HelenOS sources

U_SPECIAL          53 common/include/str.h #define U_SPECIAL      '?'
U_SPECIAL          53 kernel/generic/common/include/str.h #define U_SPECIAL      '?'
U_SPECIAL          53 uspace/lib/c/common/include/str.h #define U_SPECIAL      '?'
U_SPECIAL          53 uspace/lib/posix/include/common/str.h #define U_SPECIAL      '?'

HelenOS homepage, sources at GitHub