O_DIRECT and linux

[Gabor Forgacs <gabor@xxxxxxxxxxxxxx>]

Thanx for the answers to my previous question
I am still tweaking with direct io under linux. 

I just don't know how to get the requirements for directio 
under irix i used the 

struct dioattr dio;
if( -1 == fcntl( fd, F_DIOINFO, &dio))
{
        perror("fcntl");
        exit(1);
}

It just doesn't work under linux, dioattr and the F_DIOINFO are undefined.
I can see the O_DIRECT if i define the _GNU_SOURCE. 

What is the way under linux ? 

Thanx