Manpages are typically pretty good and provide guide-level material. C functions in libraries are documented in section 3, whereas C functions for syscalls are documented in section 2. Lots of online sites render the manpages of various Linux distros. For example: https://man.archlinux.org/ printf() is documented at https://man.archlinux.org/man/printf.3.en The C standard itself is difficult to read, but the cppreference wiki summarizes it in a more accessible fashion. The information on this site is typically very accurate and carefully distinguishes between different versions of the standard. Despite its name, it includes detailed material on C as well. https://en.cppreference.com/w/c printf() is documented at https://en.cppreference.com/w/c/io/fprintf I have had the misfortune of actually having to write Posix-compliant software. Posix extends the C standard in some ways, and adds lots of functions of its own. You can browse the Posix standard online, but it's often not written very well. Usually, manpages are more useful. https://pubs.opengroup.org/onlinepubs/9699919799/ printf() is documented at https://pubs.opengroup.org/onlinepubs/9699919799/functions/printf.html Answer from latkde on reddit.com