libaio with O_DIRECT doesn't seem to work

[Amit]

My system:

$ uname -a
Linux blackhole 3.2.0-53-generic #81-Ubuntu SMP Thu Aug 22 21:01:03 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux

Problem: The following program which uses libaio doesn't work - No errors but no data is read. Exactly same program works if I just remove the O_DIRECT flag while opening the file.

#define _GNU_SOURCE /* syscall() is not POSIX */

#include <stdio.h> /* for perror() */
#include <unistd.h> /* for syscall() */
#include <libaio.h> /* for AIO types and constants */
#include <fcntl.h> /* O_RDWR */
#include <string.h> /* memset() */
#include <inttypes.h> /* uint64_t */

int main()
{
        io_context_t ctx;
        struct iocb cb;
        struct iocb *cbs[1];
        struct io_event events[1];
        int fd;
        char *data;

        fd = open("/tmp/testfile", O_RDONLY | O_CREAT | O_DIRECT);

        ctx = 0;
        printf("Setup : %d\n", io_setup(128, &ctx));

        printf("Aligned : %d\n", posix_memalign(&data, 512, 40));
        io_prep_pread(&cb, fd, data, 40, 0);

        cbs[0] = &cb;

        printf("Submitted : %d\n", io_submit(ctx, 1, cbs));

        printf("Got Events : %d\n", io_getevents(ctx, 1, 1, events, NULL));
        printf("%ld\n", events[0].res);
        printf("%s\n", data);

        printf("Destroyed : %d\n", io_destroy(ctx));

        return 0;
}

Results:
$ ./a.out
Setup : 0
Aligned : 0
Submitted : 1
Got Events : 1
-22

Destroyed : 0

[Amit (amitdev)]

Looks like i was doing something stupid, the aligned size apparently needs to be a multiple of block size.