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