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| author | Laurent Bercot <ska-skaware@skarnet.org> | 2015-01-15 20:14:44 +0000 |
|---|---|---|
| committer | Laurent Bercot <ska-skaware@skarnet.org> | 2015-01-15 20:14:44 +0000 |
| commit | 87c5b2118efcee65eeda3f743d081ea9c2b866d9 (patch) | |
| tree | 31ca07d6134adf44bc3d58f4fcf4ea8be9cb7dbb /doc/libftrig.html | |
| parent | cd2500fcc704287c4994a3253b593593c867913e (diff) | |
| download | s6-87c5b2118efcee65eeda3f743d081ea9c2b866d9.tar.xz | |
Move Unix domain utilities and access control utilites,
as well as the accessrules library, from s6-networking to here
Diffstat (limited to 'doc/libftrig.html')
| -rw-r--r-- | doc/libftrig.html | 184 |
1 files changed, 0 insertions, 184 deletions
diff --git a/doc/libftrig.html b/doc/libftrig.html deleted file mode 100644 index da4c25b..0000000 --- a/doc/libftrig.html +++ /dev/null @@ -1,184 +0,0 @@ -<html> - <head> - <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> - <meta http-equiv="Content-Language" content="en" /> - <title>s6: libftrig</title> - <meta name="Description" content="s6 libftrig" /> - <meta name="Keywords" content="s6 libftrig" /> - <!-- <link rel="stylesheet" type="text/css" href="http://skarnet.org/default.css" /> --> - </head> -<body> - -<p> -<a href="index.html">s6</a><br /> -<a href="http://skarnet.org/software/">Software</a><br /> -<a href="http://skarnet.org/">skarnet.org</a> -</p> - -<h1> libftrig </h1> - -<p> -<t>libftrig</t> is a portable Unix C programming interface allowing a -process (the <em>subscriber</em> or <em>listener</em>) to be instantly -notified when another process (the <em>notifier</em> or <em>writer</em>) -signals an event. -</p> - -<a name="notification"> -<h2> What is notification ? </h2> -</a> - -<h3> Notification vs. polling </h3> - -<p> - Process A is <em>notified</em> of an event E when it gets a instant -notice that event E has happened; the notice may disrupt A's execution -flow. Notification is the opposite of <em>polling</em>, where A has to -periodically (every T milliseconds) check whether E happened and has no -other way to be informed of it. -</p> - -<p> - Polling is generally considered bad practice - and is inferior to -notification in practically every case - for three reasons: -</p> - -<ul> - <li> Reaction time. When event E happens, process A does not know it -instantly. It will only learn of E, and be able to react to it, when -it explicitly checks for E; and if E happened right after A performed -the check, this can take as long as T milliseconds (the <em>polling -period</em>). Polling processes have reaction delays due to the polling -periods. </li> - <li> Resource consumption. Even if <em>no</em> event ever happens, process A -will still wake up needlessly every T milliseconds. This might not seem like -a problem, but it is a serious one in energy-critical environments. Polling -processes use more CPU time than is necessary and are not energy-friendly. </li> - <li> Conflict between the two above reasons. The longer the polling period, -the more energy-friendly the process, but the longer the reaction time. The -shorter the polling period, the shorter the reaction time, but the more -resource-consuming the process. A delicate balance has to be found, and -acceptable behaviour is different in every case, so there's no general rule -of optimization. </li> -</ul> - -<p> - Notification, on the other hand, is generally optimal: reaction time is -zero, and resource consumption is minimal - a process can sleep as soon as -it's not handling an event, and only wake up when needed. -</p> - -<p> - Of course, the problem of notification is that it's often more difficult -to implement. Notification frameworks are generally more complex, involving -lots of asynchronism; polling is widely used -<a href="http://lib.store.yahoo.net/lib/demotivators/mediocritydemotivationalposter.jpg">because -it's easy.