Re: git-diff should not fire up $PAGER if there is no diff

> On Tue, Dec 16, 2008 at 01:35:53AM -0500, Stefan Karpinski wrote:
>
>> > On Mon, Dec 15, 2008 at 7:56 PM, Jeff King <peff@peff.net> wrote:
>> >  2. detect EOF before starting the pager. We in fact already delay
>> >     running the pager in the forked process until we have some activity
>> >     on the pipe, but I don't know if there is a portable way of
>> >     detecting that that activity is EOF without performing an actual
>> >     read() call (which is undesirable, since it eats the first byte of
>> >     output that should go to the pager).
>>
>> Wouldn't ungetc work? Or is that not portable enough? (It would only
>> work here because the EOF has to be the first character.)
>
> No, it won't work. ungetc works on the buffered stdio object, so it is
> useful for pushing back characters onto the buffer to be read later in
> the program from the same buffer. But in this case, we are going to
> execv() (or on Windows, spawn) the pager, meaning it will throw away
> anything that has been read() from the pipe and put in the buffer.
>
> So we would need a system call to push a character back to the OS, so
> that it was available for read() by the pager process.