Re: about including header files

[Axel Freyn <axel-freyn at gmx dot de>]

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.
    - 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 ?

Axel