Re: GCC 4.5.0 Reports invalid warning

[Dave Korn <dave dot korn dot cygwin at gmail dot com>]

On 16/07/2010 01:31, J Decker wrote:
> Oh not so bad then, I can just add at the beginning...
> 
> typedef struct a *NeverUsedDefinition;
> 
> and now it's happy?  And that makes good coding how? 

  No, that would be bad coding.  Just forward-declare the tag:

struct a;

before you try and use it in a function's formal parameter list.

  The declarations in a function's formal parameter list are in a more inner
scope than file scope, so if there isn't a tag declaration at file scope yet,
you're talking about a new tag declaration limited only to the scope in which
it is declared - the function declaration (argument list and body, if present;
just argument list in the case of a prototype).

  When you later do declare a "struct a" tag at file scope, the later
definition of f() "inherits" that one from the more global scope, just like it
would "inherit" the name of a global variable from the enclosing scope.  In
the C language spec:

6.7.2.3#4: "All declarations of structure, union, or enumerated types that
have the same scope and use the same tag declare the same type."

6.7.2.3#5: "Two declarations of structure, union, or enumerated types which
are in different scopes or use different tags declare distinct types."


    cheers,
      DaveK