Sample code for handling _IO_WRITE messages in pwrite*()

The following sample code demonstrates how to handle _IO_WRITE for the case where the client calls one of the pwrite*() functions. Keep in mind that the struct _xtype_offset information follows the struct _io_write in the sender's message buffer. This means that the data to be written follows the struct _xtype_offset information (instead of the normal case where it follows the struct _io_write). So, you must take this into account when doing the resmgr_msgget() call in order to get the data from the sender's message.

/* we are defining io_pwrite_t here to make the code below simple */
typedef struct {
    struct _io_write        write;
    struct _xtype_offset    offset;
} io_pwrite_t;

int io_write (resmgr_context_t *ctp, io_write_t *msg, RESMGR_OCB_T *ocb)
{
    off64_t offset; /* where to write */
    int     status;
    size_t  skip;   /* offset into msg to where the data resides */

    if ((status = iofunc_write_verify(ctp, msg, ocb, NULL)) != EOK) {
        return(status);
    }
    
    switch(msg->i.xtype & _IO_XTYPE_MASK) {
    case _IO_XTYPE_NONE:
        offset = ocb->offset;
        skip = sizeof(io_write_t);
        break;
    case _IO_XTYPE_OFFSET:
        /* 
         *  io_pwrite_t is defined above
         *  client is doing a one-shot write to this offset by
         *  calling one of the pwrite*() functions
         */
        offset = ((io_pwrite_t *) msg)->offset.offset;
        skip = sizeof(io_pwrite_t);
        break;
    default:
        return(ENOSYS);
    }
    ...
    
    /* 
     *  get the data from the sender's message, 
     *  skipping all possible header information
     */
    resmgr_msggetv(ctp, iovs, niovs, skip);
    ...
}