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add all thoughts and publish the idea

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Signed-off-by: Nico Schottelius <nico@kr.ethz.ch>
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blog/solution-proposal-for-the-io-select-poll-problem.mdwn
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[[!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**)
If you have used select(2) or poll(2) more than once, you may have noticed
the regular pattern that comes up 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), you have
* a function that opens the file descriptor (Let's call it ***conn_open***.) and
* another function to be executed if an event happens on the file descriptor
(***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 and belong to the
same "object" or code (***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]].
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)?
[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
We have to bring together **n times**
* open functions
* file descriptors
* file descriptors on which the following events can happen:
* data is ready to be read from the file descriptor
* data can be written to the file descriptor
* an error occured on the file descriptor
* handle functions
Furthermore, we have
* one main loop that listens for events
* **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:
I assume that every ***conn_object*** knows best, which function to use for
opening and handling and which type of event is interesting.
conn_object.open(connection_list)
Thus if we create a special function ***conn_open***
for every ***conn_object*** that
Where ***connection_list*** consists of ***connection_entries*** like this:
* returns this information to the caller, the caller can create
* a list containing the needed information and
* loop over the event list and call the corresponding handler.
The ***conn_open*** function may look like this:
conn_open(&connection_list)
Where ***connection_list*** is a list of ***connection_entries*** like this:
struct connection_list {
struct connection_entry *next;
} list;
enum type {
IN,
OUT,
ERR
};
struct connection_entry {
fd,
handler,
type,
}
int fd;
void (*handler)(struct connection_list *);
int 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 ***conn.open*** function can add or remove entries from the list.
Whether the list is an array, linked list, 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:
Before launching the main listener loop, we need to initialise
the list and run the **conn_open** function of every **conn_object**:
connection_list = init_connection_list()
struct connection_list list = init_connection_list();
# connect all conn_objects like this, which implicits opening the fd
conn_object.open(connection_list)
a_conn_open(&connection_list);
b_conn_open(&connection_list);
while(true) {
events_list = wait_for_event(connection_list)
Having done this, our main loop now looks pretty simple, doesn't it?
for(every_event_in(event_list)) {
call_corresponding_handler_for_event_from_connection_list(fd, connection_list)
}
while(1) {
/* create poll or select list from connection list, whatever you prefer */
poll_or_select_struct = connection_list_topoll_or_select(&connection_list);
changed_events = poll_or_select(&poll_or_select_struct);
for(event = changed_events; event != NULL; event = event->next) {
find_handler(event->fd, &connection_list);
}
}
Firstly we pass the **fd**, because one handler may be registered for more
one connection.
The function ***exec_handler*** would search for the registered handler
of the changed file descriptor, which could look like this:
Secondly we add the connection_list, because the handler may decide to add
conn_handle(fd, &connection_list)
Firstly we pass the **fd**, because one handler may be registered for more
than one connection.
Secondly we add the ***connection_list***, because the handler may add
more connections or remove itself.
Although I'm mainly speaking about ***poll*** and ***select***, the idea also
applies to [kqueue](http://people.freebsd.org/~jlemon/papers/kqueue.pdf),
[epoll](http://www.kernel.org/doc/man-pages/online/pages/man4/epoll.4.html)
and co.
## Further thoughts
### Combine handle and open functions
For a second I thought the functions ***conn_handle*** and
***conn_open*** could be merged into a single function,
which would get a negative file descriptor, if called the first
time.
But as this means the ***conn_handle*** would need to check every
time it is called whether the to call the open part or not,
this is probably not a good idea.
### Creating an implementation
I am currently actively (!) working on [[fui|software/fui]] and
think about creating an implementation in ruby and if it works
fine, another one in C for [[ceofhack|software/ceofhack]].
### Getting feedback
I would appreciate any feedback regading this idea, whether the
problem is no problem at all, has been solved before or the idea
may be a solution for your problem, too: You can contact me for
this special post at
**nico-io-poll-select-idea** (near) **schottelius.org**.
[[!tag programming unix]]