ICU-157 draft documentation for u_getDataDirectory() & udata_*
X-SVN-Rev: 253
This commit is contained in:
Markus Scherer
parent
401315d2ba
commit
9b0b9c9fbd
153
icu4c/docs/udata.html
Normal file
153
icu4c/docs/udata.html
Normal file
@ -0,0 +1,153 @@
|
|||||||
|
<html>
|
||||||
|
|
||||||
|
<head>
|
||||||
|
<title>ICU - Formats and API for Binary Data Files</title>
|
||||||
|
</head>
|
||||||
|
|
||||||
|
<body>
|
||||||
|
|
||||||
|
<h1>ICU - Formats and API for Binary Data Files</h1>
|
||||||
|
|
||||||
|
<p>This is a raw draft.</p>
|
||||||
|
|
||||||
|
<h2>Finding ICU data</h2>
|
||||||
|
|
||||||
|
<p>ICU data, when stored in files, is loaded from the file system
|
||||||
|
directory that is returned by <code>u_getDataDirectory()</code>.
|
||||||
|
That directory is determined sequentially by
|
||||||
|
<ul>
|
||||||
|
<li><code>getenv("ICU_DATA")</code> -
|
||||||
|
the contents of the ICU_DATA environment variable</li>
|
||||||
|
<li>on Windows, by the value named <code>"Path"</code> of the registry key
|
||||||
|
<code>HKEY_LOCAL_MACHINE "SOFTWARE\\IBM\\Unicode\\Data"</code></li>
|
||||||
|
<li>relative to the path where <code>icuuc.dll</code> or <code>libicu-uc.so</code> or similar
|
||||||
|
is loaded from: if it is loaded from <code>/some/path/lib/libicu-uc.so</code>, then
|
||||||
|
the path will be <code>/some/path/lib/../share/icu/1.3.1/</code>
|
||||||
|
where <code>"1.3.1"</code> is an example for the version of the ICU library that
|
||||||
|
is trying to locate the data directory</li>
|
||||||
|
<li>relative to the path where <code>icuuc.dll</code> or <code>libicu-uc.so</code> or similar
|
||||||
|
is found by searching the <code>PATH</code> or <code>LIBPATH</code>
|
||||||
|
as appropriate; the relative path is determined as above</li>
|
||||||
|
<li>hardcoded to <code>(system drive)/share/icu/1.3.1/</code>,
|
||||||
|
where <code>(system drive)</code> is empty or a path to the system drive, like
|
||||||
|
<code>"D:\"</code> on Windows or OS/2</li>
|
||||||
|
</ul></p>
|
||||||
|
|
||||||
|
<p>When ICU data is loaded using the <code>udata</code> API functions, then
|
||||||
|
there is a defined sequence of file locations and entry point names that are
|
||||||
|
used to locate the data. See the description in <code>icu/source/common/udata.h</code> for
|
||||||
|
details. Note that the exact data finding depends on the implementation
|
||||||
|
of this API and may differ by platform and by build configuration.
|
||||||
|
See also <code>icu/source/common/udata.c</code> for implementation details.</p>
|
||||||
|
|
||||||
|
|
||||||
|
<h2>Binary Data File Formats</h2>
|
||||||
|
|
||||||
|
<p>Data files for ICU and for applications loading their data with ICU,
|
||||||
|
should have a memory-mappable format. This means that the data should be
|
||||||
|
layed out in the file in an immediately useful way, so that the code that uses
|
||||||
|
the data does not need to parse it or copy it to allocated memory and
|
||||||
|
build additional structures (like Hashtables).
|
||||||
|
Here are some points to consider:</p>
|
||||||
|
|
||||||
|
<ul>
|
||||||
|
<li>The data memory starts at an offset within the data file
|
||||||
|
that is divisible by 16 if you use <code>unewdata.h/.c</code>
|
||||||
|
to write the data.</li>
|
||||||
|
<li>Write explicitly sized values: explicitly 32 bits with an
|
||||||
|
<code>int32_t</code>, not using an ambiguous <code>int</code>.</li>
|
||||||
|
<li>Align all values according to their data type size:
|
||||||
|
Align 16-bit integers on even offsets, 32-bit integers on
|
||||||
|
offsets divisible by 4, etc.</li>
|
||||||
|
<li>Align structures according to their largest field.</li>
|
||||||
|
<li>When writing structures directly, avoid implicit
|
||||||
|
field padding/alignment: if a field may not be aligned
|
||||||
|
within the structure according to its size, then
|
||||||
|
insert additional (reserved) fields to explicitly
|
||||||
|
size-align that field.</li>
|
||||||
|
<li>Avoid floating point values if possible. Their size and structure
|
||||||
|
may differ among platforms.</li>
|
||||||
|
<li>Avoid boolean (<code>bool_t</code>, <code>bool</code>) values
|
||||||
|
and use explictly sized integer values instead
|
||||||
|
because the size of the boolean type may vary.</li>
|
||||||
|
<li>Write offsets to sub-structures at the beginning of the data
|
||||||
|
so that those sub-structures can be accessed directly without
|
||||||
|
parsing the data that precedes them.</li>
|
||||||
|
<li>If data needs to be read linearly, then precede it with its length
|
||||||
|
rather than terminating it with a sentinel value.</li>
|
||||||
|
<li>When writing <code>char[]</code> strings, write only "invariant"
|
||||||
|
characters - avoid anything that is not common among all ASCII-
|
||||||
|
or EBCDIC-based encodings. This avoids incompatibilities and
|
||||||
|
real, heavyweight codepage conversions.
