A Unix C library contains three different sets of functions in two families to handle character set conversion. The one function family is specified in the ISO C standard and therefore is portable even beyond the Unix world.
The most commonly known set of functions, coming from the ISO C90 standard, is unfortunately the least useful one. In fact, these functions should be avoided whenever possible, especially when developing libraries (as opposed to applications).
The second family of functions got introduced in the early Unix standards (XPG2) and is still part of the latest and greatest Unix standard: Unix 98. It is also the most powerful and useful set of functions. But we will start with the functions defined in Amendment 1 to ISO C90.
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