Re: about including header files

[Andrew Haley <aph at redhat dot com>]

Axel Freyn wrote:
> On Tue, Nov 10, 2009 at 11:07:31AM +0000, Andrew Haley wrote:
> > yacson wrote:
> > > Hi everybody.
> > > what happens if a "header include" is missing? because I unexpectedly notice
> > > that  this code:
> > > main()
> > > {printf("hello");}
> > >
> > > works in linux(at least on my (mandriva spring 2007 )box) while it does not
> > > in windows(turbo C++).
> > > so finally, can I say :  #include <stdio.h> is not always necessary!
> > Strictly speaking, I think this is undefined behaviour.  So, while it
> > might work, it's not correct code, and it may break at any time.
> I think this are two questions mixed up:
> a) in this example, the function "printf" is not explicitely defined.
>    And implicit function declaration were allowed in the C 89, but
>    aren't permitted in C 99. So you have to decide which version of the
>    C standard you are writing in:-)
>    But according to my understanding of the standard, even looking at
>    C89, the behaviour of your code is undefined, as implicit function
>    declaration is only allowed for functions having a fixed number of
>    parameters - and that's not the case for printf. 
>    So I would say:
>     - for C99: it is wrong, the compiler should not translate the code.

That's not quite true.  The compiler must issue a diagnostic, but whether
or not it translates the code is up to the compiler.

Note that gcc does issue a diagnostic:

ped.c:2: warning: incompatible implicit declaration of built-in function 'printf'

>     - for C89: the behaviour is undefined, the compiler can do what he
>       wants :-)
>
> b) The second question is: do you need to include stdio.h in order to
>    declare printf - or is it sufficient to declare printf yourself?
>    According to §7.1.4.2, it's not necessary to include stdio.h: If
>    it is possible to declare a function from the standard library
>    without using a type defined in a header, it is "permissible" to
>    declare the function an use it - witout including the standard
>    header: "printf" beeing declared as 
>        int printf(const char * restrict format, ...);
>    it is sufficient to declare "printf" in your source code.
>
> So, in my opinion, the code 
>
> int printf(const char * format, ...);
> main()
> {printf("hello"); }
>
> should be perfectly valid and work with all C89 or C99 compilers -
> without the inclusion of stdio.h ?

Yes.

Andrew.