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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/libftrigr.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/libftrigr.html')
| -rw-r--r-- | doc/libftrigr.html | 283 |
1 files changed, 0 insertions, 283 deletions
diff --git a/doc/libftrigr.html b/doc/libftrigr.html deleted file mode 100644 index 79c7694..0000000 --- a/doc/libftrigr.html +++ /dev/null @@ -1,283 +0,0 @@ -<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 tain_t 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 -tain_t 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 ; -tain_t deadline, stamp ; - -tain_now(&stamp) ; -tain_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 (ftrigr_t 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_t *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_t *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> |