psql - Mailing list pgsql-sql

Hi,I have a problem. I hope someone can help me
Then, here we go:
- I have to make a C program that can conecct to a DataBase using
"PSQL".
-I do:"system("psql grp_xxx");".
but it request the password and the user.

Then How can I give by command line using "psql" the password and user.
Do I have to modified the Enviroment Variables}
if then How?

thanks a lot!

p.s:sorry for my english