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ICU-4497 update readme with u_init() usage for various ICU versions

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X-SVN-Rev: 17559
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Markus Scherer 2005-05-05 20:41:01 +00:00
parent 681fbba869
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1 changed files with 51 additions and 9 deletions
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icu4c/readme.html
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@ -10,7 +10,7 @@
<meta name="DESCRIPTION" content=
"The introduction to the International Components for Unicode with instructions on building, installation, usage and other information about ICU.">
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
<style type="text/css">
<style type="text/css">
h1 {border-width: 2px; border-style: solid; text-align: center; width: 100%; font-size: 200%; font-weight: bold}
h2 {margin-top: 3em; text-decoration: underline; page-break-before: always}
h2.TOC {page-break-before: auto}
@ -29,7 +29,7 @@
<abbr title="International Components for Unicode">ICU</abbr> 3.3
ReadMe</h1>
<p>Version: 2005-Apr-01<br>
<p>Version: 2005-May-05<br>
Copyright &copy; 1997-2005 International Business Machines Corporation and
others. All Rights Reserved.</p>
<!-- Remember that there is a copyright at the end too -->
@ -1476,13 +1476,55 @@ ADDENVVAR ENVVAR(OUTPUTDIR) VALUE('<i>libraryname</i>')</samp>
<h3><a name="ImportantNotesCPlusPlus" href="#ImportantNotesCPlusPlus" id=
"ImportantNotesCPlusPlus">Using ICU in a Multithreaded Environment</a></h3>
<p>Upon the first usage of most ICU APIs, the global mutex will get
initialized properly, but you can use the <code>u_init()</code> function
from uclean.h to ensure that it is initialized properly. Without calling
this function from a single thread, the data caches inside ICU may get
initialized more than once from multiple threads, which may cause memory
leaks and other problems. There is no harm in calling <code>u_init()</code>
in a single threaded application.</p>
<p>Some versions of ICU require calling the <code>u_init()</code> function
from <code> uclean.h</code> to ensure that ICU is initialized properly. In
those ICU versions, <code>u_init()</code> must be called before ICU is used
from multiple threads. There is no harm in calling <code>u_init()</code>
in a single-threaded application, on a single-CPU machine, or in other cases
where <code>u_init()</code> is not required.</p>
<p>In addition to ensuring thread safety, <code>u_init()</code> also
attempts to load at least one ICU data file. Assuming that all data files
are packaged together (or are in the same folder in files mode), a failure
code from <code>u_init()</code> usually means that the data cannot be
found. In this case, the data may not be installed properly, or the
application may have failed to call <code>udata_setCommonData()</code> or <code>u_setDataDirectory()</code>
which specify to ICU where it can find its data.</p>
<p>Since <code>u_init()</code> will load only one or two data files, it
cannot guarantee that all of the data that an application needs is
available. It cannot check for all data files because the set of files is
customizable, and some ICU services work without loading any data at all. An
application should always check for error codes when opening ICU service
objects (using <code>ucnv_open()</code>, <code>ucol_open()</code>, C++
constructors, etc.).</p>
<h4>ICU 3.4 and later</h4>
<p>ICU 3.4 self-initializes properly for multi-threaded use. It achieves
this without performance penalty by hardcoding the core Unicode properties
data, at the cost of some flexibility. (For details see Jitterbug 4497.)</p>
<p><code>u_init()</code> can be used to check for data loading. It tries to
load the converter alias table (<code>cnvalias.icu</code>).</p>
<h4>ICU 2.6..3.2</h4>
<p>These ICU versions require a call to <code>u_init()</code> before
multi-threaded use. The services that are directly affected are those that
don't have a service object and need to be fast: normalization and character
properties.</p>
<p><code>u_init()</code> loads and initializes the data files for
normalization and character properties (<code>unorm.icu</code> and <code>uprops.icu</code>)
and can therefore also be used to check for data loading.</p>
<h4>ICU 2.4 and earlier</h4>
<p>ICU 2.4 and earlier versions were not prepared for multithreaded use on
multi-CPU platforms where the CPUs implement weak memory coherency. These
CPUs include: Power4, Power5, Alpha, Itanium. <code>u_init()</code> was not
defined yet.</p>
<h4><a name="ImportantNotesHPUX" href="#ImportantNotesHPUX" id=
"ImportantNotesHPUX">Using ICU in a Multithreaded Environment on