As noted in linux/include/linux/ctype.h. NOTE: This ctype does not handle EOF like the standard C library.
| isalnum(c) | isalpha(c) | iscntrl(c) |
| isdigit(c) | isgraph(c) | islower(c) |
| isprint(c) | ispunct(c) | isspace(c) |
| isupper(c) | isxdigit(c) | isascii(c) |
| toascii(c) | tolower(c) | toupper(c) |
Some of these are architecture specific, therefore all cannot be expected to be available on all platforms. In order to find out, the kernels symbol table can be inspected either by running ksyms -a or by checking the System.map. Aside from these libc functions in kernel space, any of the kernel internal functions provided there symbol exported can be used.
Of course the programmer is not limited to using these string functions even if proc input and output is a character basis and these are most commonly needed; any other exported kernel function can be used. Note though that many functions can have security relevant side-effects aside from the ability to hard-lock up the system if used incorrectly. Generally speaking, some sanity checks on any values users may pass have to be performed.