On Fri, 2015-09-18 at 22:51 -0600, Jonathan Marler wrote:
> I'm curious why there wasn't another field added to the epoll_event
> struct for the application to store the descriptor's context. Any
> useful multi-plexing application will have a context that will need to
> be retrieved every time a descriptor needs to be serviced. Since the
> epoll api has no way of storing this context, it has to be looked up
> using the descriptor, which will take more time/memory as the number
> of descriptors increase. The memory saved from omitting this context
> can't be worth it since you'll have to allocate the memory in the
> application anyway, plus you're now adding the extra lookup.
>
> This "lookup" problem has always existed in multi-plexed applications.
> It was impossible to fix with older polling interfaces, however, since
> epoll is stateful, it provides an opportunity to fix this problem by
> storing the descriptor context in epoll's "state". What was the reason
> for not doing this? Was it an oversight or am I missing something?
typedef union epoll_data
{
void *ptr;
int fd;
uint32_t u32;
uint64_t u64;
} epoll_data_t;
struct epoll_event
{
uint32_t events; /* Epoll events */
epoll_data_t data; /* User data variable */
} __EPOLL_PACKED;
Application is free to use whatever is needed in poll_data_t
You can store a pointer to your own data (ptr)
Or a 32 bit cookie (u32)
Or a 64 bit cookie (u64)
(But is an union, you have to pick one of them)
Nothing forces you to use 'fd', kernel does not care.
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