date:	Tue, 21 Apr 92 17:51:22 PDT
Mailer:	Elm [revision: 66.25]
Status:	O

Laurent Duperval writes:
> I get the following errors:
> 
> cmd.c: In function `lowc':
> cmd.c:819: argument `sym' doesn't match function prototype
> cmd.c:819: a formal parameter type that promotes to `int'
> cmd.c:819: can match only `int' in the prototype
  ... more of the same ...
> 
> The only way I have found, so far, to correct the problem is to rewrite
> the definitions and the headers as standard C, i.e:
> 
> int movecmd(char);
> int movecmd(char c)
> {
> 	function body
> }
> 
> The same code compiles under BC++ 2.0 but not under DJGPP 1.06.  Exactly
> what is wrong?

What is wrong is pretty much what the compiler says is wrong.

In traditional non-ANSI C, a function that declared a parameter as
"char" or "short" really expected to get an "int" passed to it, and
the function did the necessary conversion of the int to whatever it
wanted.  The caller of the function knew to promote a "char" or
"short" parameter to "int" before pushing it on the stack, so
everything worked out.

In ANSI C, a function can use a new-style function declaration to
specify that it really wants the unpromoted "char" or "short"
parameter.  And a new-style function prototype can tell the caller of
the function that the function doesn't want a "char" or "short"
parameter promoted to "int".

The problem comes when there is a new-style function prototype (which
tells the caller to push a char or short), but the function itself is
declared in the old-style that doesn't have the parameter types inside
the parentheses.  The caller then pushes a "char" or "short", but the
function itself is expecting to receive an "int" on the stack.

The compiler could deal with this, but this is not correct code
according to ANSI C, and this is why the compiler is giving you the
error.  Apparently BC++ 2.0 works around this coding error without
telling you about it.

Moral: don't mix old-style function declaration with new-style
function prototypes.
--
				Eric Backus
				ericb%hplsla AT hplabs DOT hp DOT com
				(206) 335-2495

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