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.

On a ubuntu system they are in the packages

manpages-posix-dev (headers)
manpages-dev (functions)

Ubuntu + others; You can also do a search in aptitude. I.e:

:~$ aptitude search manpages
i   asr-manpages          - alt.sysadmin.recovery manual pages
i   csound-manpages       - manual pages for csound
i   erlang-manpages       - Erlang/OTP manual pages
i   freebsd-manpages      - Manual pages for a GNU/kFreeBSD system
i   funny-manpages        - more funny manpages
i   gmt-manpages          - Manpages for the Generic Mapping Tools
i   manpages              - Manual pages about using a GNU/Linux system
p   manpages-cs           - Czech version of the manual pages
p   manpages-de           - German manpages
p   manpages-de-dev       - German development manpages
i   manpages-dev          - Manual pages about using GNU/Linux for development
p   manpages-es           - Spanish man pages
p   manpages-es-extra     - Spanish extra manpages
p   manpages-fr           - French version of the manual pages about using GNU/Linux
p   manpages-fr-dev       - French version of the development manual pages
...

Where "i" mean the package is installed and "p" means purged/not installed.

~$ man aptitude

And then i.e:

~$ sudo apt-get install manpages-es-extra

Other systems has similar functions.