iconv and non-identical/non-reversible conversions
According to the man-page, iconv(3C) would return the number of non-identical (or non-reversible in Gnu's vocabulary), when that happens. But what's the non-identical/non-reversible conversion? Try the following example:$ echo "abc测试" | iconv -f UTF-8 -t ASCII
abc??
As you can see, the last two characters are out-of scope of ASCII, however they are legal/valid UTF-8 characters. In this case, iconv(3C) shall convert them to another two characters (non-identical-conversion character, in some cases it's '?'), and returns 2. However, Gnu's iconv raises an error (return -1, and set errno to EILSEQ) in this case. Though, according to its manpage, EILSEQ is set to indicate there is an invalid multibyte sequence in the input.
So, it not a portable way to use this method to tell if the target encoding is capable to represent the source contents. And you should not either rely on the non-identical conversion numbers. A successful conversion may return -1 and with E2BIG when output buffer is exhausted, meanwhile non-identical conversions may happen. And there is no flag in iconv(3C)/iconv_open(3C) to control whether to perform non-identical conversions or to raise an error.
In gernal, iconv(3C) is not a well-defined interface.
P.S., this post is a summary of the discussion between JDS/Evolution team and myself, to locate/isolate an iconv(3C) related bug.
