Buffered input/output.

[Peteris Krumins]

Hello,

I am struggling with the following problem:
Two unrelated programs communicate via two fifos (is this a good way of
communicating between two programs?).
The first program opens one fifo for reading (R) and the second fifo
for writing (W), the second program does the same but opens fifos vice
versa, the first fifo for writing and the second fifo for reading.
(Also, can i open these fifos with fopen() without blocking? (I know
about open() and O_NONBLOCK but can i do the same with fopen()?))

The first program writes something to its W fifo, calls fflush(W) and
the second program meanwhile has issued select() on its R fifo,
select() successfully returns, then a fread() follows on R fifo but
it just blocks.

I assume this has something to do with buffered input?
How do i read from the second programs R fifo after the first program
has successfully written something to it?
i tried calling setvbuf(R,NULL,_IONBF,0) but it still does not work.
Also, i am curious - how to auto-flush output (In my case R fifo), so i
didnt have to call fflush() after each write to that fifo?


P.Krumins

[Floyd Davidson]

Peteris Krumins <pkruminsREMOVETHIS@inbox.lv> wrote:

>Hello,
>
> I am struggling with the following problem:
> Two unrelated programs communicate via two fifos (is this a good way of
> communicating between two programs?).
> The first program opens one fifo for reading (R) and the second fifo
> for writing (W), the second program does the same but opens fifos vice
> versa, the first fifo for writing and the second fifo for reading.
> (Also, can i open these fifos with fopen() without blocking? (I know
> about open() and O_NONBLOCK but can i do the same with fopen()?))
>
> The first program writes something to its W fifo, calls fflush(W) and
> the second program meanwhile has issued select() on its R fifo,
> select() successfully returns, then a fread() follows on R fifo but
> it just blocks.

You are using select(), which tells you when a *file descriptor*
will block. But then you say you are using fread(), which reads
from a stream (which doesn't block at the same times).

You can't expect to mix calls using fd's and fp's.

> I assume this has something to do with buffered input?
> How do i read from the second programs R fifo after the first program
> has successfully written something to it?

Use read() instead of fread(), for example.

> i tried calling setvbuf(R,NULL,_IONBF,0) but it still does not work.
> Also, i am curious - how to auto-flush output (In my case R fifo), so i
> didnt have to call fflush() after each write to that fifo?

What's wrong with using fflush()?

--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) [email]floyd@barrow.com[/email]

[Peteris Krumins]

Floyd Davidson <floyd@barrow.com> wrote in
news:87r84kemx2.fld@barrow.com:

> Peteris Krumins <pkruminsREMOVETHIS@inbox.lv> wrote:

>>Hello,
>>

[...]

> You are using select(), which tells you when a *file descriptor*
> will block. But then you say you are using fread(), which reads
> from a stream (which doesn't block at the same times).
> You can't expect to mix calls using fd's and fp's.

Oh, thanks for this information. I didn't know this!

>> I assume this has something to do with buffered input?
>> How do i read from the second programs R fifo after the first
>> program has successfully written something to it?

>
> Use read() instead of fread(), for example.

Yes, it solved the problem.

>> i tried calling setvbuf(R,NULL,_IONBF,0) but it still does not work.
>> Also, i am curious - how to auto-flush output (In my case R fifo),
>> so i didnt have to call fflush() after each write to that fifo?

>
> What's wrong with using fflush()?

Nothing is wrong, i just thought there was a method to auto-flush output
buffer on each write, so i didnt waste time with fflush() calls.


P.Krumins