zkString.h is a portable wrapper for the Standard C header <string.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.

zkString.h - Supported Functions from <string.h>
memchr
memcmp
memcpy
memmove
memset
strcat
strchr
strcmp
strcpy
strcspn
strlen
strncat
strncmp
strncpy
strpbrk
strrchr
strspn
strstr

zkString.h - Supported Functions not in <string.h>
Function Reason
strcasecmp Not in Standard C.
strdup Not in Standard C.
strncasecmp Not in Standard C.
strndup Not in Standard C.

zkString.h - Deprecated Functions from <string.h>
Don't Call This Instead, Call This Reason
strcoll zkStrcoll Standard C doesn't define strcoll as setting errno.
strerror zkStrerror The ZK code base assumes that this function can return meaningful strings for the values of errno defined by POSIX. This is not the case on a non-POSIX platform.
strtok no replacement Not Reentrant.
strxfrm zkStrxfrm Standard C doesn't define strxfrm as setting errno.