Re: how to implement tail -n

[Eric Bambach <eric@cisu.net>]

On Sunday 23 January 2005 11:49 pm, you wrote:
> Hi,
>
> I would like to know how to implement "tail -n" in C.
>
> The "-n" option will be used to print the last n lines of a file -
> rather than the default option of printing the last 10 lines of a file.
>
> I wish to do this in a single pass. I can use lseek() to go to the end
> of the file. How to traverse backwards from there ? Is there any
> function that does this ?

Not that I know of. There is no GOOD way to do this in a single pass since the 
lines can be of varying length. If by single pass you meant you want 
performace, then dont read a char at a time, rather just seek to the end of 
the file,  read in about 4K( a reasonable buffer) or so and see if you can 
find (n+1) newlines. If you dont, read in 4 more k and keep going backwards 
till you find them.

P.S. Please dont hi-jack subjects. Type in a new message instead of replying 
to something and deleting the original message; it screws with threading.


> thanks,
> venkatesh
> -
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-- 
----------------------------------------
--EB

> All is fine except that I can reliably "oops" it simply by trying to read
> from /proc/apm (e.g. cat /proc/apm).
> oops output and ksymoops-2.3.4 output is attached.
> Is there anything else I can contribute?

The latitude and longtitude of the bios writers current position, and
a ballistic missile.

                --Alan Cox LKML-December 08,2000 

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