diff options
Diffstat (limited to 'erts/doc')
37 files changed, 358 insertions, 98 deletions
diff --git a/erts/doc/src/Makefile b/erts/doc/src/Makefile index 89d7c85a86..d4c6fe67d2 100644 --- a/erts/doc/src/Makefile +++ b/erts/doc/src/Makefile @@ -78,6 +78,7 @@ XML_CHAPTER_FILES = \ erl_ext_dist.xml \ erl_dist_protocol.xml \ communication.xml \ + time_correction.xml \ notes.xml \ notes_history.xml diff --git a/erts/doc/src/absform.xml b/erts/doc/src/absform.xml index e512c19e05..55aef9f8ab 100644 --- a/erts/doc/src/absform.xml +++ b/erts/doc/src/absform.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE chapter SYSTEM "chapter.dtd"> <chapter> diff --git a/erts/doc/src/alt_dist.xml b/erts/doc/src/alt_dist.xml index 038950b54d..e4912576f7 100644 --- a/erts/doc/src/alt_dist.xml +++ b/erts/doc/src/alt_dist.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE chapter SYSTEM "chapter.dtd"> <chapter> <header> <copyright> - <year>2000</year><year>2011</year> + <year>2000</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/book.xml b/erts/doc/src/book.xml index 00a2888685..dc02edc5c6 100644 --- a/erts/doc/src/book.xml +++ b/erts/doc/src/book.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE book SYSTEM "book.dtd"> <book xmlns:xi="http://www.w3.org/2001/XInclude"> <header titlestyle="normal"> <copyright> - <year>1997</year><year>2009</year> + <year>1997</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/communication.xml b/erts/doc/src/communication.xml index 61a9b0e716..02040c9edb 100644 --- a/erts/doc/src/communication.xml +++ b/erts/doc/src/communication.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE chapter SYSTEM "chapter.dtd"> <chapter> diff --git a/erts/doc/src/crash_dump.xml b/erts/doc/src/crash_dump.xml index 8f5515baca..c59741f250 100644 --- a/erts/doc/src/crash_dump.xml +++ b/erts/doc/src/crash_dump.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE chapter SYSTEM "chapter.dtd"> <chapter> diff --git a/erts/doc/src/driver.xml b/erts/doc/src/driver.xml index 52283879c7..616703fdef 100644 --- a/erts/doc/src/driver.xml +++ b/erts/doc/src/driver.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE chapter SYSTEM "chapter.dtd"> <chapter> <header> <copyright> - <year>2001</year><year>2011</year> + <year>2001</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/driver_entry.xml b/erts/doc/src/driver_entry.xml index c37138e7b1..48dfcb09b1 100644 --- a/erts/doc/src/driver_entry.xml +++ b/erts/doc/src/driver_entry.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE cref SYSTEM "cref.dtd"> <cref> <header> <copyright> - <year>2001</year><year>2012</year> + <year>2001</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> @@ -110,6 +110,8 @@ <p>When the driver has passed the <c>driver_entry</c> over to the emulator, the driver is <em>not</em> allowed to modify the <c>driver_entry</c>.</p> + <p>If compiling a driver for static inclusion via --enable-static-drivers you + have to define STATIC_ERLANG_DRIVER before the DRIVER_INIT declaration.</p> <note> <p>Do <em>not</em> declare the <c>driver_entry</c> <c>const</c>. This since the emulator needs to modify the <c>handle</c>, and the <c>handle2</c> diff --git a/erts/doc/src/epmd.xml b/erts/doc/src/epmd.xml index 3e7005410f..963d35c3c8 100644 --- a/erts/doc/src/epmd.xml +++ b/erts/doc/src/epmd.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE comref SYSTEM "comref.dtd"> <comref> <header> <copyright> - <year>1996</year><year>2011</year> + <year>1996</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/erl.xml b/erts/doc/src/erl.xml index 528a2d95aa..bf0d132955 100644 --- a/erts/doc/src/erl.xml +++ b/erts/doc/src/erl.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE comref SYSTEM "comref.dtd"> <comref> diff --git a/erts/doc/src/erl_dist_protocol.xml b/erts/doc/src/erl_dist_protocol.xml index 84f4be208d..890293d802 100644 --- a/erts/doc/src/erl_dist_protocol.xml +++ b/erts/doc/src/erl_dist_protocol.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="iso-8859-1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE chapter SYSTEM "chapter.dtd"> <chapter> diff --git a/erts/doc/src/erl_driver.xml b/erts/doc/src/erl_driver.xml index c055d1ca9e..b453a4861e 100644 --- a/erts/doc/src/erl_driver.xml +++ b/erts/doc/src/erl_driver.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE cref SYSTEM "cref.dtd"> <cref> diff --git a/erts/doc/src/erl_ext_dist.xml b/erts/doc/src/erl_ext_dist.xml index c6849f3326..28e6f1d05e 100644 --- a/erts/doc/src/erl_ext_dist.xml +++ b/erts/doc/src/erl_ext_dist.