Just to clarify the reason for this further, the manpage is in section 2 because it is a system call (implemented more or less directly as part of the kernel, rather than the C library).
This distinction can seem somewhat arbitrary, especially with older system calls that are now library functions (fork is still in section 2 even though it is now a wrapper for clone), unless you already know it. In general, look in section 3 first, then try section 2 if you can't find it or it looks like it might not be relevant. Also, some of the functions in section 2 are internal or obsolete linux-specific functions that are not supposed to be called by normal programs (e.g. getdents, gettid).
You can also install the manpages-posix-dev package to get a set of manpages that is written from a portable perspective rather than containing linux-specific information. In this package, all of the manpages provided for C functions are in section 3p.