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| author | Laurent Bercot <ska-skaware@skarnet.org> | 2014-12-05 22:26:11 +0000 |
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| committer | Laurent Bercot <ska-skaware@skarnet.org> | 2014-12-05 22:26:11 +0000 |
| commit | 90b12bd71bb9fc79a4640b9112c13ef529d0196a (patch) | |
| tree | 523b3f4ee2969e7a729bab2ba749c4b924ae62af /doc/libftrigr.html | |
| download | s6-90b12bd71bb9fc79a4640b9112c13ef529d0196a.tar.xz | |
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diff --git a/doc/libftrigr.html b/doc/libftrigr.html new file mode 100644 index 0000000..27ffd61 --- /dev/null +++ b/doc/libftrigr.html @@ -0,0 +1,283 @@ +<html> + <head> + <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> + <meta http-equiv="Content-Language" content="en" /> + <title>s6: the ftrigr library interface</title> + <meta name="Description" content="s6: the ftrigr library interface" /> + <meta name="Keywords" content="s6 ftrig notification subscriber listener libftrigr ftrigr library interface" /> + <!-- <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> The <tt>ftrigr</tt> library interface </h1> + +<p> + The <tt>ftrigr</tt> library provides an API for listeners, i.e. +programs that want to subscribe to fifodirs and be instantly +notified when the proper sequence of events happens. +</p> + +<h2> Compiling </h2> + +<ul> + <li> Make sure the s6 headers, as well as the skalibs headers, +are visible in your header search path. </li> + <li> Use <tt>#include <s6/ftrigr.h></tt> </li> +</ul> + +<h2> Linking </h2> + +<ul> + <li> Make sure the s6 libraries, as well as the skalibs libraries, +are visible in your library search path. </li> + <li> Link against <tt>-ls6</tt> and <tt>-lskarnet</tt>. </li> +</ul> + +<h2> Programming </h2> + +<p> + Check the <tt>s6/ftrigr.h</tt> header for the +exact function prototypes. +</p> + +<p> + Make sure your application is not disturbed by children it doesn't +know it has. This means paying some attention to the SIGCHLD handler, +if any, and to the way you perform <tt>waitpid()</tt>s. The best +practice is to use a +<a href="http://www.skarnet.org/software/skalibs/libstddjb/selfpipe.html">self-pipe</a> +to handle SIGCHLD (as well as other signals the application needs to trap), +and to <em>always</em> use <tt>wait_nohang()</tt> to reap children, +simply ignoring pids you don't know. +</p> + +<p> + If your (badly programmed) application has trouble handling unknown +children, consider using a ftrigrd service. +</p> + +<h3> A programming example </h3> + +<p> + The <tt>src/pipe-tools/s6-ftrig-listen1.c</tt> and +<tt>src/supervision/s6-svwait.c</tt> files in the s6 package, +for instance, illustrate how to use the ftrigr library. +</p> + + +<h3> Synchronous functions with a specified maximum execution time </h3> + +<ul> + <li> Synchronous functions take a <tt>tain_t const *</tt> +(<em>deadline</em>) parameter and a <tt>tain_t *</tt> (<em>stamp</em>) +parameter. Those are pointers to taia structures containing absolute times; +the former represents a deadline (in most cases, this time will be in the +future) and the latter must be an accurate enough timestamp. These +structures can be filled using the <tt>tain_</tt> primitives declared in +<a href="http://skarnet.org/software/skalibs/libstddjb/tai.html">skalibs/tai.h</a>. </li> + <li> ("Accurate enough" means that <strong>no blocking system call must have +been made</strong> since the last time <em>stamp</em> was updated (by +<tt>tain_now(&stamp)</tt>). It's a good policy to always update +<em>stamp</em> right after a (potentially) blocking system call like +<tt>select()</tt>returns. And unless the application is extremely CPU-intensive +(think calculus for physicists or astronomers) updating <em>stamp</em> more +frequently is unnecessary.) </li> + <li> If such a synchronous function still hasn't returned when the deadline +occurs, then it will immediately return a failure code and set errno to ETIMEDOUT. +It is possible to pass null pointers to the function instead of pointers to +taia structures, in which case the function will never timeout. </li> + <li> If a timeout occurs, the library does not guarantee proper interprocess +communication later on; the application should either die, or at least close +the communication channel and open a new one. </li> + <li> If any waiting occurred, the <em>stamp</em> structure is automatically +updated by the called function, so it always represents an accurate enough estimation +of the current time. This allows the programmer to call several such functions +in a sequence without modifying the <em>deadline</em> and <em>stamp</em> +parameters: then the whole sequence is bound in execution time. </li> + <li> This is a general safety mechanism implemented in +<a href="http://skarnet.org/software/skalibs/libunixonacid/">libunixonacid</a>: +in interprocess communication, purely synchronous primitives are dangerous +because they make the calling process rely on proper behaviour of the called +process. Giving synchronous primitives the ability to timeout allows developers +to write reliable programs even when interacting with software they have no +control on. </li> +</ul> + + +<h3> Starting and ending a session </h3> + +<pre> +ftrigr_t a = FTRIGR_ZERO ; +struct taia deadline, stamp ; + +taia_now(&stamp) ; +taia_addsec(&deadline, &stamp, 2) + +// char const *path = FTRIGR_IPCPATH ; +// ftrigr_start(&a, path, &deadline, &stamp) ; +ftrigr_startf(&a, &deadline, &stamp) ; +</pre> + +<p> +<tt>ftrigr_start</tt> starts a session with a ftrigrd service listening on +<em>path</em>. <br /> +<tt>ftrigr_startf</tt> starts a session with a ftrigrd process as a child +(which is the simplest usage). <br /> +<tt>a</tt> is a ftrigr_t structure that must be declared in the stack and +initialized to FTRIGR_ZERO. +<tt>stamp</tt> must be an accurate enough timestamp. <br /> +If the session initialization fails, the function returns 0 and errno is set; +else the function returns 1. +</p> +<p> +If the absolute time <tt>deadline</tt> is reached and the function +has not returned yet, it immediately returns 0 with errno set to ETIMEDOUT. + +Only local interprocess communications are involved; unless your system is +heavily overloaded, the function should return near-instantly. One or two +seconds of delay between <tt>stamp</tt> and <tt>deadline</tt> should be +enough: if the function takes more than that to return, then there is a +problem with the underlying processes. +</p> + +<p> + You can have more than one session open in parallel, by declaring +several distinct <tt>ftrigr_t</tt> structures and calling +<tt>ftrigr_startf</tt> (or <tt>ftrigr_start</tt>) more than once. +However, this is useless, since one single session can handle +virtually as many concurrent fifodirs as your application needs. +</p> + +<pre> +ftrigr_end(&a) ; +</pre> + +<p> +<tt>ftrigr_end</tt> frees all the resources used by the session. The +<tt>a</tt> structure is then reusable for another session. +</p> + +<h3> Subscribing to a fifodir </h3> + +<pre> +char const *path = "/var/lib/myservice/fifodir" ; +char const *re = "a.*b|c*d" ; +uint32 options = 0 ; + +uint16 id = ftrigr_subscribe (&a, path, re, options, &deadline, &stamp) ; +</pre> + +<p> +<tt>ftrigr_subscribe</tt> instructs the +<a href="s6-ftrigrd.html">s6-ftrigrd daemon</a>, related to the open +session represented by the <tt>a</tt> structure, to subscribe to the +<tt>path</tt> fifodir, and to notify the application when it receives +a series of events that matches the <tt>re</tt> regexp. +<tt>options</tt> can be 0 or FTRIGR_REPEAT. If it is 0, the daemon will +automatically unsubscribe from <tt>path</tt> once <tt>re</tt> has been +matched by a series of events. If it is FTRIGR_REPEAT, it will remain +subscribed until told otherwise. +</p> + +<p> + <tt>ftrigr_subscribe()</tt> returns 0 and sets errno in case of failure, or +a nonzero 16-bit number identifying the subscription in case of success. +</p> + +<p> +<tt>ftrigr_subscribe</tt> should return near-instantly, but if +<em>deadline</em> is reached, it will return 0 ETIMEDOUT. If +<tt>ftrigr_subscribe</tt> returns successfully, then the +s6-ftrigrd daemon is guaranteed to be listening on <tt>path</tt>, +and events can be sent without the risk of a race condition. +</p> + +<h3> Synchronously waiting for events </h3> + +<pre> +uint16 list[1] ; +unsigned int n = 1 ; +char trigger ; +list[0] = id ; + +// r = ftrigr_wait_and(&a, list, n, &deadline) ; +r = ftrigr_wait_or(&a, list, n, &deadline, &trigger) ; +</pre> + +<p> + <tt>ftrigr_wait_and()</tt> waits for <em>all</em> the <tt>n</tt> fifodirs +whose ids are listed in <tt>list</tt> to receive an event. It returns -1 +in case of error or timeout, or a non-negative integer in case of success. <br /> + <tt>ftrigr_wait_or()</tt> waits for <em>one</em> of the <tt>n</tt> fifodirs +whose ids are listed in <tt>list</tt> to receive an event. It returns -1 +in case of error or timeout; if it succeeds, the return value is the +position in <tt>list</tt>, starting at 0, of the identifier that received +an event; and <tt>trigger</tt> is set to the character that triggered that +event, i.e. the last character of a sequence that matched the regular +expression <tt>re</tt> used in the subscription. +</p> + +<h3> Asynchronously waiting for events </h3> + +<p> +<em> (from now on, the functions are listed with their prototypes instead +of usage examples.) </em> +</p> + +<pre> +int ftrigr_fd (struct ftrigr const *a) +</pre> + +<p> + Returns a file descriptor to select on for reading. Do not +<tt>read()</tt> it though. +</p> + +<pre> +int ftrigr_update (ftrigr_ref a) +</pre> + +<p> + Call this function whenever the fd checks readability: it will +update <em>a</em>'s internal structures with information from the +<a href="s6-ftrigrd.html">s6-ftrigrd</a> daemon. It returns -1 if an error +occurs; in case of success, it returns the number of identifiers for +which something happened. +</p> + +<p> + When <tt>ftrigr_update</tt> returns, +<tt>genalloc_s(uint16, &a->list)</tt> points to an array of +<tt>genalloc_len(uint16, &a->list)</tt> 16-bit unsigned +integers. Those integers are ids waiting to be passed to +<tt>ftrigr_check</tt>. +</p> + +<pre> +int ftrigr_check (ftrigr_ref a, uint16 id, char *what) +</pre> + +<p> + Checks whether an event happened to <em>id</em>. Use after a +call to <tt>ftrigr_update()</tt>. +</p> + +<ul> + <li> If an error occurred, returns -1 and sets errno. The error +number may have been transmitted from +<a href="s6-ftrigrd.html">s6-ftrigrd</a>. </li> + <li> If no notification happened yet, returns 0. </li> + <li> If something happened, writes the character that triggered the +latest notification into <em>what</em> and returns the number of +times that an event happened to this identifier since the last +call to <tt>ftrigr_check()</tt>. </li> +</ul> + +</body> +</html> |