how do I search all memory of a process in gdb?

I'm trying to find a certain value (integer magic number (654321)) in a running process that I'm currently attached to using GDB. I found a find [/sn] start_addr, +len, val1 [, val2, ...] command, but it requires start and end address, yet I don't know where my process memory starts and where it ends.

This seems to be very common problem, yet I remember googling for it a year ago, and I repeated that now with no success, so.. How do I know where process memory starts and where it ends?