mirror of https://github.com/odrling/Aegisub
65 lines
3.3 KiB
Plaintext
65 lines
3.3 KiB
Plaintext
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While some other iconv(3) implementations - like FreeBSD iconv(3) - choose
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the "many small shared libraries" and dlopen(3) approach, this implementation
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packs everything into a single shared library. Here is a comparison of the
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two designs.
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* Run-time efficiency
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1. A dlopen() based approach needs a cache of loaded shared libraries.
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Otherwise, every iconv_open() call will result in a call to dlopen()
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and thus to file system related system calls - which is prohibitive
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because some applications use the iconv_open/iconv/iconv_close sequence
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for every single filename, string, or piece of text.
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2. In terms of virtual memory use, both approaches are on par. Being shared
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libraries, the tables are shared between any processes that use them.
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And because of the demand loading used by Unix systems (and because libiconv
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does not have initialization functions), only those parts of the tables
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which are needed (typically very few kilobytes) will be read from disk and
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paged into main memory.
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3. Even with a cache of loaded shared libraries, the dlopen() based approach
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makes more system calls, because it has to load one or two shared libraries
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for every encoding in use.
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* Total size
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In the dlopen(3) approach, every shared library has a symbol table and
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relocation offset. All together, FreeBSD iconv installs more than 200 shared
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libraries with a total size of 2.3 MB. Whereas libiconv installs 0.45 MB.
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* Extensibility
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The dlopen(3) approach is good for guaranteeing extensibility if the iconv
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implementation is distributed without source. (Or when, as in glibc, you
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cannot rebuild iconv without rebuilding your libc, thus possibly
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destabilizing your system.)
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The libiconv package achieves extensibility through the LGPL license:
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Every user has access to the source of the package and can extend and
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replace just libiconv.so.
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The places which have to be modified when a new encoding is added are as
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follows: add an #include statement in iconv.c, add an entry in the table in
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iconv.c, and of course, update the README and iconv_open.3 manual page.
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* Use within other packages
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If you want to incorporate an iconv implementation into another package
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(such as a mail user agent or web browser), the single library approach
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is easier, because:
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1. In the shared library approach you have to provide the right directory
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prefix which will be used at run time.
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2. Incorporating iconv as a static library into the executable is easy -
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it won't need dynamic loading. (This assumes that your package is under
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the LGPL or GPL license.)
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All conversions go through Unicode. This is possible because most of the
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world's characters have already been allocated in the Unicode standard.
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Therefore we have for each encoding two functions:
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- For conversion from the encoding to Unicode, a function called xxx_mbtowc.
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- For conversion from Unicode to the encoding, a function called xxx_wctomb,
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and for stateful encodings, a function called xxx_reset which returns to
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the initial shift state.
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All our functions operate on a single Unicode character at a time. This is
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obviously less efficient than operating on an entire buffer of characters at
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a time, but it makes the coding considerably easier and less bug-prone. Those
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who wish best performance should install the Real Thing (TM): GNU libc 2.1
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or newer.
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