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.
