If different character sets are used by the locale categories, the results achieved by an application utilizing these categories are undefined. Likewise, if different codesets are used for the data being processed by interfaces whose behavior is dependent on the current locale, or the codeset is different from the codeset assumed when the locale was created, the result is also undefined.

If these variables specify locale categories that are not based upon the same underlying codeset, the results are unspecified.

If different character sets are used by the locale categories, the results achieved by an application utilizing these categories are undefined. Likewise, if different codesets are used for the data being processed by interfaces whose behavior is dependent on the current locale, or the codeset is different from the codeset assumed when the locale was created, the result is also undefined.

If incompatible character sets are used by the locale categories, the results achieved by an application utilizing these categories are undefined. Two locale categories have incompatible character sets if one of the categories is LC_CTYPE and the locale data associated with the other category includes at least one character that either is not in the character set used by LC_CTYPE or has a different encoding than the same character in the character set used by LC_CTYPE.

Likewise, unless specified otherwise, if different codesets are used by a particular category of the selected locale and by the data being processed by an interface whose behavior is dependent on that category of the selected locale, the results are undefined.

If these variables specify locale categories that are not based upon the same underlying codeset, the results are unspecified.

See [xref to Section 7.1] for the consequences of setting these variables to locales with different character sets.

The locale settings of individual categories cannot be truly independent and still guarantee correct results. For example, when collating two strings, characters must first be extracted from each string (governed by LC_CTYPE) before being mapped to collating elements (governed by LC_COLLATE) for comparison. That is, if LC_CTYPE is causing parsing according to the rules of a large, multi-byte code set (potentially returning 20 000 or more distinct character codeset values), but LC_COLLATE is set to handle only an 8-bit codeset with 256 distinct characters, meaningful results are obviously impossible.

Earlier versions of this standard stated that if different character sets are used by the locale categories, the results achieved by an application utilizing these categories are undefined. This was felt to be overly restrictive. For example, when setting:

LANG=en_US.utf8
LC_TIME=POSIX

on a system where the codeset for the POSIX locale is ASCII and the codeset for en_US.utf8 is UTF-8, all of the characters used in the LC_TIME locale data exist, with the same encoding, in the codeset used for LC_CTYPE (via LANG), so there is no reason for the behavior to be undefined in this case. This standard now has more precise requirements in this area.