question on accessing PostgreSQL with C

Hello,
I'm a newbee in PostgreSQL, Linux world.  Currently facing some problem
in building a simple application.  I'm trying to write a C program that
get input from user, then update (insert / change) the database, or
retrieve selected data.

Questions:
(1)  Where to get more example of communicating with PostgreSQL with C
program?

(2)  How to put variables into the SQL?
      I use C program to access PostgreSQL with libpq, and use PQexec( )
to send the query. One of the failed example of mine,
                    :
           /* get input from user, a string and an integer */
                    :
        PQexec(conn,"insert into NameList (name,id) values (%s,%d)
.......
                    :
           /* I'm trying to put variables of string and integer into the
query */


(3)  How to check who are the users in PostgreSQL system?

Please advise.
Thanks & Regards,
LCH