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| author | Laurent Bercot <ska-skaware@skarnet.org> | 2014-09-18 18:55:44 +0000 |
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| committer | Laurent Bercot <ska-skaware@skarnet.org> | 2014-09-18 18:55:44 +0000 |
| commit | 3534b428629be185e096be99e3bd5fdfe32d5544 (patch) | |
| tree | 210ef3198ed66bc7f7b7bf6a85e4579f455e5a36 /doc/libstddjb/tai.html | |
| download | skalibs-3534b428629be185e096be99e3bd5fdfe32d5544.tar.xz | |
initial commit with rc for skalibs-2.0.0.0
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| -rw-r--r-- | doc/libstddjb/tai.html | 462 |
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diff --git a/doc/libstddjb/tai.html b/doc/libstddjb/tai.html new file mode 100644 index 0000000..3524c05 --- /dev/null +++ b/doc/libstddjb/tai.html @@ -0,0 +1,462 @@ +<html> + <head> + <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> + <meta http-equiv="Content-Language" content="en" /> + <title>skalibs: the tai library interface</title> + <meta name="Description" content="skalibs: the tai library interface" /> + <meta name="Keywords" content="skalibs c unix tai library libstddjb" /> + <!-- <link rel="stylesheet" type="text/css" href="http://skarnet.org/default.css" /> --> + </head> +<body> + +<p> +<a href="index.html">libstddjb</a><br /> +<a href="../libskarnet.html">libskarnet</a><br /> +<a href="../index.html">skalibs</a><br /> +<a href="http://skarnet.org/software/">Software</a><br /> +<a href="http://skarnet.org/">skarnet.org</a> +</p> + +<h1> The <tt>tai</tt> library interface </h1> + +<p> + The following functions are declared in the <tt>skalibs/tai.h</tt> header, +and implemented in the <tt>libskarnet.a</tt> or <tt>libskarnet.so</tt> library. +</p> + +<h2> General information </h2> + +<p> + <tt>tai</tt> is a set of data structures and primitives to represent +and perform operations on time. +</p> + +<p> + The point of <tt>tai</tt> is to handle time without ever having to +deal with annoyances such as Y2K, Y2038, NTP limits, non-linear +clocks, and the like. By using the <tt>tai</tt> interface, you ensure +your program will behave properly no matter what. +</p> + +<h3> What is the problem ? </h3> + +<p> + The standard APIs for time management under Unix are broken in more +or less subtle ways. The most obvious thing is that they do not pass +year 2038. A less obvious problem is that they do not handle leap +seconds correctly. Here are a few references you should read to +understand what is going on: +</p> + +<ul> + <li> <a href="http://www.madore.org/~david/misc/time.html">David Madore's page +on time</a>. It's outdated (there was a leap second in 2009), but complete. </li> + <li> From David Madore again, more to the point: a +<a href="http://www.madore.org/~david/computers/unix-leap-seconds.html">page +about leap seconds, UTC and TAI</a>. </li> + <li> The skalibs <a href="../flags.html#clockistai">--enable-tai-clock</a> +and <a href="../flags.html#tzisright">--enable-right-tz</a> documentation. </li> + <li> <a href="http://cr.yp.to/proto/utctai.html">Dan J. Bernstein's page +on UTC, TAI and Unix time</a>. </li> +</ul> + +<p> + The meat and potatoes of all this is that programmers cannot simply rely on +standard Unix APIs such as +<a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/gettimeofday.html">gettimeofday()</a> +(which, by the way, is marked as obsolescent, but it's not going to disappear tomorrow) +to measure time intervals or even to give precise absolute time, and in +any case those APIs will become obsolete in 2038. +</p> + +<h3> So what does <tt>tai</tt> do ? </h3> + +<p> + <tt>tai</tt> implements - among other things - the +<a href="http://cr.yp.to/libtai/tai64.html">TAI64 and TAI64N</a> +formats, which are used in all of skalibs. This gives a programmer access +to precise <em>linear absolute time</em>, which