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libiconv

    1: While some other iconv(3) implementations - like FreeBSD iconv(3) - choose
    2: the "many small shared libraries" and dlopen(3) approach, this implementation
    3: packs everything into a single shared library. Here is a comparison of the
    4: two designs.
    5: 
    6: * Run-time efficiency
    7:   1. A dlopen() based approach needs a cache of loaded shared libraries.
    8:   Otherwise, every iconv_open() call will result in a call to dlopen()
    9:   and thus to file system related system calls - which is prohibitive
   10:   because some applications use the iconv_open/iconv/iconv_close sequence
   11:   for every single filename, string, or piece of text.
   12:   2. In terms of virtual memory use, both approaches are on par. Being shared
   13:   libraries, the tables are shared between any processes that use them.
   14:   And because of the demand loading used by Unix systems (and because libiconv
   15:   does not have initialization functions), only those parts of the tables
   16:   which are needed (typically very few kilobytes) will be read from disk and
   17:   paged into main memory.
   18:   3. Even with a cache of loaded shared libraries, the dlopen() based approach
   19:   makes more system calls, because it has to load one or two shared libraries
   20:   for every encoding in use.
   21: 
   22: * Total size
   23:   In the dlopen(3) approach, every shared library has a symbol table and
   24:   relocation offset. All together, FreeBSD iconv installs more than 200 shared
   25:   libraries with a total size of 2.3 MB. Whereas libiconv installs 0.45 MB.
   26: 
   27: * Extensibility
   28:   The dlopen(3) approach is good for guaranteeing extensibility if the iconv
   29:   implementation is distributed without source. (Or when, as in glibc, you
   30:   cannot rebuild iconv without rebuilding your libc, thus possibly
   31:   destabilizing your system.)
   32:   The libiconv package achieves extensibility through the LGPL license:
   33:   Every user has access to the source of the package and can extend and
   34:   replace just libiconv.so.
   35:   The places which have to be modified when a new encoding is added are as
   36:   follows: add an #include statement in iconv.c, add an entry in the table in
   37:   iconv.c, and of course, update the README and iconv_open.3 manual page.
   38: 
   39: * Use within other packages
   40:   If you want to incorporate an iconv implementation into another package
   41:   (such as a mail user agent or web browser), the single library approach
   42:   is easier, because:
   43:   1. In the shared library approach you have to provide the right directory
   44:   prefix which will be used at run time.
   45:   2. Incorporating iconv as a static library into the executable is easy -
   46:   it won't need dynamic loading. (This assumes that your package is under
   47:   the LGPL or GPL license.)
   48: 
   49: 
   50: All conversions go through Unicode. This is possible because most of the
   51: world's characters have already been allocated in the Unicode standard.
   52: Therefore we have for each encoding two functions:
   53: - For conversion from the encoding to Unicode, a function called xxx_mbtowc.
   54: - For conversion from Unicode to the encoding, a function called xxx_wctomb,
   55:   and for stateful encodings, a function called xxx_reset which returns to
   56:   the initial shift state.
   57: 
   58: 
   59: All our functions operate on a single Unicode character at a time. This is
   60: obviously less efficient than operating on an entire buffer of characters at
   61: a time, but it makes the coding considerably easier and less bug-prone. Those
   62: who wish best performance should install the Real Thing (TM): GNU libc 2.1
   63: or newer.
   64: