Re: [Qemu-devel] What "opaque" stand for?

[Peter Maydell]

On 7 November 2014 14:42, Kaiyuan <address@hidden> wrote:
>
> Hello, all
>
> I am reviewing source of Qemu. I can see parameter called "opaque" from a lot 
> of function like this:
> //@hw/timer/xilinx_timer.c
> static uint64_t timer_read(void *opaque, hwaddr addr, unsigned int size)
>
> {
> ...
> }
>
>
> , and its meaning confuse me. What does "opaque" stand for?

It's a general term (not specific to QEMU) that means "this is some
data (structure) which we cannot see into". (The word 'opaque' means
"not able to be seen through"; it's the opposite of 'transparent'.)
Usually there is an API layer involved which lets you register a
callback function together with some data to be passed to it
(in this case that API is memory_region_init_io()). The memory
region code does not know or care what the data pointer is and
cannot see inside it (because it's typed as a void*) -- the data
is opaque to that code. When the callback function timer_read()
is called, it is passed the opaque data, and the first thing it
has to do is cast that pointer to the correct type so it can be used.

-- PMM