|
||||||
|
Even on the same platform, the default encoding may not always
|
||||||
|
be the same one, and every "non-invariant" character
|
||||||
|
may change.<br>
|
||||||
|
(The term "invariant characters" is from
|
||||||
|
<a href="http://www.unicode.org/unicode/reports/tr16/">
|
||||||
|
Unicode Technical Report 16 (UTF-EBCDIC)</a>.)</li>
|
||||||
|
</ul>
|
||||||
|
|
||||||
|
|
||||||
|
<h2>Platform-dependency of Binary Data Files</h2>
|
||||||
|
|
||||||
|
<p>Data files with formats as described above should be portable among
|
||||||
|
machines with the same set of relevant properties:</p>
|
||||||
|
|
||||||
|
<ul>
|
||||||
|
<li>Byte ordering: If the data contains values other than byte arrays.<br>
|
||||||
|
Example: <code>uint16_t</code>, <code>int32_t</code>.</li>
|
||||||
|
<li>Character set family: Some data files contain <code>char[]</code>.
|
||||||
|
Such strings should contain only "invariant characters", but
|
||||||
|
are even so only portable among machines with the same character set
|
||||||
|
family, i.e., they must share for example the ASCII or EBCDIC
|
||||||
|
graphic characters.</li>
|
||||||
|
<li>Unicode Character size: Some data files contain <code>UChar[]</code>.
|
||||||
|
In principle, Unicode characters are stored using UTF-8, UTF-16, or UTF-32.
|
||||||
|
Thus, Unicode strings are directly compatible if the code unit size is the same.
|
||||||
|
ICU uses only UTF-16 at this point.</li>
|
||||||
|
</ul>
|
||||||
|
|
||||||
|
<p>All of these properties can be verified by checking the
|
||||||
|
<code>UDataInfo</code> structure of the data, which is done
|
||||||
|
best in a <code>UDataMemoryIsAcceptable()</code> function passed into
|
||||||
|
the <code>udata_openChoice()</code> API function.</p>
|
||||||
|
|
||||||
|
<p>If a data file is loaded on a machine with different relevant properties
|
||||||
|
than the machine where the data file was generated, then the using
|
||||||
|
code could adapt by detecting the differences and reformatting the
|
||||||
|
data on the fly or in a copy in memory.
|
||||||
|
This would improve portability of the data files but significantly
|
||||||
|
decrease performance.</p>
|
||||||
|
|
||||||
|
<p>"Relevant" properties are those that affect the portability of the
|
||||||
|
data in the particular file.</p>
|
||||||
|
|
||||||
|
<p>For example, a flat (memory-mapped) binary data file
|
||||||
|
that contains 16-bit and 32-bit integers and is
|
||||||
|
created for a typical, big-endian Unix machine, can be used
|
||||||
|
on an OS/390 system or any other big-endian machine.<br>
|
||||||
|
If the file also contains <code>char[]</code> strings,
|
||||||
|
then it can be easily shared among all big-endian <em>and</em>
|
||||||
|
ASCII-based machines, but not with (e.g.) an OS/390.<br>
|
||||||
|
OS/390 and OS/400 systems, however, could easily share such
|
||||||
|
a data file.</p>
|
||||||
|
|
||||||
|
<p>To make sure that the relevant platform properties of
|
||||||
|
the data file and the loading machine match, the
|
||||||
|
<code>udata_openChoice()</code> API function should be used with a
|
||||||
|
<code>UDataMemoryIsAcceptable()</code> function that checks for
|
||||||
|
these properties.</p>
|
||||||
|
|
||||||
|
<p>Some data file loading mechanisms prevent using data files generated on
|
||||||
|
a different platform to begin with, especially data files packaged as DLLs
|
||||||
|
(shared libraries).</p>
|
||||||
|
|
||||||
|
|
||||||
|
<h2>Writing a binary data file</h2>
|
||||||
|
|
||||||
|
<p>... Use <code>icu/source/tools/toolutil/unewdata.h|.c</code> to write data files,
|
||||||
|
can include a copyright statement or other comment...</p>
|
||||||
|
|
||||||
|
</body>
|
||||||
|
|
||||||
|
</html>
|
||||||
Loading…
Reference in New Issue
Block a user