From: Simon Strandgaard <0bz63fz3m1qt3001@...>
Date: 2003-05-12T07:16:51+09:00
Subject: Re: RCR for child execution

On Sun, 11 May 2003 19:12:17 +0900, Brian Candler wrote:

> On Sun, May 11, 2003 at 09:29:43AM +0900, Simon Strandgaard wrote:
>>         stdout = fdopen(new_stdout, "a"); // BOOM!
>>         stderr = fdopen(new_stderr, "a"); // BOOM!
> 
> The particular BOOM I get is:
> 
> main.cpp:40: non-lvalue in assignment
> main.cpp:41: non-lvalue in assignment
> 
> That's because stdout is not a variable: on my FreeBSD box it is defined as

All my boxes run FreeBSD5.0 :-)
I don't get any warnings/errors.

 
>   extern FILE __sF[];
>   ...
>   #define stdout  (&__sF[1])
> 
> (so it's not a pointer variable which can be reassigned, it is a pointer to
> a static piece of storage)
> 
> To reopen stdout you need either 'freopen' (at the stdio.h layer) or 'dup2'
> (at the Unix API layer). 

from man freopen
     FILE *
     freopen(const char *path, const char *mode, FILE *stream);

I don't know what "path" I should initialize freopen with, therefore
I use dup2 + fdopen.

int new_stdout = dup2(bak_stdout, 1);
stdout = fdopen(new_stdout, "a"); 



 
>> Well, there is no thread to flush the pipe.. but this is not important
>> here!  What is important is that system('ls a.c xx')'s output goes to
>> stdout/stderr,  is mixed up with C's output.
> 
> Yes, of course it is.
> 
> If you want the child to have a different stdout/stderr than yours, then
> what you need to do is:
>     - fork
>       - (in child) reopen stdout/stderr
>       - (in child) exec
>     - (in parent) wait for child to finish

I know im terrible expressing myself in english, so be gentle with me :-)

Separation of ruby stdout/stderr from C/C++ stdout/stderr is what I want.
I run Ruby in the same thread as C++ (ruby embedded into C++).
This separation is working fine, as long as I don't use system or
backquote.

Using system or backquote, then it goes BOOM. The problem as I see it is
localized in Rubys code - not mine!  Ruby spawns a child which inherits
stdout/stderr.  This child should instead inherit rb_stdout/rb_stderr
instead!!!    Can this issue be fixed on Ruby ???

 
> It sounds to me like you are actually asking for changes to Unix, but I
> can't see exactly what.

I have no problems with Unix.. I think you underestimate me because of my
broken english... I have been programming my hole life.. I just recently
started to poke around with unix-pipes.  I am very familiar with sockets
so no big difference here. Thanks for putting so much effort in it anyway :-)
 

--
Simon Strandgaard