linux kernel aio functionality

I am testing kernel asynchronous io functions (not posix aio) and am trying to figure out how it works. The code below is a complete program where I simply write an array repeatedly to a file opened using O_DIRECT. I get an error in the callback function "write missed bytes expect 1024 got 0" (see the fprintf statement in work_done()).

For those not familiar with kernel aio, the code below does the following:

1. Init some structs
2. Prepare aio (io_prep_pwrite)
3. Submit io requests (io_submit)
4. Check for event completion (io_getevents)
5. Call a callback function to see if everything went ok.

I get an error at step 5. If I do not open the file using O_DIRECT, things work fine, but it beats the purpose of having async writes. Can someone tell me what I am doing wrong? Is this the correct usage of kernel aio, for example, is my use of callbacks correct? Are there any restrictions on the usage of O_DIRECT?

I compile using 'gcc -Wall test.c -laio'

Thanks in advance.

/* 
 * File:   myaiocp.c
 * Author: kmehta
 *
 * Created on July 11, 2011, 12:50 PM
 *
 *
 * Testing kernel aio. 
 * Program creates a 2D matrix and writes it multiple times to create a file of desired size. 
 * Writes are performed using kernel aio functions (io_prep_pwrite, io_submit, etc.)
 */
#define _GNU_SOURCE
#define _XOPEN_SOURCE 600

#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>
#include <pthread.h>
#include <fcntl.h>
#include <string.h>
#include <sys/uio.h>
#include <sys/time.h>
#include <omp.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <libaio.h>

char ** buf;
long seg_size;
int seg_rows;
double total_size;
char * filename;
static int wait_count = 0;

void io_task();
void cleanup();
void allocate_2D_matrix(int[]);
int file_open(char *);
void wr_done(io_conte