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="iso-8859-1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE chapter SYSTEM "chapter.dtd"> <chapter> diff --git a/erts/doc/src/erl_nif.xml b/erts/doc/src/erl_nif.xml index 864b91946a..7ac8181d47 100644 --- a/erts/doc/src/erl_nif.xml +++ b/erts/doc/src/erl_nif.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE cref SYSTEM "cref.dtd"> <cref> @@ -330,6 +330,8 @@ ok <c>upgrade</c> will be called to initialize the library. <c>unload</c> is called to release the library. They are all described individually below.</p> + <p>If compiling a nif for static inclusion via --enable-static-nifs you + have to define STATIC_ERLANG_NIF before the ERL_NIF_INIT declaration.</p> </item> <tag><marker id="load"/>int (*load)(ErlNifEnv* env, void** priv_data, ERL_NIF_TERM load_info)</tag> diff --git a/erts/doc/src/erl_prim_loader.xml b/erts/doc/src/erl_prim_loader.xml index 9f5b3f385b..6751deda4d 100644 --- a/erts/doc/src/erl_prim_loader.xml +++ b/erts/doc/src/erl_prim_loader.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE erlref SYSTEM "erlref.dtd"> <erlref> <header> <copyright> - <year>1996</year><year>2011</year> + <year>1996</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/erlang.xml b/erts/doc/src/erlang.xml index cbb25c2cf2..a498fc24df 100644 --- a/erts/doc/src/erlang.xml +++ b/erts/doc/src/erlang.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE erlref SYSTEM "erlref.dtd"> <erlref> @@ -2613,7 +2613,28 @@ os_prompt% </pre> <desc> <p>Returns a port identifier as the result of opening a new Erlang port. A port can be seen as an external Erlang - process. <c><anno>PortName</anno></c> is one of the following:</p> + process. + </p> + <p>The name of the executable as well as the arguments + given in <c>cd</c>, <c>env</c>, <c>args</c> and <c>arg0</c> is subject to + Unicode file name translation if the system is running + in Unicode file name mode. To avoid + translation or force i.e. UTF-8, supply the executable + and/or arguments as a binary in the correct + encoding. See the <seealso + marker="kernel:file">file</seealso> module, the + <seealso marker="kernel:file#native_name_encoding/0"> + file:native_name_encoding/0</seealso> function and the + <seealso marker="stdlib:unicode_usage">stdlib users guide + </seealso> for details.</p> + + <note><p>The characters in the name (if given as a list) + can only be > 255 if the Erlang VM is started in + Unicode file name translation mode, otherwise the name + of the executable is limited to the ISO-latin-1 + character set.</p></note> + + <p><c><anno>PortName</anno></c> is one of the following:</p> <taglist> <tag><c>{spawn, <anno>Command</anno>}</c></tag> <item> @@ -2668,25 +2689,6 @@ os_prompt% </pre> executed, the appropriate command interpreter will implicitly be invoked, but there will still be no command argument expansion or implicit PATH search.</p> - - <p>The name of the executable as well as the arguments - given in <c>args</c> and <c>arg0</c> is subject to - Unicode file name translation if the system is running - in Unicode file name mode. To avoid - translation or force i.e. UTF-8, supply the executable - and/or arguments as a binary in the correct - encoding. See the <seealso - marker="kernel:file">file</seealso> module, the - <seealso marker="kernel:file#native_name_encoding/0"> - file:native_name_encoding/0</seealso> function and the - <seealso marker="stdlib:unicode_usage">stdlib users guide - </seealso> for details.</p> - - <note><p>The characters in the name (if given as a list) - can only be > 255 if the Erlang VM is started in - Unicode file name translation mode, otherwise the name - of the executable is limited to the ISO-latin-1 - character set.</p></note> <p>If the <c><anno>FileName</anno></c> cannot be run, an error exception, with the posix error code as the reason, is @@ -2762,11 +2764,7 @@ os_prompt% </pre> strings. The one exception is <c><anno>Val</anno></c> being the atom <c>false</c> (in analogy with <c>os:getenv/1</c>), which removes the environment variable. - </p> - <p>If Unicode filename encoding is in effect (see the <seealso - marker="erts:erl#file_name_encoding">erl manual - page</seealso>), the strings (both <c>Name</c> and - <c>Value</c>) may contain characters with codepoints > 255.