cppreference [1] has also entries for the C standard library. POSIX/SuS is directly available from opengroup [2]. glibc is better browsed from local man pages, or for a more organic discussion, info pages. Searching 'man function-name' also usually works.
[1] https://en.cppreference.com/w/c.html
[2] https://pubs.opengroup.org/onlinepubs/9699919799.2018edition...