153 lines
5 KiB
Markdown
153 lines
5 KiB
Markdown
[[!meta title="Solution proposal for the io select/poll problem"]]
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## The situation
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If you have used select(2) or poll(2) more than once, you may have noticed
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the regular pattern that comes up again and again:
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* The main task of a program is to listen and react on multiple input/output connections.
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* For each i/o connection (file descriptor), you have
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* a function that opens the file descriptor (Let's call it ***conn_open***.) and
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* another function to be executed if an event happens on the file descriptor
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(***conn_handle***).
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* After ***conn_handle*** has been called, the number of connections may have changed:
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***conn_handle*** may
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* add (think of accept(2))
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* or remove (think of close(2)) connections.
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* ***conn_open*** and ***conn_handle*** are closely related and belong to the
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same "object" or code (***conn_object***).
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## The problem
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Each and every time this situation occurs, you have to (re-)write
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code to handle that case. I have seen it in some applications I have
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been writing, for instance [[software/ceofhack]] or [[software/fui]].
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## The solution proposal
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Write a solution to the problem
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[once and only once](http://c2.com/xp/OnceAndOnlyOnce.html),
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so you and me
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[don't reapeat ourselves](http://en.wikipedia.org/wiki/Don%27t_repeat_yourself).
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First of all, begin with the obvious part:
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### What do we have?
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We have to bring together **n times**
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* open functions
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* file descriptors on which the following events can happen:
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* data is ready to be read from the file descriptor
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* data can be written to the file descriptor
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* an error occured on the file descriptor
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* handle functions
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Furthermore, we have
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* **one** main loop that listens for events
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### How to connect them properly?
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I assume that every ***conn_object*** knows best, which function to use for
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opening and handling and which type of event is interesting.
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Thus if we create a special function ***conn_open***
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for every ***conn_object*** that
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* returns this information to the caller, the caller can create
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* a list containing the needed information and
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* loop over the event list and call the corresponding handler.
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The ***conn_open*** function may look like this:
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connection_entry = conn_open();
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Where ***connection_list*** is a list of ***connection_entries*** like this:
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struct connection_list {
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struct connection_entry *next;
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} list;
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enum type {
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IN,
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OUT,
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ERR
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};
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struct connection_entry {
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int fd;
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void (*handler)(struct connection_list *);
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int type;
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};
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The ***conn_open*** function can add or remove entries from the list.
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Whether the list is an array, linked list, hash or whatever may be implementation specific.
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### The main loop
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Before launching the main listener loop, we need to initialise
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the list and run the **conn_open** function of every **conn_object**:
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struct connection_list list = init_connection_list();
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connection_add(&list, a_conn_open());
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connection_add(&list, b_conn_open());
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Having done this, our main loop now looks pretty simple, doesn't it?
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while(1) {
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/* create poll or select list from connection list, whatever you prefer */
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poll_or_select_struct = connection_list_topoll_or_select(&connection_list);
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changed_events = poll_or_select(&poll_or_select_struct);
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for(event = changed_events; event != NULL; event = event->next) {
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exec_handler(event->fd, &connection_list);
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}
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}
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The function ***exec_handler*** would search for the registered handler
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of the changed file descriptor, which could look like this:
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conn_handle(fd, &connection_list)
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Firstly we pass the **fd**, because one handler may be registered for more
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than one connection.
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Secondly we add the ***connection_list***, because the handler may add
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more connections or remove itself.
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Although I'm mainly speaking about ***poll*** and ***select***, the idea also
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applies to [kqueue](http://people.freebsd.org/~jlemon/papers/kqueue.pdf),
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[epoll](http://www.kernel.org/doc/man-pages/online/pages/man4/epoll.4.html)
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and co.
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## Further thoughts
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### Combine handle and open functions
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For a second I thought the functions ***conn_handle*** and
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***conn_open*** could be merged into a single function,
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which would get a negative file descriptor, if called the first
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time.
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But as this means the ***conn_handle*** would need to check every
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time it is called whether the to call the open part or not,
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this is probably not a good idea.
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### Creating an implementation
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I am currently actively (!) working on [[fui|software/fui]] and
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think about creating an implementation in ruby and if it works
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fine, another one in C for [[ceofhack|software/ceofhack]].
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### Getting feedback
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I would appreciate any feedback regading this idea, whether the
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problem is no problem at all, has been solved before or the idea
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may be a solution for your problem, too: You can contact me for
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this special post at
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**nico-io-poll-select-idea** (near) **schottelius.org**.
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[[!tag programming unix]]
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