/* */
#include <stdio.h>
#define __USE_XOPEN2K
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define __USE_GNU
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>


#define	BUFSIZE	8192
#define	NBLKS	10

int main(int argc, char *argv[])
{
	int	fd;
	int	i, n;
	char	*buf, *ptr;

	/* Open file */
	if ((fd = open("file", O_DIRECT|/*O_SYNC|*/O_RDWR|O_CREAT, 0666)) < 0) {
		perror("open failed: ");
		exit(-1);
	}

	/* From: Terje Eggestad (terje.eggestad@scali.no)
	 * Date: Sun Dec 16 2001 - 08:57:53 EST
	 *
	 * The problem is that the kernel that don't support O_DIRECT has
	 * erronous handling of the O_DIRECT flag. Meaning they happily accept
	 * it.
	 *
	 * In order to figure out if the running kernel support O_DIRECT you
	 * MUST attempt an unaligned read/write, if it succed the kernel DON'T
	 * support O_DIRECT.
	 *
	 * Martin Habets: man open states for O_DIRECT the buffer must be
	 * aligned to 512 bytes at least.
	 */
	ptr = malloc(16);
	buf = ptr;
	/* Got an aligned buffer? Make it unaligned. */
	if ((((unsigned) buf) & (512-1)) == 0) {
		buf = &ptr[8];
	}

	memset(buf, 1, 8);
	n = write(fd, buf, 8);
	if (n == -1) {
		if (errno != EINVAL) {
			perror("unaligned write");
			goto out;
		}
		printf("unaligned write failed, O_DIRECT is supported.\n");
	} else if (n == 8) { 
		printf("unaligned write suceeded, O_DIRECT is not supported.\n");
		goto out;
	}
	free(ptr);
	lseek(fd, 0, SEEK_SET);

	/* Allocate aligned buffer, Create output file. 
	 */
	n = posix_memalign((void **)&buf, 512, BUFSIZE*NBLKS);
	if (n != 0) {
		perror("posix_memalign() failed:");
		exit(-1);
	}
	for (i = 1; i < NBLKS; i++) {
		memset(&buf[i*BUFSIZE], (i % 256), BUFSIZE);
		n = write(fd, &buf[i*BUFSIZE], BUFSIZE);
		if (n != BUFSIZE) { 
			printf("write failed: %d %s\n", n, strerror(errno));
			break;
		}
/* This does not seem to help
		if (fsync(fd) != 0) {
			printf("sync failed: %s\n", strerror(errno));
			break;
		}
		sleep(1);
*/
	}

out:
	/* Cleanup */
	close(fd);
	free(buf);
	return 0;
}