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Two sets of characters and their associated collating sequences shall be defined: the set in which source files are written (the source character set), and the set interpreted in the execution environment (the execution character set). Each set is further divided into a basic character set, whose contents are given by this subclause, and a set of zero or more locale-specific members (which are not members of the basic character set) called extended characters. The combined set is also called the extended character set. The values of the members of the execution character set are implementation-defined.
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There are several national variants of ASCII. Therefore, the original ASCII is often referred as *US-ASCII*. The international standard _ISO 646_ defines a character set similar to US-ASCII, but with code positions corresponding to US-ASCII characters {{@\[\]\{\|\}}} as "national use positions". It also gives some liberties with characters {{\#$^`\~}}. In _ISO 646_, several "national variants of ASCII" have been defined, assigning different letters and symbols to the "national use" positions. Thus, the characters that appear in those positions - including those in *US-ASCII* are somewhat "unsafe" in international data transfer. Thus, due to the "national variants," some characters are less "safe" than others--they might be transferred or interpreted incorrectly. |
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