New to SQL; hopefully simple question

I have a situation where I want to keep a bunch of ordinals in a row
of a table maintained by the database. From this table, I want to
obtain a number, and increment it in the table. This is the general
idea, abstracted to the key parts (and it works, as far as it goes:

"select valueone from ordtable"
VALUE = PQgetvalue(res,0,0)
...compute VALUE = VALUE + 1
"update ordtable SET valueone = VALUE"

...as I said, this works. however, as far as I understand it, it's
divisible and thus subject to multi-user fugue. What I need,
conceptually, is:

lock the table so anyone else waits for the unlock (not fails, waits)
do the above
unlock the table

Can anyone point me in the right direction for this? I looked at lock,
but it seems to only apply across one DB action. Yes? No?

Thanks!

Walt

Walt
Software Engineer
Black Belt Systems
pages: http://www.blackbeltsystems.com/
email: http://www.blackbeltsystems.com/contact.html