If you unselect the "frequently used in CS50" selection, the CS50 Manual pages for the C standard library, the C POSIX library, and the CS50 Library is a great digital and searchable reference for the C Programming language.

Might be a bit different than what you're looking for but outside of printed books, it seems to be the most complete resource for C. And the CS50 Library, though not standard, is a well written extension to the standard library which may be helpful for common use cases.