How to find the beginning of last line of a big text file ?

[MRAB]

Barak, Ron wrote:
> Hi,
>  
> I have a _very_ big text file: I need to find the place where the last 
> line begins (namely, the offset of the one-before-the-last '\n' + 1).
>  
> Could you suggest a way to do that without getting all the file into 
> memory (as I said, it's a big file), or heaving to readline() all lines 
> (ditto) ?
>  
You could seek() to near the end of the file before reading lines with 
readline(). Remember that the seek will almost certainly put the file 
pointer somewhere in the middle of a line, but that doesn't matter 
provided that it's not the last line (ie if the second readline() 
returns "" then the first readline() started somewhere in middle of the 
last line of the file). If you find that the seek put the file pointer 
somewhere in the middle of the last line, then try again, but this time 
seeking further back from the end of file before reading. Repeat as 
necessary.