94 lines
2.7 KiB
Markdown
94 lines
2.7 KiB
Markdown
[[!meta title="Solution proposal for the io select/poll problem]]
|
|
|
|
## The situation
|
|
If you have used select(2) or poll(2) more than time, you may have noticed
|
|
the regular pattern that arrives again and again:
|
|
|
|
* The main task of a program is to listen and react on multiple input/output connections.
|
|
* For each i/o connection (file descriptor, **fd**), you have
|
|
* a function that opens the **fd** (Let's call it ***conn_open***.)
|
|
* another function to be executed if an event happens on the **fd** (***conn_handle***)
|
|
* After ***conn_handle*** has been called, the number of connections may have changed:
|
|
***conn_handle*** may
|
|
* add (think of accept(2))
|
|
* or remove (think of close(2)) connections
|
|
* ***conn_open*** and ***conn_handle*** are closely related (**conn_object**)
|
|
|
|
## The problem
|
|
|
|
Each and every time this situation occurs, you have to (re-)write
|
|
code to handle that case. I have seen it in some applications I have
|
|
been writing, for instance [[software/ceofhack]] or
|
|
[[software/fui]].
|
|
|
|
## The solution proposal
|
|
|
|
Write a solution to the problem
|
|
[once and only once](http://c2.com/xp/OnceAndOnlyOnce.html),
|
|
so you and me
|
|
[don't reapeat ourselves](http://en.wikipedia.org/wiki/Don%27t_repeat_yourself)?
|
|
|
|
First of all, begin with the obvious part:
|
|
|
|
### What do we have?
|
|
|
|
We have to bring together n times
|
|
|
|
* open functions
|
|
* file descriptors
|
|
* handle functions
|
|
|
|
Furthermore, we have
|
|
|
|
* one main loop that listens for events
|
|
|
|
### How to connect them properly?
|
|
|
|
If every **conn_object** supports the following interface, it's
|
|
very easy to write a generic main loop:
|
|
|
|
conn_object.open(connection_list)
|
|
|
|
Where ***connection_list*** consists of ***connection_entries*** like this:
|
|
|
|
struct connection_entry {
|
|
fd,
|
|
handler,
|
|
type,
|
|
}
|
|
|
|
The type attribute describes, which events to wait for:
|
|
|
|
* input
|
|
* output
|
|
* errors
|
|
|
|
The ***conn_object.open*** function can add or remove entries from the list.
|
|
Whether the list is an array, hash, or whatever may be implementation specific.
|
|
|
|
|
|
### The main loop
|
|
|
|
Before launching the main listener loop, we probably need to initialise
|
|
the list, but afterwards it's all up to the handlers:
|
|
|
|
connection_list = init_connection_list()
|
|
|
|
# connect all conn_objects like this, which implicits opening the fd
|
|
conn_object.open(connection_list)
|
|
|
|
while(true) {
|
|
events_list = wait_for_event(connection_list)
|
|
|
|
for(every_event_in(event_list)) {
|
|
call_corresponding_handler_for_event_from_connection_list(fd, connection_list)
|
|
}
|
|
}
|
|
|
|
Firstly we pass the **fd**, because one handler may be registered for more
|
|
one connection.
|
|
|
|
Secondly we add the connection_list, because the handler may decide to add
|
|
more connections or remove itself.
|
|
|
|
|