</p> + </p> </item> <tag><c>{args, [ string() | binary() ]}</c></tag> <item> @@ -2794,21 +2792,6 @@ os_prompt% </pre> should not be given in this list. The proper executable name will automatically be used as argv[0] where applicable.</p> - <p>When the Erlang VM is running in Unicode file name - mode, the arguments can contain any Unicode characters and - will be translated into whatever is appropriate on the - underlying OS, which means UTF-8 for all platforms except - Windows, which has other (more transparent) ways of - dealing with Unicode arguments to programs. To avoid - Unicode translation of arguments, they can be supplied as - binaries in whatever encoding is deemed appropriate.</p> - - <note><p>The characters in the arguments (if given as a - list of characters) can only be > 255 if the Erlang - VM is started in Unicode file name mode, - otherwise the arguments are limited to the - ISO-latin-1 character set.</p></note> - <p>If one, for any reason, wants to explicitly set the program name in the argument vector, the <c>arg0</c> option can be used.</p> @@ -2824,9 +2807,6 @@ os_prompt% </pre> responds to this is highly system dependent and no specific effect is guaranteed.</p> - <p>The unicode file name translation rules of the - <c>args</c> option apply to this option as well.</p> - </item> <tag><c>exit_status</c></tag> diff --git a/erts/doc/src/erlc.xml b/erts/doc/src/erlc.xml index 81dffe45cf..10cab344b0 100644 --- a/erts/doc/src/erlc.xml +++ b/erts/doc/src/erlc.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE comref SYSTEM "comref.dtd"> <comref> <header> <copyright> - <year>1997</year><year>2012</year> + <year>1997</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/erlsrv.xml b/erts/doc/src/erlsrv.xml index 365ae21d39..71cee714a5 100644 --- a/erts/doc/src/erlsrv.xml +++ b/erts/doc/src/erlsrv.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE comref SYSTEM "comref.dtd"> <comref> <header> <copyright> - <year>1998</year><year>2012</year> + <year>1998</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/erts_alloc.xml b/erts/doc/src/erts_alloc.xml index 9dab5b3876..8dd9ed1398 100644 --- a/erts/doc/src/erts_alloc.xml +++ b/erts/doc/src/erts_alloc.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE cref SYSTEM "cref.dtd"> <cref> diff --git a/erts/doc/src/escript.xml b/erts/doc/src/escript.xml index 9e2a87dde6..180447cac4 100644 --- a/erts/doc/src/escript.xml +++ b/erts/doc/src/escript.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE comref SYSTEM "comref.dtd"> <comref> diff --git a/erts/doc/src/fascicules.xml b/erts/doc/src/fascicules.xml index cae197a516..1c371bd9c8 100644 --- a/erts/doc/src/fascicules.xml +++ b/erts/doc/src/fascicules.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE fascicules SYSTEM "fascicules.dtd"> <fascicules> diff --git a/erts/doc/src/inet_cfg.xml b/erts/doc/src/inet_cfg.xml index 2a033c037c..d40bc5f9ee 100644 --- a/erts/doc/src/inet_cfg.xml +++ b/erts/doc/src/inet_cfg.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE chapter SYSTEM "chapter.dtd"> <chapter> <header> <copyright> - <year>2004</year><year>2010</year> + <year>2004</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/init.xml b/erts/doc/src/init.xml index d5c43f6e57..09b5493341 100644 --- a/erts/doc/src/init.xml +++ b/erts/doc/src/init.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE erlref SYSTEM "erlref.dtd"> <erlref> <header> <copyright> - <year>1996</year><year>2011</year> + <year>1996</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/match_spec.xml b/erts/doc/src/match_spec.xml index bdcf9c3816..334b47d34c 100644 --- a/erts/doc/src/match_spec.xml +++ b/erts/doc/src/match_spec.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE chapter SYSTEM "chapter.dtd"> <chapter> <header> <copyright> - <year>1999</year><year>2012</year> + <year>1999</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/notes_history.xml b/erts/doc/src/notes_history.xml index cc3b938c86..4420311912 100644 --- a/erts/doc/src/notes_history.xml +++ b/erts/doc/src/notes_history.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE chapter SYSTEM "chapter.dtd"> <chapter> <header> <copyright> - <year>2006</year><year>2009</year> + <year>2006</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/part.xml b/erts/doc/src/part.xml index fa50329cad..7b17b5b551 100644 --- a/erts/doc/src/part.xml +++ b/erts/doc/src/part.