Cursors for update.., we have to port an informix 9.x appication using cursors for update

We have to port an application from Informix 9 CSQL to Postgres 8.1 
using Ecpg.

Informix application uses cursors for update to lock all the rows 
involved in those cursor. Then an update my occur ussing Current of.

How we can do this with postgres?

Thanks for the reply.
Jorge