What does this mean

[Philipp Dunkel]

I'm not really understanding a compiler warning I get compiling my .so

warning: 'name-of-function' used but never declared

The reason I don't get it, because the line it gives me is the line of
the prototype of the function in a header file.

Could anyone please tell me what this warning is supposed to mean,
and/or where I can look up the meanings of warning/error-cryptography,
since I did not find a resource on the topic.

Thanks everyone in advance for taking the time to help me out.

TTY Philipp

P.S.: As I have not yet subscribed to gcc-help@gcc.gnu.org I would
appreciate any help to go to philipp@dunkel.org.