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