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE part SYSTEM "part.dtd"> <part xmlns:xi="http://www.w3.org/2001/XInclude"> @@ -32,6 +32,7 @@ <p>The Erlang Runtime System Application <em>ERTS</em>.</p> </description> <xi:include href="communication.xml"/> + <xi:include href="time_correction.xml"/> <xi:include href="match_spec.xml"/> <xi:include href="crash_dump.xml"/> <xi:include href="alt_dist.xml"/> diff --git a/erts/doc/src/part_notes.xml b/erts/doc/src/part_notes.xml index 4f183999e6..b5c8f0af09 100644 --- a/erts/doc/src/part_notes.xml +++ b/erts/doc/src/part_notes.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE part SYSTEM "part.dtd"> <part xmlns:xi="http://www.w3.org/2001/XInclude"> <header> <copyright> - <year>2004</year><year>2009</year> + <year>2004</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/part_notes_history.xml b/erts/doc/src/part_notes_history.xml index 1b9bcca773..a99fa4a17f 100644 --- a/erts/doc/src/part_notes_history.xml +++ b/erts/doc/src/part_notes_history.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE part SYSTEM "part.dtd"> <part xmlns:xi="http://www.w3.org/2001/XInclude"> <header> <copyright> - <year>2006</year><year>2009</year> + <year>2006</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/ref_man.xml b/erts/doc/src/ref_man.xml index e55923c344..8ed7090a61 100644 --- a/erts/doc/src/ref_man.xml +++ b/erts/doc/src/ref_man.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE application SYSTEM "application.dtd"> <application xmlns:xi="http://www.w3.org/2001/XInclude"> diff --git a/erts/doc/src/run_erl.xml b/erts/doc/src/run_erl.xml index c9784299b3..684f7b1ddd 100644 --- a/erts/doc/src/run_erl.xml +++ b/erts/doc/src/run_erl.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE comref SYSTEM "comref.dtd"> <comref> <header> <copyright> - <year>1999</year><year>2011</year> + <year>1999</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/specs.xml b/erts/doc/src/specs.xml index e5c2f4783f..41a3984659 100644 --- a/erts/doc/src/specs.xml +++ b/erts/doc/src/specs.xml @@ -1,4 +1,4 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <specs xmlns:xi="http://www.w3.org/2001/XInclude"> <xi:include href="../specs/specs_erl_prim_loader.xml"/> <xi:include href="../specs/specs_erlang.xml"/> diff --git a/erts/doc/src/start.xml b/erts/doc/src/start.xml index 5dc33deb2a..e9a5714f93 100644 --- a/erts/doc/src/start.xml +++ b/erts/doc/src/start.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE comref SYSTEM "comref.dtd"> <comref> <header> <copyright> - <year>1999</year><year>2009</year> + <year>1999</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/start_erl.xml b/erts/doc/src/start_erl.xml index 92d87b095a..fe808f7737 100644 --- a/erts/doc/src/start_erl.xml +++ b/erts/doc/src/start_erl.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE comref SYSTEM "comref.dtd"> <comref> <header> <copyright> - <year>1998</year><year>2011</year> + <year>1998</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/time_correction.xml b/erts/doc/src/time_correction.xml new file mode 100644 index 0000000000..d52cc7f3e2 --- /dev/null +++ b/erts/doc/src/time_correction.xml @@ -0,0 +1,274 @@ +<?xml version="1.0" encoding="utf8" ?> +<!DOCTYPE chapter SYSTEM "chapter.dtd"> + +<chapter> + <header> + <copyright> + <year>1999</year><year>2013</year> + <holder>Ericsson AB. All Rights Reserved.</holder> + </copyright> + <legalnotice> + The contents of this file are subject to the Erlang Public License, + Version 1.1, (the "License"); you may not use this file except in + compliance with the License. You should have received a copy of the + Erlang Public License along with this software. If not, it can be + retrieved online at http://www.erlang.org/. + + Software distributed under the License is distributed on an "AS IS" + basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See + the License for the specific language governing rights and limitations + under the License. + + </legalnotice> + + <title>Time and time correction in Erlang</title> + <prepared>Patrik Nyblom</prepared> + <responsible></responsible> + <docno></docno> + <approved></approved> + <checked></checked> + <date>2013-08-28</date> + <rev>PA1</rev> + <file>time_correction.xml</file> + </header> + <p>Time is vital to an Erlang program and, more importantly, <em>correct</em> + time is vital to an Erlang program. As Erlang is a language with + soft real time properties and we have the possibility to express + time in our programs, the Virtual Machine and the language has to be + very careful about what is considered a correct point in time and in + how time functions behave.