Saving file descriptors

[Peteris Krumins]

Hello,

I have the following problem,

I close fileno(stdout) and fileno(stderr), then dup() the closed
descriptors to the descriptor pointing "/dev/null", so there was no output
from the program.

But now, at certain situation i want to restore stdout and stderr, so i
could output something to stderr or stdout.

How to do it?

I tried saving stdout and stderr, both as file pointers or just
descriptor numbers and later restoring them, but it didnt work.


P.Krumins

[Barry Margolin]

In article <Xns93BC42D72A49whitesuneapollolv@130.133.1.4>,
Peteris Krumins <pkruminsREMOVETHIS@inbox.lv> wrote:

> But now, at certain situation i want to restore stdout and stderr, so i
>could output something to stderr or stdout.
>
> How to do it?

dup() them before closing them. Then dup2() them back to restore.

--
Barry Margolin, [email]barry.margolin@level3.com[/email]
Level(3), Woburn, MA
*** DON'T SEND TECHNICAL QUESTIONS DIRECTLY TO ME, post them to newsgroups.
Please DON'T copy followups to me -- I'll assume it wasn't posted to the group.

[Peteris Krumins]

Barry Margolin <barry.margolin@level3.com> wrote in
news:HIERa.256$0z4.155@news.level3.com:

> In article <Xns93BC42D72A49whitesuneapollolv@130.133.1.4>,
> Peteris Krumins <pkruminsREMOVETHIS@inbox.lv> wrote:

>> But now, at certain situation i want to restore stdout and stderr,
>> so i
>>could output something to stderr or stdout.
>>
>> How to do it?

>
> dup() them before closing them. Then dup2() them back to restore.
>

Thanks!


P.Krumins

[Eric Sosman]

Barry Margolin wrote:

>
> In article <Xns93BC42D72A49whitesuneapollolv@130.133.1.4>,
> Peteris Krumins <pkruminsREMOVETHIS@inbox.lv> wrote:

> > But now, at certain situation i want to restore stdout and stderr, so i
> >could output something to stderr or stdout.
> >
> > How to do it?

>
> dup() them before closing them. Then dup2() them back to restore.

If there's any chance that output was generated with
the <stdio.h> functions (printf(), putc(), perror(), ...),
you'll also need to "synchronize" the stdio buffers with
the file descriptors. The best way to do this is probably

dup() the original descriptors
fclose() both streams
fdopen() both streams to the "/dev/null" descriptor
...
fclose() both streams
dup2() to restore the descriptors
fdopen() both streams to the original descriptors,
possibly using "a" instead of "w".

Another thing: if you ever seek employment with a sane
software company, don't admit that you implemented this awful
mess. The interviewer will admire your ingenuity, but will
make the sign of the Cross as you depart and will offer the
job to somebody else. (1/2 ;-)

--
[email]Eric.Sosman@sun.com[/email]

[Geoff Clare]

Eric Sosman <Eric.Sosman@sun.com> writes:

>> The best way is just to use fflush() before messing with the
>> underlying file descriptors.

> If I understand you correctly, your suggestion amounts to
>using fflush() to drain buffers, then fiddling with the file
>descriptors "behind the back" of the stdio implementation
>and proceeding as if nothing untoward had happened.

> This will most likely work on many implementations, but it
>also embodies some guesses about how the implementation works,
>guesses that (AFAIK) aren't promises by any of the relevant
>standards. For example, a FILE* must mediate between the
>different notions of "current position" that ftell() and
>lseek() return.

Oops, yes I forgot about the offset. In some circumstances
you also need to do an fseek() after fiddling with the
file descriptors. The circumstances are defined by POSIX in
the (lengthy) description of how to coordinate the use of
multiple "handles" (file descriptors and stdio streams) that
refer to the same open file description:

"If any previous active handle has been used by a function that
explicitly changed the file offset, except as required above for
the first handle, the application shall perform an lseek() or
fseek() (as appropriate to the type of handle) to an appropriate
location."

--
Geoff Clare <nospam@gclare.org.uk>