nonblocking lock?

Is it possible to do a nonblocking lock? That is, 
I want several clients to execute,

    begin
    if table A is locked
    then
        go around doing stuff on other tables
    else
        lock A and do stuff on A
    endif

the problem is, if I use normal lock, then 
after one client has locked and is doing stuff on A
the other one will block and thus it won't be able
to go around doing stuff on other tables. Is it
possible to do a nonblocking lock that will just
fail if the table is locked already? 


NOTE: I tried using PQrequestCancel but it won't
cancel the request. It still blocks for as long
as the lock lasts. The only way around I've found so 
far is to use PQreset. That's crude but works. But 
it leaves a dangling postmaster process that lives
until the orignal lock is freed. Any other ideas? 

Thanks a lot 

Pablo Funes
Brandeis University
pablo@cs.brandeis.edu