who can help me?

Hi, all hackers,
I can startup database using command "/usr/local/pgsql/bin/postmaster -D /usr/local/pgsql/data".
But when I debug it using gdb and set args -D /usr/local/pgsql/data, it can't startup database. it says;
FATAL:  Database "postgres" does not exist in the system catalog.

Is there any difference between them?

Thanks a lot.

Jinqiang Han