How to install man pages for C standard library functions in Ubuntu?

Install the manpages-dev and manpages-posix-dev (thanks ChristopheD) packages. You should be able to find them in synaptic, or type

apt-get install manpages-dev
apt-get install manpages-posix-dev

at the command line.

For Fedora, you can install it using yum:

yum install man-pages libstdc++-docs

You may find glibc-doc package useful as well. From http://packages.ubuntu.com/jaunty/glibc-doc:

Contains The GNU C Library Reference manual in info and html format as well as the man pages for libpthread functions and the complete GNU C Library ChangeLog.