How to seek negative off the current position in file?

Question

I'm trying to seek 2 bytes backwards from current position in ruby. The following code tries to find previous end of line.

    1.9.3-p448 :003 > while file.getc != '\n'
    1.9.3-p448 :004?>   file.seek(-2,IO::SEEK_CUR)
    1.9.3-p448 :005?>   end
    Errno::EINVAL: Invalid argument - test
    from (irb):4:in `seek'
    from (irb):4
    from /usr/local/rvm/rubies/ruby-1.9.3-p448/bin/irb:16:in `<main>'

I'm unable to understand why I'm getting this error. It seeks perfectly outside loop (the same statement). test is the filename.

Answer 1

I'd guess that your problem is that you're not two bytes into the file yet. I just did this with a simple text file:

>> fp = File.new('pancakes.txt', 'r')
>> fp.readline
>> fp.seek(-2, IO::SEEK_CUR)
=> 0
>> fp.seek(-20, IO::SEEK_CUR)
Errno::EINVAL: Invalid argument - x.txt

The first line of the file was only a couple characters long so -20 would try to put me before the beginning of the file.

I think you just need to throw a pos check into the mix: if file.pos == 0 then you're at the beginning and a negative seek will raise an exception.