Displaying database names from within application

I'm writing an application program which uses Postgresql
functionality via calls to the library libpq.

One of the things I'd like to do is present the user with a list of
possible database names to choose from, nicely formatted from within
a GUI, but I can't seem to find a way to access the database names
through system calls.

(Of course, I could make a system call, asking Unix to interpret the
string "psql -l", but I'd like to thing there is a more direct route
available to me, performing the same call to the Postgresql backend
that the command "psql -l" must do itself.)

I see in the Developer's Guide that there are catalogs with this
information, but I don't know how to query the catalogs.

Can someone help?

Thanks!

     -- Bill