Are there man pages for data structures?
 

Are there manpages available for data structures?. I am looking
for ones like sockaddr_in, hostent etc. I don't any any, at least
on my machine, so I am not sure if I never installed those, or
they are not a part of documentation at all.

If they are not documented by man, how to find out online help
for data structures?

Thanks in advance,
Ravi

Try man -k sockaddr_in

You could also post to the gnu / gcc mailing list or news-group(s)

There are functions described in man, not structures.
I don't know a good source of about data structures used for socket programming, but they are all defined in /usr/include (directories sys, netinet etc), usually with comments. Example:
sockaddr_in /usr/include/netinet/in.h
hostent /usr/include/netdb.h

The info pages of libc contain a lot of useful info. You might want to look in there