erm, thats not the whole backtrace, [expletive deleted] GDB pages it and needs you to keep pressing until you get it all.
Oh sorry. I thought I had pressed 'c' to get the whole lot. I'll be more careful next time. Yes, in this case I had a lot of data (plain text) in the file (about 1.4 million integers in 1 line) . Wrapping was on. I'll continue with another file which is not so extreme.