freopen on both stdout and stderr

G

Guillaume Dargaud

Hello all,
a while ago I was pointed towards freopen as a way to redirect stderr to a
log file. It works great, but apparently the app also writes a few lines to
stdout. Now I could redirect to 2 separate files, but would rather keep the
2 flows together.

Is it correct to do this:
stderr=freopen(LogFile, "w", stderr);
stdout=freopen(LogFile, "a", stdout);

I can see plenty of reasons why it would fail (buffering and flushes come to
mind).
Any advice ?
--
Guillaume Dargaud
http://www.gdargaud.net/Climbing/
"Faith can move mountains but let them happily fall down on the heads of
other people. What's the point in moving mountains when it's so simple to
climb over them ?" - Boris Vian, surrealist French writer and singer, En
verve.
 
M

Mark Bluemel

Guillaume said:
Hello all,
a while ago I was pointed towards freopen as a way to redirect stderr to a
log file. It works great, but apparently the app also writes a few lines to
stdout. Now I could redirect to 2 separate files, but would rather keep the
2 flows together.

Is it correct to do this:
stderr=freopen(LogFile, "w", stderr);
stdout=freopen(LogFile, "a", stdout);

It's not in accordance with the standard to do this, as "stderr" and
"stdout" are not, as I recall, guaranteed to be lvalues...

If you can assign them, and you're prepared to accept non-portability,
then you could perhaps try the following, untested, hackery

stderr=freopen(LogFile, "w", stderr);
fclose(stdout);
stdout=stderr;

Otherwise, I'm not convinced you have many options available to you.
 
F

Flash Gordon

Mark Bluemel wrote, On 23/11/07 12:01:
It's not in accordance with the standard to do this, as "stderr" and
"stdout" are not, as I recall, guaranteed to be lvalues...

If you can assign them, and you're prepared to accept non-portability,
then you could perhaps try the following, untested, hackery

stderr=freopen(LogFile, "w", stderr);
fclose(stdout);
stdout=stderr;

Otherwise, I'm not convinced you have many options available to you.

Personally I would use a mechanism outside the program in order to
achieve this and not redirect either stdout or stderr within the
program. E.g.
prog >log 2>&1
The precise mechanism (and whether it is even possible) is dependant on
the system being used.
 
K

Keith Thompson

Flash Gordon wrote:
[...]
Personally I would use a mechanism outside the program in order to
achieve this and not redirect either stdout or stderr within the
program. E.g.
prog >log 2>&1
The precise mechanism (and whether it is even possible) is dependant on
the system being used.

That's fine if you want to redirect stderr and stdout to the same place
for the entire run of the program. If you want to redirect it only for
part of the program, it's more difficult.
 
S

santosh

Flash Gordon wrote:
[...]
Personally I would use a mechanism outside the program in order to
achieve this and not redirect either stdout or stderr within the
program. E.g.
prog >log 2>&1
The precise mechanism (and whether it is even possible) is dependant
on the system being used.

That's fine if you want to redirect stderr and stdout to the same
place for the entire run of the program. If you want to redirect it
only for part of the program, it's more difficult.

As far as I can see, the only fully portable solution is to design the
program from the ground up in such a way that all I/O functions take
explicit stream parameters instead of being hard-wired to a particular
stream.

Also when stdout is redirected there is no way to recover the path to
the original device.
 
J

Justin Spahr-Summers

It's not in accordance with the standard to do this, as "stderr" and
"stdout" are not, as I recall, guaranteed to be lvalues...

If you can assign them, and you're prepared to accept non-portability,
then you could perhaps try the following, untested, hackery

stderr=freopen(LogFile, "w", stderr);
fclose(stdout);
stdout=stderr;

Otherwise, I'm not convinced you have many options available to you.

N1256 seems to specify that, on success, freopen() just returns its
third argument, so there'd be no need to assign the result to stdout/
stderr when passing them as the stream. There's even a footnote:

"The primary use of the freopen function is to change the file
associated with a standard text stream (stderr, stdin, or stdout), as
those identifiers need not be modifiable lvalues to which the value
returned by the fopen function may be assigned."
 
C

Charlie Gordon

Guillaume Dargaud said:
Hello all,
a while ago I was pointed towards freopen as a way to redirect stderr to a
log file. It works great, but apparently the app also writes a few lines
to stdout. Now I could redirect to 2 separate files, but would rather keep
the 2 flows together.

Is it correct to do this:
stderr=freopen(LogFile, "w", stderr);
stdout=freopen(LogFile, "a", stdout);

Do not store the result of freopen into stderr or stdout, just check that it
is not NULL.
In order to preserve proper flows for both streams, you should make both
unbuffered via setvbuf.
You might also want to first create the LogFile, and then freopen both
streams in append mode.
 

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