open() not working with O_DIRECT flag on UBUNTU

Question

I'm trying to open a file with the O_DIRECT flag. On Ubuntu, the program fails with errno 22. I just installed Fedora on the same machine with dual boot, and this exact same code runs there smoothly.

I'm running Ubuntu 13.10 with kernel 3.12.6 and g++ of version 4.8.1 and file system ext4. The Fedora I just installed is version 20 with kernel 3.12.6.

#include <unistd.h>
#include <fcntl.h>
#include <iostream>
#include <errno.h>

using namespace std;

int main(void)
{
    int filedesc = open("testfile.txt", O_RDWR | O_CREAT | O_APPEND | O_DIRECT);
    if (filedesc < 0) {
    std::cout << "fail with errno: " << errno << std::endl;
        return -1;
    }
    return 0;
}

Answer 1

You probably have the data journalling ext4 feature enabled. With data being journalled, writes must be buffered (think about it), so O_DIRECT will fail with EINVAL.