is suitable for both +timestamping (<em>wallclock</em> usage) and time interval measurements +(<em>stopwatch</em> usage). Additionally, TAI64 passes Y2038 (it can +represent dates exceeding the estimated lifespan of the universe). +</p> + +<p> + <tt>tai</tt> has been inspired by Dan J. Bernstein's +<a href="http://cr.yp.to/libtai.html">libtai</a> library, but does not +borrow any code from it. +</p> + +<h2> Data structures </h2> + +<p> + A <tt>tai_t</tt> structure holds an absolute date with a one-second +precision. A <tt>tain_t</tt> structure holds an absolute date with a +maximum of one-nanosecond precision, as permitted by the underlying system +call. If <a href="../flags.html#usert">flag-usert</a> is clear, the system +clock will be read via +<a href="http://www.opengroup.org/onlinepubs/9699919799/functions/gettimeofday.html">gettimeofday()</a> +system call, which has a one-microsecond precision; if it is set, the +system clock will be read via the +<a href="http://www.opengroup.org/onlinepubs/9699919799/functions/clock_gettime.html">clock_gettime()</a> +system call, which has a one-nanosecond precision. In either case, a current +<tt>tain_t</tt> will be unable to be more precise than the underlying +implementation. +</p> + +<p> + A <tt>tai_t</tt>, as well as a <tt>tain_t</tt>, can also +hold a (possibly negative) relative time, i.e. a difference of absolute +dates. It is up to the programmer to make sure that a relative time is +never interpreted as an absolute TAI64 date, and vice-versa. +</p> + +<h3> The leap second table </h3> + +<p> + skalibs provides a <tt>src/etc/leapsecs.dat</tt> file, +which is copied to <tt>/etc/leapsecs.dat</tt> at installation time +(or wherever you specified with the <tt>--prefix</tt> or <tt>--datadir</tt> +options to configure). +<strong>Make sure this file is always present and readable.</strong> +This file contains the <em>leap second table</em>, which is needed for +conversions between TAI and UTC. If you call a function that needs such +a conversion (for instance, you call <tt>tain_sysclock()</tt> and your +system clock is set to UTC) and the file cannot be read, the function +call will fail. +</p> + +<p> + The leap second table is read once in every process that needs it +(the first time a TAI ↔ UTC conversion is made) and then is +stored in memory. If the <tt>leapsecs.dat</tt> file changes, long-lived +processes will need to be restarted to take the change into account. +</p> + +<h2> Functions </h2> + +<h3> Wallclock operations </h3> + +<p> +<code> int tai_now (tai_t *t) </code> <br /> +Writes the current time as a TAI value to *<em>t</em>, with a +1-second precision. The current time is based on the system clock. +Make sure skalibs has been compiled with or without the +<a href="../flags.html#clockistai">--enable-tai-clock</a> configure option according +to your system clock synchronization method: skalibs supports a +system clock set to TAI-10 or to UTC. +The function returns 1 if it succeeds, or 0 (and sets errno) if +it fails. +</p> + +<p> +<code> int sysclock_get (tain_t *a) </code> <br /> +Reads the current value of the system clock into *<em>a</em>, with +a 1-nanosecond (resp. 1-microsecond ) precision if skalibs has been +configured with (resp. without) the +<a href="../flags.html#usert">--enable-clock</a> option. +Returns 1 if it succeeds or 0 (and sets errno) if it +fails. Note that despite being a <tt>tain_t</tt>, *<em>a</em> +<strong>does not contain a TAI value</strong> - it only contains +an internal, Y2038-safe representation of the value of the system +clock, which should be either TAI-10 or UTC. You should not use +this function directly unless you know exactly what you are doing. +</p> + +<p> +<code> int sysclock_set (tain_t const *a) </code> <br /> +Sets the system clock to *<em>a</em>, provided *<em>a</em> has +the correct internal representation. You