connecting with libpq interface

I installed postgresql in two places - MacOS X and Ubuntu Linux
7.10. Connections with libpq works fine on OS X, but not on Ubuntu
and I can't figure out why. Here's the code: On MacOS X, the
Connected!! message is displayed, on Ubuntu 7.10 I get Connect error.

#include </usr/local/pgsql/include/libpq-fe.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
PGconn *cn;

cn = PQconnectdb("dbname=bdp");
if(PQstatus(cn) != CONNECTION_OK)
printf("Connect error\n");
else
printf("Connected!!\n");
}

I am using libpqxx, which is built on libpq and it gives me this
detailed message, so I ran the above test program to determine if it
was libpqxx or libpq. Apparently, libpq is not connecting for some
reason:

terminate called after throwing an instance of 'pqxx::broken_connection'
what(): could not connect to server: No such file or directory
Is the server running locally and accepting
connections on Unix domain socket "/var/run/
postgresql/.s.PGSQL.5432"?

the server is running and I can connect to it with psql -p 5432. I
have these environment variables set, but I don't see how they could
be the problem.

export PGDATA=~/bdp/db
export PGDATABASE=bdp

--
Vincent