[Gambas-user] Help with SHELL and EXEC

[Benoit Minisini]

On dimanche 12 octobre 2008, Doriano Blengino wrote:
> Kari Laine ha scritto:
> > I got interested this writing process input thing. I can read process
> > output without problem. Although it seems that reading of the stdout will
> > start only when process ends. I made a little filter program in C and a
> > gambas project to try to use it but I can not get the wrriting work so
> > that when written the filter would return something to gambas-project.
> > This got to do something that same program cannot both write and read to
> > process - or what? Well here they are if someone is also interested to
> > test. I would be very great full for any advice how to write to certain
> > process.
>
> If you look carefully at the docs about EXEC:
>
>   [ _Process_ *=* ] *EXEC* _Command_ [ *WAIT* ] [ *FOR* { { *READ* |
> *INPUT* } | { *WRITE* | *OUTPUT* } } ] [ *AS* _Name_ ]
>
> you'll notice that you can't specify both READ & WRITE --------^
>
> I have also a big book about Linux programming and, when I first read
> that book, I was wondered because it said that you can't open a pipe
> both for reading and for writing. Doing such things is a completely
> different matter, it said.
> Still I think that it should be possible with the right calls but, don't
> knowing what gambas does behind the scenes, the only thing I can say
> now, is that the syntax explanation does not permit that. But gambas
> happily lets you specify both READ & WRITE...

Guess what: Gambas internally uses -two- pipes. One for reading the process 
output, and another for writing to it. Of course only one pipe is used if you 
specify only READ or WRITE.

>
> Then, reading more carefully, I noticed that the docs say: "if you
> specify INPUT & OUTPUT instead of READ & WRITE, then the process is
> executed inside a virtual terminal". Remembering that that book was
> talking, if I well remember, about virtual terminals to be necessary to
> do such things, I tried in gambas. And it works!

I'm happy to know that. :-)

This feature is needed, because many command-line tools need to be run inside 
a terminal to be really useful.

Regards,

-- 
Benoit Minisini