Subject: Re: bug#11631: closed (Re: bug#11631: Head command does not position file pointer correctly for negative line count)

From: Jim Meyering <[email-masked]>
Date: Thu, 07 Jun 2012 19:38:58 +0200

Anoop Sharma wrote:
> The thought behind the proposed change was that lseek should reflect
> the amount of data that head has actually been able to print.
>
> For example, how do we want head to behave in a situation like the
> following where files more than a particular size are not allowed
> (with bash shell on a machine with block size of 1024 bytes)? This
> situation can be handled by applying this patch. I agree this example
> is custom designed to illustrate my point but what do we gain by not
> making the check?:
>
> ulimit -f 1; trap '' SIGXFSZ
> (stdbuf -o0 head -n -1025 >someOutFile; cat) <someIpFile
>
> What should cat print now?
>
> By detecting fwrite failure, we can increment file pointer by the
> amount that was written successfully.
> That was what I originally wanted to accomplish. However, I looked at
> the existing implementation of head.c and found that a stock behavior
> on fwrite failures was to exit and afraid to rock the boat too much, I
> proposed that.
>
> I agree that the checking for fwrite failure is not fool-proof. But it
> looks better than ignoring the return value.

While head is ignoring that return value,
it is not really ignoring the failure.  That would be a bug.

Rather, head is relying on the fact that the stream records the failure,
and that our atexit-invoked close_stdout function will detect the prior
failure (via ferror(stdout)) and diagnose it.

In practice, testing for fwrite failure will make no difference,
other than adding a small amount to the size of "head".

Regarding your example, what you've done above (turning off buffering)
is very unusual.  That doesn't seem like a case worth catering to.
And besides, since in general we don't know how much a failing fwrite
function has actually written, there can be no guarantee that the input
stream position somehow reflects what was written.