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The ISO C standard first appeared in 1989 (the standard is often called ANSI C). It added several new features to the C language, most notably function prototypes . This led to many years of portability issues when deciding whether to use ISO C features.
We think that programs written today can assume the presence of an ISO C compiler. Therefore, we will not discuss issues related to the differences between ISO C compilers and older compilers--often called K&R compilers, from the first book on C by Kernighan and Ritchie. You may see these differences handled in older programs.
There is a newer C standard called `C9X'. Because compilers that support it are not widely available as of this writing, this discussion does not cover it.