connecting alternative output stream to terminal

[Christof Warlich]

Hi,

besides stdout and stderr, I would like to connect a third
output stream to my terminal, as I want to be able to redirect
stdout, stderr and the third output stream separately. But the
following attempt:

#include <unistd.h>
#include <stdio.h>
int main(void) {
FILE *stdalt = fdopen(dup(fileno(stderr)), "w");
printf("Alternative file descriptor: %d\n", fileno(stdalt));
fprintf(stdout, "STDOUT\n");
fprintf(stderr, "STDERR\n");
fprintf(stdalt, "STDALT\n");
return 0;
}

does not do what I intended: When I run the program, I see that
the newly created descriptor has number 3 as expected:

$ ./a.out
Alternative file descriptor: 3
STDOUT
STDERR
STDALT

but when I redirect stderr, stdalt gets redirected as well:

../a.out 2>/dev/null
Alternative file descriptor: 3
STDOUT

But I'd like to get something like this:

$ ./a.out 2>/dev/null
Alternative file descriptor: 3
STDOUT
STDALT

$ ./a.out 3>/dev/null
Alternative file descriptor: 3
STDOUT
STDERR

Can anyone tell how to do this right?

Thanks,

Christof

 

[Falcon Kirtaran]

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Hash: SHA1

Hi,

besides stdout and stderr, I would like to connect a third
output stream to my terminal, as I want to be able to redirect
stdout, stderr and the third output stream separately. But the
following attempt:

#include <unistd.h>
#include <stdio.h>
int main(void) {
FILE *stdalt = fdopen(dup(fileno(stderr)), "w");
printf("Alternative file descriptor: %d\n", fileno(stdalt));
fprintf(stdout, "STDOUT\n");
fprintf(stderr, "STDERR\n");
fprintf(stdalt, "STDALT\n");
return 0;
}

does not do what I intended: When I run the program, I see that
the newly created descriptor has number 3 as expected:

$ ./a.out
Alternative file descriptor: 3
STDOUT
STDERR
STDALT

but when I redirect stderr, stdalt gets redirected as well:

./a.out 2>/dev/null
Alternative file descriptor: 3
STDOUT

But I'd like to get something like this:

$ ./a.out 2>/dev/null
Alternative file descriptor: 3
STDOUT
STDALT

$ ./a.out 3>/dev/null
Alternative file descriptor: 3
STDOUT
STDERR

Can anyone tell how to do this right?

Thanks,

Christof

In POSIX, when file number 3 is redirected, it should be open when
main() is called. Thus, you should be able to try dup(3) and check the
return code (you can close() the file number dup() gave you if you
wish). If it failed, the redirect is not in use (and so you could set
your stdalt to standard output or dup() standard output or something
like that). It's a bit of a hack, though...

- --
- --Falcon Darkstar Kirtaran
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[Christof Warlich]

In POSIX, when file number 3 is redirected, it should be open when
main() is called.

Thanks, yes, I just found that out meanwhile as well .

Thus, you should be able to try dup(3) and check the
return code (you can close() the file number dup() gave you if you
wish). If it failed, the redirect is not in use (and so you could set
your stdalt to standard output or dup() standard output or something
like that). It's a bit of a hack, though...

Oh yes, seems to be ... But thanks for the advice, the following code
does the job:

#include <unistd.h>
#include <stdio.h>
int main(void) {
FILE *stdalt;
int tmp = dup(3);
if(tmp == -1) {
stdalt = fdopen(dup(2), "w");
stdalt = freopen("/dev/tty", "w", stdalt);
}
else {
close(tmp);
stdalt = fdopen(3, "w");
}
printf("Alternative file descriptor: %d\n", fileno(stdalt));
fprintf(stdout, "STDOUT\n");
fprintf(stderr, "STDERR\n");
fprintf(stdalt, "STDALT\n");
return 0;
}

$ ./a.out 1>xxx
STDERR
STDALT
$ ./a.out 2>xxx
Alternative file descriptor: 3
STDOUT
STDALT
$ ./a.out 3>xxx
Alternative file descriptor: 3
STDOUT
STDERR

I'd really like to know why I have to use freopen on stdalt with
/dev/tty though. Whithout it, I get

../a.out 2>xxx
Alternative file descriptor: 3
STDOUT

Everything else stays the same as above. Any ideas?

Thanks a lot,

Christof

 

[Flash Gordon]

Thanks, yes, I just found that out meanwhile as well .

Everything else stays the same as above. Any ideas?

You need to ask over in comp.unix.programmer as this is a Unix issue
rather than a C issue.

 

[Richard]

You need to ask over in comp.unix.programmer as this is a Unix issue
rather than a C issue.

Actually it could be a C issue. Since he didn't include the header for
fdopen ....

You missed a good chance there.

In addition, since when was POSIX only Unix?

If you do not know enough about a subject it is better you do not
redirect people to the wrong place.

"Portable Operating System Interface"

 

[Flash Gordon]

Actually it could be a C issue. Since he didn't include the header for
fdopen ....

You missed a good chance there.

True. However I knew that someone else here would address any ossible C
issues that I missed.

In addition, since when was POSIX only Unix?

