zkCtype.h is a portable wrapper for the Unix 98 header <ctype.h>. It is part of the libzk library because there are parts of the Zero Knowledge code base that assume it contains functionality beyond that provided by Standard C, namely POSIX or platform-specific features.

zkCtype.h - Supported Functions from <ctype.h>
isalnum
isalpha
iscntrl
isdigit
isgraph
islower
isprint
ispunct
isspace
isupper
isxdigit
tolower
toupper