16.1.4.3 c-strcaseeq

The c-strcaseeq module contains an optimized case-insensitive string comparison function operating on single-byte character strings, that operate as if the locale encoding was ASCII. (The "C" locale on many systems has the locale encoding "ASCII".)

The functions is actually implemented as a macro:

extern int STRCASEEQ (const char *s1, const char *s2,
                      int s20, int s21, int s22, int s23, int s24, int s25,
                      int s26, int s27, int s28);

s2 should be a short literal ASCII string, and s20, s21, ... the individual characters of s2.

For case conversion here, only ASCII characters are considered to be upper case or lower case.