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- 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.
-
-