Sample code for handling _IO_WRITE messages

QNX SDP8.0Writing a Resource ManagerDeveloper

The following is a code snippet that can be added to one of the simple resource manager examples. It prints out whatever it's given (making the assumption that it's given only character text):

int io_write (resmgr_context_t *ctp, io_write_t *msg, RESMGR_OCB_T *ocb)
{
    int     status;
    char    *buf;
    size_t  nbytes;

    if ((status = iofunc_write_verify(ctp, msg, ocb, NULL)) != EOK)
        return (status);

    if ((msg->i.xtype & _IO_XTYPE_MASK) != _IO_XTYPE_NONE)
        return(ENOSYS);

    /* Extract the length of the client's message. */
    nbytes = _IO_WRITE_GET_NBYTES(msg);

    /* Filter out malicious write requests that attempt to write more
       data than they provide in the message. */
    if(nbytes > (size_t)ctp->info.srcmsglen - (size_t)ctp->offset - sizeof(io_write_t)) {
        return EBADMSG;
    }

    /* set up the number of bytes (returned by client's write()) */
    _IO_SET_WRITE_NBYTES (ctp, nbytes);

    buf = (char *) malloc(nbytes + 1);
    if (buf == NULL)
        return(ENOMEM);

    /*
     *  Read the data from the sender's message.
     *  We're not assuming that all of the data fit into the
     *  resource manager library's receive buffer.
     */
    resmgr_msgget(ctp, buf, nbytes, sizeof(msg->i));
    buf [nbytes] = '\0'; /* just in case the text is not NULL terminated */
    printf ("Received %zu bytes = '%s'\n", nbytes, buf);
    free(buf);

    if (nbytes > 0)
        ocb->attr->flags |= IOFUNC_ATTR_MTIME | IOFUNC_ATTR_CTIME;

    return (_RESMGR_NPARTS (0));
}

Of course, we'll have to give the resource manager library the address of our io_write handler so that it'll know to call it. In the code for main() where we called iofunc_func_init(), we'll add a line to register our io_write handler:

/* initialize functions for handling messages */
iofunc_func_init(_RESMGR_CONNECT_NFUNCS, &connect_funcs,
                 _RESMGR_IO_NFUNCS, &io_funcs);
io_funcs.write = io_write;

You may also need to add the following prototype:

int io_write (resmgr_context_t *ctp, io_write_t *msg, RESMGR_OCB_T *ocb);

At this point, if you were to run the resource manager (our simple resource manager used the name /dev/sample), you could write to it by doing echo Hello > /dev/sample as follows:

# echo Hello > /dev/sample
Received 6 bytes = 'Hello'

Notice how we passed the last argument to resmgr_msgget() (the offset argument) as the size of the input message buffer. This effectively skips over the header and gets to the data component.

If the buffer you supplied wasn't big enough to contain the entire message from the client (e.g., you had a 4 KB buffer and the client wanted to write 1 megabyte), you'd have to read the buffer in stages, using a for loop, advancing the offset passed to resmgr_msgget() by the amount read each time.

Unlike the io_read handler sample, this time we didn't do anything with ocb->offset. In this case there's no reason to. The ocb->offset would make more sense if we were managing things that had advancing positions such as a file position.

The reply is simpler than with the io_read handler, since a write() call doesn't expect any data back. Instead, it just wants to know if the write succeeded and if so, how many bytes were written. To tell it how many bytes were written we used the _IO_SET_WRITE_NBYTES() macro. It takes the nbytes that we give it and stores it in the context structure (ctp). Then when we return to the library, the library takes this nbytes and passes it as the second parameter to the MsgReplyv(). The second parameter tells the kernel what the MsgSend() should return. And since the write() function is calling MsgSend(), that's where it finds out how many bytes were written.

Since we're writing to the device, we should also update the modification, and potentially, the creation time. For details on updating the modification and change of file status times, see the section on Updating the time for reads and writes below.

Page updated: March 05, 2025