It isn't and I've used it else where. So?

If you do not know enough about a subject it is better you do not
redirect people to the wrong place.

"Portable Operating System Interface"

Ah, so you are claiming not to know that they discus POSIX on
comp.unix.programmer. I, on the other hand, know that they do. If you
don't know enough about what is acceptable on a group don't try claiming
it is the wrong place.

 

[Falcon Kirtaran]

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Thanks, yes, I just found that out meanwhile as well .
Oh yes, seems to be ... But thanks for the advice, the following code
does the job:

#include <unistd.h>
#include <stdio.h>
int main(void) {
FILE *stdalt;
int tmp = dup(3);
if(tmp == -1) {
stdalt = fdopen(dup(2), "w");
stdalt = freopen("/dev/tty", "w", stdalt);
}
else {
close(tmp);
stdalt = fdopen(3, "w");
}
printf("Alternative file descriptor: %d\n", fileno(stdalt));
fprintf(stdout, "STDOUT\n");
fprintf(stderr, "STDERR\n");
fprintf(stdalt, "STDALT\n");
return 0;
}

$ ./a.out 1>xxx
STDERR
STDALT
$ ./a.out 2>xxx
Alternative file descriptor: 3
STDOUT
STDALT
$ ./a.out 3>xxx
Alternative file descriptor: 3
STDOUT
STDERR

I'd really like to know why I have to use freopen on stdalt with
/dev/tty though. Whithout it, I get

./a.out 2>xxx
Alternative file descriptor: 3
STDOUT

Everything else stays the same as above. Any ideas?

Thanks a lot,

Christof

If you fopen() on 2, but 2 is redirected, whatever you've fopen()ed will
correspond to the same file as 2 does. You could probably replace

stdalt = fdopen(dup(2), "w");
stdalt = freopen("/dev/tty", "w", stdalt);

with

stdalt = fopen("/dev/tty", "w");

- --
- --Falcon Darkstar Kirtaran
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[Christof Warlich]

If you fopen() on 2, but 2 is redirected, whatever you've fopen()ed will
correspond to the same file as 2 does. You could probably replace

stdalt = fdopen(dup(2), "w");
stdalt = freopen("/dev/tty", "w", stdalt);

with

stdalt = fopen("/dev/tty", "w");

Yes, that works as well and is much easier to understand.
Again, many thanks for the advice.

Regards,

Christof

 

[Christof Warlich]

You need to ask over in comp.unix.programmer as this is a Unix issue
rather than a C issue.

You are right, this would probably have been the better place to ask.
But anyway, I'm quite happy with the advice I got from Falcon Kirtaran.

Regards,

Christof

 

[Falcon Kirtaran]

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Hash: SHA1

Yes, that works as well and is much easier to understand.
Again, many thanks for the advice.

Regards,

Christof

Yay! Anytime.

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[Christof Warlich]

You can make the test less clumsy through the use of dup2(). Here's
an idiom that many real-world C programmers use:

-----------------------------------------------------
#include <stdio.h>
#include <unistd.h>

int main(void)
{
if(dup2(3, 3) == -1) /* 3 invalid; EBADF? */
puts("not already open!");
else /* does nothing */
puts("already open!");

return 0;
}
-----------------------------------------------------

Yes, this makes things even more simple, as it avoids the need
for the close() call. So to sum up again, my code would the look
like this:

#include <unistd.h>
#include <stdio.h>
int main(void) {
FILE *stdalt;
if(dup2(3, 3) == -1) {
stdalt = fopen("/dev/tty", "w");
}
else {
stdalt = fdopen(3, "w");
}
printf("Alternative file descriptor: %d\n", fileno(stdalt));
fprintf(stdout, "STDOUT\n");
fprintf(stderr, "STDERR\n");
fprintf(stdalt, "STDALT\n");
return 0;
}

Thanks for this further improvement as well,

Christof

 

[CBFalconer]

.... snip ...

Yes, this makes things even more simple, as it avoids the need
for the close() call. So to sum up again, my code would the look
like this:

#include <unistd.h>
#include <stdio.h>
int main(void) {

Please don't encourage Hans off-topic postings. unistd.h is not a
standard C header, and thus is off-topic on c.l.c. We attempt to
limit things to the portable C language, that is not OS dependant,
but does depend on the C standard. comp.unix.programmer would
probably be a suitable newsgroup.

 

[Antoninus Twink]

We attempt to limit things [snip usual blather]

Ah, the royal "we" again.

This is not a moderated group, and each person can make his or her own
decision about what is appropriate to post here.

 

[Richard]

We attempt to limit things [snip usual blather]

Ah, the royal "we" again.

This is not a moderated group, and each person can make his or her own
decision about what is appropriate to post here.

Also I have a header called "unistd.h" which contains

#define H_FLAG 0


Why is this off topic?

 

[CBFalconer]

It is not. However, I would agree with Chuck that the majority
of this thread is off topic, as the primary focus of the thread
is dup, dup2, and shell redirections. Those topics have little
to do with C as a language.

He referred to <unistd.h>, which is not a standard C header.
"unistd.h" is in the users domain, and is another matter.