should not use this +function directly unless you know exactly what you are doing. +</p> + +<p> +<code> int tain_sysclock (tain_t *a) </code> <br /> +Reads the current time into *<em>a</em>, as a TAI64N value, +with a 1-nanosecond (resp. 1-microsecond) precision if skalibs +has been configured with (resp. without) the +<a href="../flags.html#usert">--enable-clock</a> +option. Returns 1 if it succeeds or 0 (and sets errno) if it +fails. + Here <em>a</em> contains a valid TAI stamp, no matter what the +system clock is set to: arithmetic operations can be performed +on it. +</p> + +<p> +<code> int tain_setnow (tain_t const *a) </code> <br /> +Sets the current time to *<em>a</em>, with a 1-nanosecond +(resp. 1-microsecond) precision if skalibs has been configured +with (resp. without) the +<a href="../flags.html#usert">--enable-clock</a> +option. Returns 1 if it succeeds or 0 (and sets errno) if it +fails. <em>a</em> must contain a valid TAI stamp; proper +operations will be automatically run to convert that stamp into +the right format for the system clock. +</p> + +<h3> Stopwatch operations </h3> + +<p> + The following 3 operations are only defined if your system +provides the +<a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/clock_gettime.html">clock_gettime()</a> +primitive with the CLOCK_MONOTONIC option. +</p> + +<p> +<code> int tain_clockmon_init (tain_t *offset) </code> <br /> +Initializes a stopwatch in *<em>offset</em>. The actual value of +*<em>offset</em> is meaningless to the user; <em>offset</em>'s only +use is to be given as a second parameter to <tt>tain_clockmon()</tt>. +The function returns 1 if it succeeds or 0 (and sets errno) if it fails. +</p> + +<p> + What <tt>tain_clockmon_init()</tt> does is synchronize the "stopwatch +clock" (CLOCK_MONOTONIC) to the system clock. Right after +<tt>tain_clockmon_init()</tt> has been called, the absolute times given +by <tt>tain_clockmon()</tt> and <tt>tain_sysclock()</tt> are similar. Then, +depending on the accuracy of the system clock, a drift may appear; calling +<tt>tain_clockmon_init()</tt> again resets that drift to zero. +</p> + +<p> +<code> int tain_clockmon (tain_t *a, tain_t const *offset) </code> <br /> + Gives the absolute time, as a TAI64N value, in *<em>a</em>. This +absolute time is computed as a linear increase (as measured with +CLOCK_MONOTONIC) since the last time <tt>tain_clockmon_init()</tt> +was called with parameter <em>offset</em>. <tt>tain_clockmon()</tt> +guarantees precise time interval measurements; however, the time it +gives can slightly differ from the result of <tt>tain_sysclock()</tt>. +The function returns 1 if it succeeds or 0 (and sets errno) if it fails. +</p> + +<h3> All-purpose time reading </h3> + +<p> +<code> int tain_init (void) </code> <br /> +If skalibs has been configured with the +<a href="../flags.html#usemon">--enable-monotonic</a> option: this +function initializes a process-global stopwatch, that future +<tt>tain_now</tt> invocations will depend on. +Without the <a href="../flags.html#usemon">--enable-monotonic</a> option: this +function does nothing. +The function returns 1 if it succeeds or 0 (and sets errno) if it fails. +</p> + +<p> +<code> int tain_now (tain_t *a) </code> <br /> +Writes the current time, as a TAI value, to *<em>a</em>. This is the +function you should use to read time by default. It returns 1 if it succeeds or +0 (and sets errno) if it fails. +</p> + +<p> + If skalibs has been configured with the +<a href="../flags.html#usemon">--enable-monotonic</a> option: +<tt>tain_now()</tt> is computed as a linear increase from the last time +<tt>tain_init()</tt> was called. (If <tt>tain_init()</tt> has never +been called before, the first invocation of <tt>tain_now()</tt> +automatically calls <tt>tain_init()</tt>.) + Without the <a href="../flags.html#usemon">--enable-monotonic</a> option: +<tt>tain_now()</tt> is the same as <tt>tain_sysclock()</tt>. +</p> + +<p> + If the above is unclear to you: just use <tt>tain_now()</tt> +everytime you need to read time, and you will always get a reasonable +approximation of the current time, in a format suited for arithmetic +computations. +</p> + +<h3> Converting to/from libc representations </h3> + +<p> +<code> int tai_from_timeval (tai_t *t, struct timeval const *tv) <br /> +int tai_from_timespec (tai_t *t, struct timespec const *ts) <br /> +int tai_relative_from_timeval (tai_t *t, struct timeval const *tv) <br /> +int tai_relative_from_timespec (tai_t *t, struct timespec const *ts) </code> <br /> +Those functions convert an absolute (resp. relative) time in a +struct timeval (resp. struct timespec) to an absolute (resp. relative) +time in a tai_t, with a 1-second precision. They return 1. +</p> + +<p> +<code> int timeval_from_tai (struct timeval *tv, tai_t const *t) <br /> +int timespec_from_tai (struct timespec *ts, tai_t const *t) <br /> +int timeval_from_tai_relative (struct timeval *tv, tai_t const *t) <br /> +int timespec_from_tai_relative (struct timespec *ts, tai_t const *t) </code> <br /> +Those functions do the opposite conversion. They normally return 1; +however, <tt>struct timeval</tt> and <tt>struct timespec</tt> cannot +represent an absolute date before the Epoch, or a negative relative time; +if *<em>t</em> cannot be converted, 0 EINVAL is returned. +</p> + +<p> +<code> int tain_from_timeval (tain_t *a, struct timeval const *tv) <br /> +int tain_from_timespec (tain_t *a, struct timespec const *ts) <br /> +int tain_relative_from_timeval (tain_t *a, struct timeval const *tv) <br /> +int tain_relative_from_timespec (tain_t *a, struct timespec const *ts) <br /> +int timeval_from_tain (struct timeval *tv, tain_t const *a) <br /> +int timespec_from_tain (struct timespec *ts, tain_t const *a) <br /> +int timeval_from_tain_relative (struct timeval *tv, tain_t const *a) <br /> +int timespec_from_tain_relative (struct timespec *ts, tain_t const *a) </code> <br /> +Same conversion operations, but with a <tt>tain_t</tt>. The 1-microsecond +(for <tt>struct timeval</tt>) or 1-nanosecond (for <tt>struct timespec</tt>) +precision is preserved. +</p> + +<p> +<code> void tain_uint (tain_t *a, unsigned int c) </code> <br /> +Stores a relative time of <em>c</em> seconds into <em>a</em>. +</p> + +<p> +<code> int tain_from_millisecs (tain_t *a, int ms) </code> <br /> +This function makes a <tt>tain_t</tt> representing a relative +time of <em>ms</em> milliseconds. <em>ms</em> must be non-negative. +The function returns 1, unless <em>ms</em> is negative, in which case +it returns 0 EINVAL. +</p> + +<p> +<code> int tain_to_millisecs (tain_t const *a) </code> <br /> +If *<em>a</em> contains a non-negative relative time that fits into +a 31-bit integer number of milliseconds, the function returns that +number. Else it returns -1 EINVAL. +</p> + +<h3> Time computations </h3> + +<p> +<code> void tai_add (tai_t *t, tai_t const *t1, tai_t const *t2) </code> <br /> +Stores *<em>t1</em> + *<em>t2</em> into <em>t</em>. Of course, *<em>t1</em> +and *<em>t2</em> must not both represent absolute times. +</p> + +<p> +<code> void tai_sub (tai_t *t, tai_t const *t1, tai_t const *t2) </code> <br /> +Stores *<em>t1</em> - *<em>t2</em> into <em>t</em>. Of course, *<em>t1</em> +and *<em>t2</em> must be of the same type (relative or absolute), and +*<em>t</em> will always be relative. +</p> + +<p> +<code> void tain_add (tain_t *a, tain_t const *a1, tain_t const *a2) <br /> +void tain_sub (tain_t *a, tain_t const *a1, tain_t const *a2) </code> <br /> +Same thing with <tt>tain_t</tt>. +</p> + +<p> +<code> void tain_addsec (tain_t *a, tain_t const *a1, int c) </code> <br /> +Adds <em>c</em> seconds to *<em>a1</em> and stores the result into <em>a</em>. +<em>c</em> may be