</a> -</p> - -<h3> Notifications and Unix </h3> - -<p> - Unix provides several frameworks so that a process B (or the kernel) can -notify process A. -</p> - -<ul> - <li> Signals. The simplest Unix notification mechanism. Sending events amounts -to a <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/kill.html">kill()</a> -call, and receiving events amounts to installing a signal handler (preferrably -using a <a href="http://skarnet.org/software/skalibs/libstddjb/selfpipe.html">self-pipe</a> -if mixing signals with an event loop). Unfortunately, Unix signals, even the more -recent and powerful real-time Posix signals, have important limitations when it's -about generic notification: - <ul> - <li> non-root processes can only send signals to a very restricted and -implementation-dependent set of processes (roughly, processes with the same UID). This is a problem when -designing secure programs that make use of the Unix privilege separation. </li> - <li> you need to know the PID of a process to send it signals. This is generally -impractical; process management systems that do not use supervisor processes have -to do exactly that, and they resort to unreliable, ugly hacks (.pid files) to track -down process PIDs. </li> - </ul> </li> - <li> BSD-style IPCs, i.e. file descriptors to perform select()/poll() system -calls on, in an <em>asynchronous event loop</em>. This mechanism is very widely used, -and rightly so, because it's extremely generic and works in every ordinary situation; -you have to be doing <a href="http://www.kegel.com/c10k.html">very specific stuff</a> -to reach its limits. If process A is reading on -fd <em>f</em>, it is notified everytime another process makes <em>f</em> readable - -for instance by writing a byte to the other end if <em>f</em> is the reading end -of a pipe. And indeed, this is how libftrig works internally; but libftrig is needed -because direct use of BSD-style IPCs also has limitations. - <ul> - <li> Anonymous pipes are the simplest and most common BSD-style IPC. If there is a -pipe from process B to process A, then B can notify A by writing to the pipe. The -limitation is that A and B must have a common ancestor that created the pipe; two -unrelated processes cannot communicate this way. </li> - <li> Sockets are a good many-to-one notification system: once a server is up, it -can be notified by any client, and notify all its clients. The limitation of sockets -is that the server must be up before the client, which prevents us from using them -in a general notification scheme. </li> - </ul> </li> - <li> System V IPCs, i.e. message queues and semaphores. The interfaces to those IPCs -are quite specific and can't mix with select/poll loops, that's why nobody in their -right mind uses them. </li> -</ul> - -<h3> What we want </h3> - -<p> - We need a general framework to: -</p> - -<ul> - <li> Allow an event-generating process to broadcast notifications to every process -that asked for one, without having to know their PIDs </li> - <li> Allow a process to subscribe to a "notification channel" and be instantly, -asynchronously notified when an event occurs on this channel. </li> -</ul> - -<p> - This requires a many-to-many approach that Unix does not provide natively, and -that is what libftrig does. -</p> - -<a name="bus"> -<h2> That's what a bus is for. D-Bus already does all this. </h2> -</a> - -<p> - Yes, a bus is a good many-to-many notification mechanism indeed. However, -a Unix bus can only be implemented via a daemon - you need a long-running -process, i.e. a <em>service</em>, to implement a bus. And s6 is a -<em>supervision suite</em>, i.e. a set of programs designed to manage -services; we would like to be able to use notifications in the supervision -suite, to be able to wait for a service to be up or down... <em>without</em> -relying on a particular service to be up. libftrig provides a notification -mechanism that <em>does not need</em> a bus service to be up, that's its -main advantage over a bus. -</p> - -<p> - If you are not concerned with supervision and can depend on a bus service, -though, then yes, by all means, use a bus for your notification needs. -There is a <a href="http://skarnet.org/software/skabus/">skabus</a> -project in the making, which aims to be simpler, smaller and more -maintainable than D-Bus. -</p> - -<h2> How to use libftrig </h2> - -<p> - <tt>libftrig</tt> is really a part of <tt>libs6</tt>: all the functions -are implemented in the <tt>libs6.a</tt> archive, or the <tt>libs6.so</tt> -dynamic shared object. However, the interfaces are different for notifiers -and listeners: -</p> - -<ul> -<li> Notifiers use the <a href="libftrigw.html">libftrigw</a> interface. </li> -<li> Listeners use the <a href="libftrigr.html">libftrigr</a> interface. </li> -</ul> - -</body> -</html> |