Message-based file I/O

/* Create an unnamed message queue for replies to I/O requests */
mqueue_t *io_reply_queue = mqueue_create(NULL);

/* Open the file */
file_t *notes = file_open("~/Documents/notes.txt", FILE_READ);

/* Issue multiple asynchronous read requests, with request identifiers */
char buffer1[512], buffer2[512];
file_read_async(notes, buffer1, 0, 512, io_reply_queue, 0);
file_read_async(notes, buffer2, 512, 512, io_reply_queue, 1);

/* Number of writes completed */
int num_writes_done = 0;

/* Wait for responses */
while(1)
{
    /* Block for replies */
    message_t *msg = mqueue_recv(io_reply_queue);

    /* Which type of operation completed? */
    switch(msg->method)
    {
    case FILE_READ_FINISH:
        /* Read failed */
        if (msg->reply < 0)
        {
            printf("Read failed\n");
            return 1;
        }

        /* First read request */
        switch(msg->id)
        {
        case 0:
            buffer1[0] = 'A';
            file_write_async(notes, buffer1, 0, 512, io_reply_queue, 0);
            break;
        case 1:
            buffer2[0] = 'B';
            file_write_async(notes, buffer2, 512, 512, io_reply_queue, 1);
            break;
        default:
            printf("Invalid read reply\n");
            return 1;
        }

        break;
    case FILE_WRITE_FINISH:
        /* Write failed */
        if (msg->reply < 0)
        {
            printf("Write failed\n");
            return 1;
        }

        /* One more write finished */
        num_writes_done++;
        if (num_writes_done == 2)
        {
            printf("Done\n");
            return 0;
        }

        break;
    default:
        printf("Invalid message\n");
}