</p> + + <p>In the beginning, Erlang was constructed assuming that the wall + clock time in the system showed a monotonic time moving forward at + exactly the same pace as the definition of time. That more or less + meant that an atomic clock (or better) was expected to be attached + to your hardware and that the hardware was then expected to be + locked away from any human (or unearthly) tinkering for all + eternity. While this might be a compelling thought, it's simply + never the case.</p> + + <p>A "normal" modern computer can not keep time. Not on itself and + not unless you actually have a chip level atomic clock wired to + it. Time, as perceived by your computer, will normally need to be + corrected. Hence the NTP protocol that together with the ntpd + process will do it's best to keep your computers time in sync with + the "real" time in the universe. Between NTP corrections, usually a + less potent time-keeper than an atomic clock is used.</p> + + <p>But NTP is not fail safe. The NTP server can be unavailable, the + ntp.conf can be wrongly configured or your computer may from time to + time be disconnected from the internet. Furthermore you can have a + user (or even system administrator) on your system that thinks the + right way to handle daylight saving time is to adjust the clock one + hour two times a year (a tip, that is not the right way to do + it...). To further complicate things, this user fetched your + software from the internet and has never ever thought about what's + the correct time as perceived by a computer. The user simply does + not care about keeping the wall clock in sync with the rest of the + universe. The user expects your program to have omnipotent knowledge + about the time.</p> + + <p>Most programmers also expect time to be reliable, at least until + they realize that the wall clock time on their workstation is of by + a minute. Then they simply set it to the correct time, maybe or + maybe not in a smooth way. Most probably not in a smooth way.</p> + + <p>The amount of problems that arise when you expect the wall clock + time on the system to always be correct may be immense. Therefore Erlang + introduced the "corrected estimate of time", or the "time + correction" many years ago. The time correction relies on the fact + that most operating systems have some kind of monotonic clock, + either a real time extension or some built in "tick counter" that is + independent of the wall clock settings. This counter may have + microsecond resolution or much less, but generally it has a drift + that is not to be ignored.</p> + + <p>So we have this monotonic ticking and we have the wall clock + time. Two unreliable times that together can give us an estimate of + an actual wall clock time that does not jump around and that + monotonically moves forward. If the tick counter has a high + resolution, this is fairly easy to do, if the counter has a low + resolution, it's more expensive, but still doable down to + frequencies of 50-60 Hz (of the tick counter).</p> + + <p>So the corrected time is the nearest approximation of an atomic + clock that is available on the computer. We want it to have the + following properties:</p> + <taglist> + <tag>Monotonic</tag> + <item>The clock should not move backwards</item> + <tag>Intervals should be near the truth</tag> + <item>We want the actual time (as measured by an atomic clock or + an astronomer) that passes between two time stamps, T1 and T2, to be as + near to T2 - T1 as possible.</item> + <tag>Tight coupling to the wall clock</tag> + <item>We want a timer that is to be fired when the wall clock + reaches a time in the future, to fire as near to that point in + time as possible</item> + </taglist> + <p>To meet all the criteria, we have to utilize both times in such a + way that Erlangs "corrected time" moves slightly slower or slightly + faster than the wall clock to get in sync with it. The word + "slightly" means a maximum of 1% difference to the wall clock time, + meaning that a sudden change in the wall clock of one minute, takes + 100 minutes to fix, by letting all "corrected time" move 1% slower + or faster.</p> + + <p>Needless to say, correcting for a faulty handling of daylight + saving time may be disturbing to a user comparing wall clock + time to for example calendar:now_to_local_time(erlang:now()). But + calendar:now_to_local_time/1 is not supposed to be used for presenting wall + clock time to the user.