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Re: How to dup a FILE*

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On Tue, 06 Dec 2011 20:38:47 +0000, Rainer Weikusat wrote about Re: How
to dup a FILE*:

>David W Noon <dwnoon@spamtrap.ntlworld.com> writes:
>> On Tue, 06 Dec 2011 09:51:43 +0100, Josef Moellers wrote about How to
>> dup a FILE*:
>>
>>>I need to juggle with stderr: redirecting it from one file to another
>>>and back.
>> [snip]
>>
>> I think you're over-complicating things here.  After all, stderr is
>> simply a pointer.  So, cache the old pointer; open a new file stream
>> stored in stderr; fclose(stderr) when done; reinstate the original
>> pointer.
>
>That's a thoroughly stupid idea because it won't affect output to
>'standard error filedescriptor' which doesn't go through the stderr
>stream *and* while the glibc documentation claims that this should
>work, it actually doesn't (or at least didn't for some random version
>where I tried this about ten years ago -- only to be told that I
>"should use freopen instead" by Ulrich Drepper [which I couldn't do
>because I was trying to divert the output to a socket]).

The C Standard Library does not provide "file descriptors" with any
specified meaning, only as opaque integers.  Consequently, all error
logging *should* be done using the stderr stream, not fd of 2.  It is
only a happenstance of platform-specific shells and run-time libraries
(e.g. glibc) that stderr defaults to fd of 2 on UNIX-like systems.

My best suggestion is that you try it and see.

Note also that freopen() closes the existing stream and opens a new
one, so using that will make your initial stderr useless.  The
underlying file descriptor can change too, as a call to fileno() before
and after freopen() will probably demonstrate.

Again, just try it and see.
- -- 
Regards,

Dave  [RLU #314465]
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dwnoon@spamtrap.ntlworld.com (David W Noon)
Remove spam trap to reply by e-mail.
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Thread

How to dup a FILE* Josef Moellers <josef.moellers@ts.fujitsu.com> - 2011-12-06 09:51 +0100
  Re: How to dup a FILE* Richard Kettlewell <rjk@greenend.org.uk> - 2011-12-06 09:16 +0000
    Re: How to dup a FILE* Josef Moellers <josef.moellers@ts.fujitsu.com> - 2011-12-06 10:41 +0100
  Re: How to dup a FILE* Jasen Betts <jasen@xnet.co.nz> - 2011-12-06 13:28 +0000
    Re: How to dup a FILE* Josef Moellers <josef.moellers@ts.fujitsu.com> - 2011-12-06 16:57 +0100
  Re: How to dup a FILE* David W Noon <dwnoon@spamtrap.ntlworld.com> - 2011-12-06 19:53 +0000
    Re: How to dup a FILE* Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-12-06 20:38 +0000
      Re: How to dup a FILE* Richard Kettlewell <rjk@greenend.org.uk> - 2011-12-06 22:03 +0000
      Re: How to dup a FILE* David W Noon <dwnoon@spamtrap.ntlworld.com> - 2011-12-06 21:46 +0000
        Re: How to dup a FILE* Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-12-07 17:52 +0000

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