63 significant initial characters in an internal identifier or a macro name (each
universal character name or extended source character is considered a single
character)
? 31 significant initial characters in an external identifier (each universal character name
specifying a short identifier of 0000FFFF or less is considered 6 characters, each
universal character name specifying a short identifier of 00010000 or more is
considered 10 characters, and each extended source character is considered the same
number of characters as the corresponding universal character name, if any)
Restriction of the significance of an external name to fewer than 255 characters
(considering each universal character name or extended source character as a single
character) is an obsolescent feature that is a concession to existing implementations.
References
- ISO/IEC 9899-1999 5.2.4.1 Translation limits
- MISRA 04 Rule 3.5