Re: capture stdout and stderr

[Thomas Danckaert <post@thomasdanckaert.be>]

From: Stefan Israelsson Tampe <stefan.itampe@gmail.com>
Subject: Re: capture stdout and stderr
Date: Thu, 24 May 2018 15:25:51 +0200

> I think that your code whould work but doesn't because string ports 
> are not file ports and system commands pipes out to file ports.
>
> 'open-input-pipe' uses 'scm_open_process' that returns only a 
> read-port and a write-port, the write port is then taped the output 
> string in your run-command.
> and it works.
>
> ALso when you create a err-port with pipe, you create a file port 
> nad this will be used translated as an error port for the system 
> comand and it all works.
> However the failing program uses a string port which is not a file 
> port and it does not end up capturing the error output in the 
> system command.
>
> One would wish that guile if the error port is not a file port make 
> a file port, as done with 'pipe' and create a thread that pipes the 
> data from the file port to the non file port and your code
> would work. Also similarly behavior for read and write ports 
> towards the shell.
>
> the relevant code is in posix.c, scm_open_process

Hi! Thanks for the message.  Indeed, that makes sense.  In fact I had 
some vague intuition that some magic would be needed to channel the 
stderr output into such a string port.  I think the manual could be 
more explicit about that, though? (e.g. in the section on  default 
ports 
https://www.gnu.org/software/guile/manual/html_node/Default-Ports.html#Default-Ports 
).  Maybe I should propose a patch to the docs, stating that those 
"with-x-to-port" methods only work on file ports.

Thomas