negative. +</p> + +<p> +<code> void tain_half (tain_t *a, tain_t const *b) </code> <br /> +Stores *<em>b</em>/2 into <em>a</em>. *<em>b</em> must be relative. +</p> + +<h3> Comparing dates or durations </h3> + +<p> +<code> int tai_less (tai_t const *t1, tai_t const *t2) <br /> +int tain_less (tain_t const *t1, tain_t const *t2) </code> <br /> +Those functions return nonzero iff *<em>t1</em> is lesser than *<em>t2</em>. +*<em>t1</em> and *<em>t2</em> must be both relative, or both absolute. +</p> + +<h3> Packing and unpacking </h3> + +<p> +<code> void tai_pack (char *s, tai_t const *t) </code> <br /> +Marshals *<em>t</em> into the buffer <em>s</em> points to, which +must be preallocated with at least TAI_PACK (8) characters. Afterwards, +the buffer contains the +<a href="http://cr.yp.to/libtai/tai64.html#tai64">external TAI64 format</a> +representation of *<em>t</em>. +</p> + +<p> +<code> void tai_unpack (char const *s, tai_t *t) </code> <br /> +Unmarshals the +<a href="http://cr.yp.to/libtai/tai64.html#tai64">external TAI64 format</a> +label pointed to by <em>s</em> (at least TAI_PACK characters) and stores +the result into <em>t</em>. +</p> + +<p> +<code> void tain_pack (char *s, tain_t const *a) <br /> +void tain_unpack (char const *s, tain_t *a) <br /> +void tain_pack (char *s, tain_t const *a) <br /> +void tain_unpack (char const *s, tain_t *a) </code> <br /> +Same thing with +<a href="http://cr.yp.to/libtai/tai64.html#tai64n">external TAI64N format</a>, +using TAIN_PACK (12) characters. +</p> + +<h3> Formatting and scanning </h3> + +<p> +<code> unsigned int tain_fmt (char *s, tain_t const *a) </code> <br /> +Writes into <em>s</em> an ASCII representation of *<em>a</em> in external +TAI64N format. <em>s</em> must point to a preallocated buffer of at least +TAIN_PACK*2 (24) characters. The function returns the number of bytes that +have been written to <em>s</em> (24). +</p> + +<p> +<code> unsigned int tain_scan (char const *s, tain_t *a) </code> <br /> +Reads 24 characters from <em>s</em>; if those characters are a valid ASCII +representation of the external TAI64N format of some time value, this value +is stored into <em>a</em>, and 24 is returned. Else 0 is returned. +</p> + +<a name="timestamp"><h3> Timestamping </h3></a> + +<p> + A <em>TAI64N timestamp</em> is a string of 25 characters: a single '@' +character followed by the ASCII representation of the TAI64N external +format of an absolute date. +</p> + +<p> +<code> unsigned int timestamp_fmt (char *s, tain_t const *a) </code> <br /> +Writes a TAI64N timestamp representing the absolute date *<em>a</em> +into the 25 characters pointed to by <em>s</em>. Returns 25. +</p> + +<p> +<code> unsigned int timestamp_scan (char const *s, tain_t *a) </code> <br /> +Reads 25 characters at <em>s</em>. If those characters are a valid TAI64N +timestamp, stores the absolute date in <em>a</em> and returns 25. Else, +returns 0. +</p> + +<p> +<code> int timestamp (char *s) </code> <br /> +Writes the current time (read from the system clock) as a TAI64N timestamp +into <em>s</em>. Returns 1 if it succeeds or 0 (and sets errno) if it fails. +</p> + +<p> + TAI64N timestamps are an efficient, robust, and easy-to-use way of +timestampping log lines. They're easy to recognize in automatic data +parsers. Log files where every line starts with a TAI64N timestamp can +be merged and alphanumerically sorted: the resulting file will be +chronologically sorted. +</p> + +<p> + The <a href="http://skarnet.org/software/s6/">s6</a> package +provides tools to convert TAI64N timestamps into human-readable +dates. Please do not embed human-readable dates in your log files, +thus making parsing tools unnecessarily hard to write; +use TAI64N timestamps instead, design tools that can parse them, +and translate them to human-readable form at human analysis time. +</p> + +</body> +</html> |