</p> + + <p>Time correction is not perfect, but it saves you from the havoc + of clocks jumping around, which would make timers in your program + fire far to late or far to early and could bring your whole system + to it's knees (or worse) just because someone detected a small error + in the wall clock time of the server where your program runs. So + while it might be confusing, it is still a really good feature of + Erlang and you should not throw it away using time functions which + may give you higher benchmark results, not unless you really know + what you're doing.</p> + + <section> + <title>What does time correction mean in my system?</title> + <p>Time correction means that Erlang estimates a time from current + and previous settings of the wall clock, and it uses a fairly + exact tick counter to detect when the wall clock time has jumped + for some reason, slowly adjusting to the new value.</p> + + <p>In practice, this means that the difference between two calls + to time corrected functions, like erlang:now(), might differ up to + one percent from the corresponding calls to non time corrected + functions (like os:timestamp()). Furthermore, if comparing + calendar:local_time/0 to calendar:now_to_local_time(erlang:now()), + you might temporarily see a difference, depending on how well kept your + system is.</p> + + <p>It is important to understand that it is (to the program) + always unknown if it is the wall clock time that moves in the + wrong pace or the Erlang corrected time. The only way to determine + that, is to have an external source of universally correct time. If + some such source is available, the wall clock time can be kept + nearly perfect at all times, and no significant difference will be + detected between erlang:now/0's pace and the wall clock's.</p> + + <p>Still, the time correction will mean that your system keeps + it's real time characteristics very well, even when the wall clock + is unreliable.</p> + </section> + <section> + <title>Where does Erlang use corrected time?</title> + <p>For all functionality where real time characteristics are + desirable, time correction is used. This basically means:</p> + <taglist> + <tag>erlang:now/0</tag> + <item>The infamous erlang:now/0 function uses time correction so + that differences between two "now-timestamps" will correspond to + other timeouts in the system. erlang:now/0 also holds other + properties, discussed later.</item> + <tag>receive ... after</tag> + <item>Timeouts on receive uses time correction to determine a + stable timeout interval.</item> + <tag>The timer module</tag> + <item>As the timer module uses other built in functions which + deliver corrected time, the timer module itself works with + corrected time.</item> + <tag>erlang:start_timer/3 and erlang:send_after/3</tag> + <item>The timer BIF's work with corrected time, so that they + will not fire prematurely or too late due to changes in the wall + clock time.</item> + </taglist> + + <p>All other functionality in the system where erlang:now/0 or any + other time corrected functionality is used, will of course + automatically benefit from it, as long as it's not "optimized" to + use some other time stamp function (like os:timestamp/0).</p> + + <p>Modules like calendar and functions like erlang:localtime/0 use + the wall clock time as it is currently set on the system. They + will not use corrected time. However, if you use a now-value and + convert it to local time, you will get a corrected local time + value, which may or may not be what you want. Typically older code + tend to use erlang:now/0 as a wall clock time, which is usually + correct (at least when testing), but might surprise you when + compared to other times in the system.</p> + </section> + <section> + <title>What is erlang:now/0 really?