[syslog-ng] need help understanding logqueue-fifo.c code

[Gergely Nagy]

Evan Rempel <erempel at uvic.ca> writes:

> I am having difficulties understanding the code that handles the LogQueueFifo objects.
>
> For instance
>
> static void log_queue_fifo_push_head(LogQueue *s, LogMessage *msg, const LogPathOptions *path_options)
> {
>     LogQueueFifo *self = (LogQueueFifo *) s;
>     LogMessageQueueNode *node;
>
> ...
>
>
> I don't understand how a LogQueue*  can be passed in, and then cast to a LogQueueFifo* when the
> structs are not the same. Then fields in the LogQueueFifo* are referenced but they won't have any meaning
> because the LogQueue* didn't have the same data at the same location.
>
> I am a little rusty in C, so can someone explain this to me please?

What happens here, is that this function is a callback, which has a
generic prototype, but one that will always get a LogQueueFifo passed to
it. A LogQueueFifo can also serve as a LogQueue, since its first member
*is* a LogQueue struct, so when cast over, it will Just Work.

LogQueueFifo is a child object of LogQueue. And due to the way these
callbacks work (see log_queue_fifo_new()), we can be sure that a
callback like this will always get a LogQueueFifo, therefore, the cast
from parent type to child type is safe. Since we passed a pointer, the
rest of the LogQueueFifo structure will be there in the memory too, and
once we did the cast, they become accessible too.

Hope that sheds some light on it.

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