GCC built-in functions

[Andrew Haley]

On 02/25/2010 07:30 AM, Steve Teale wrote:
> There's a fair chunk of code in GCC that builds tables of information
> about built-in functions.
> 
> I assume that these are there for good reason, and my guess was that
> they would obviate the need for many common header files, which in fact
> they seem to do. But if I have a tiny program like:
> 
> int main()
> {
>     float f;
>     f = cos(1.0);
>     printf("%f\n", f);
>     return 0;
> }
> 
> compilation produces warnings:
> 
> steve@Ubuntu:~/scratch$ gcc nohead.c
> nohead.c: In function ‘main’:
> nohead.c:4: warning: incompatible implicit declaration of built-in
> function ‘cos’
> nohead.c:5: warning: incompatible implicit declaration of built-in
> function ‘printf’
> steve@Ubuntu:~/scratch$ ./a.out
> 0.540302
> 
> The program runs. Why is gcc warning me about its own internals? Also,
> incompatible with what?

Without a declaration, C assumes that a function is implicitly int f(),
which is wrong for cos().  This is true even though cos() is built-in
to gcc.  You need the header files.

Andrew.