</title> + <p>erlang:now/0 is a function designed to serve multiple purposes + (or a multi-headed beast if you're a VM designer). It is expected + to hold the following properties:</p> + <taglist> + <tag>Monotonic</tag> + <item>erlang:now() never jumps backwards - it always moves + forward</item> + <tag>Interval correct</tag> + <item>The interval between two erlang:now() calls is expected to + correspond to the correct time in real life (as defined by an + atomic clock, or better)</item> + <tag>Absolute correctness</tag> + <item>The erlang:now/0 value should be possible to convert to an + absolute and correct date-time, corresponding to the real world + date and time (the wall clock)</item> + <tag>System correspondence</tag> + <item>The erlang:now/0 value converted to a date-time is + expected to correspond to times given by other programs on the + system (or by functions like os:timestamp/0)</item> + <tag>Unique</tag> + <item>No two calls to erlang:now on one Erlang node should + return the same value</item> + </taglist> + <p>All these requirements are possible to uphold at the same + time if (and only if):</p> + <taglist> + <tag>The wall clock time of the system is perfect</tag> + <item>The system (Operating System) time needs to be perfectly + in sync with the actual time as defined by an atomic clock or + a better time source. A good installation using NTP, and that is + up to date before Erlang starts, will have properties that for + most users and programs will be near indistinguishable from the + perfect time. Note that any larger corrections to the time done + by hand, or after Erlang has started, will partly (or + temporarily) invalidate some of the properties, as the time is + no longer perfect.</item> + <tag>Less than one call per microsecond to erlang:now/0 is + done</tag> + <item>This means that at <em>any</em> microsecond interval in + time, there can be no more than one call to erlang:now/0 in the + system. However, for the system not to loose it's properties + completely, it's enough that it on average is no more than one + call per microsecond (in one Erlang node).</item> + </taglist> + <p>The uniqueness property of erlang:now/0 is the most limiting + property. It means that erlang:now() maintains a global state and + that there is a hard-to-check property of the system that needs to + be maintained. For most applications this is still not a problem, + but a future system might very well manage to violate the + frequency limit on the calls globally. The uniqueness property is + also quite useless, as there are globally unique references that + provide a much better unique value to programs. However the + property will need to be maintained unless a really subtle + backward compatibility issue is to be introduced.</p> + </section> + <section> + <title>Should I use erlang:now/0 or os:timestamp/0</title> + <p>The simple answer is to use erlang:now/0 for everything where + you want to keep real time characteristics, but use os:timestamp + for things like logs, user communication and debugging (typically + timer:ts uses os:timestamp, as it is a test tool, not a real world + application API). The benefit of using os:timestamp/0 is that it's + faster and does not involve any global state (unless the operating + system has one). The downside is that it will be vulnerable to wall + clock time changes.</p> + </section> + <section> + <title>Turning off time correction</title> + <p>If, for some reason, time correction causes trouble and you are + absolutely confident that the wall clock on the system is nearly + perfect, you can turn off time correction completely by giving the + <c>+c</c> option to <c>erl</c>. The probability for this being a + good idea, is very low.</p> + </section> +</chapter> + diff --git a/erts/doc/src/tty.xml b/erts/doc/src/tty.xml index b16523e085..db15195f65 100644 --- a/erts/doc/src/tty.xml +++ b/erts/doc/src/tty.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE chapter SYSTEM "chapter.dtd"> <chapter> <header> <copyright> - <year>1996</year><year>2010</year> + <year>1996</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/werl.xml b/erts/doc/src/werl.xml index 1494d91da8..49cc45e745 100644 --- a/erts/doc/src/werl.xml +++ b/erts/doc/src/werl.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE comref SYSTEM "comref.dtd"> <comref> <header> <copyright> - <year>1998</year><year>2009</year> + <year>1998</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> diff --git a/erts/doc/src/zlib.xml b/erts/doc/src/zlib.xml index 8917ab5c3a..afc597b729 100644 --- a/erts/doc/src/zlib.xml +++ b/erts/doc/src/zlib.xml @@ -1,10 +1,10 @@ -<?xml version="1.0" encoding="latin1" ?> +<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE erlref SYSTEM "erlref.dtd"> <erlref> <header> <copyright> - <year>2005</year><year>2011</year> + <year>2005</year><year>2013</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> |