freopen()

[John Devereux]

Hi,

I am a bit confused about the behaviour of freopen() and the standard
i/o streams.

I am trying to redirect a stream (stderr) to a particular "file". But,
if the call fails I want it to stay as it was, or at least be able to
redirect it somewhere else (e.g. stdout).

For example I would like to do,

..
stderr = freopen("/dev/errors","w", stderr); /* may fail */
if(!stderr) freopen("/dev/screen", "w", stderr);

 

[Richard Tobin]

I am trying to redirect a stream (stderr) to a particular "file". But,
if the call fails I want it to stay as it was, or at least be able to
redirect it somewhere else (e.g. stdout).

According to the standard, "The freopen function first attempts to
close any file that is associated with the specified stream". So if
opening the new file fails, it's too late to revert to the old one.

(You may be able to do this using something operating-system specific
instead, such as dup2() in unix.)

I see nothing that implies that you can't attempt to freopen() the
same stream again. (This may explain the "... any file ..." wording.)

stderr = freopen("/dev/errors","w", stderr); /* may fail */

Don't do this. You can't assign to stderr. freopen() changes the file
associated with the stream rather than creating a new stream. Just
do

if(!freopen("/dev/errors","w", stderr))
...

-- Richard

 

[John Devereux]

According to the standard, "The freopen function first attempts to
close any file that is associated with the specified stream". So if
opening the new file fails, it's too late to revert to the old one.

(You may be able to do this using something operating-system specific
instead, such as dup2() in unix.)

I see nothing that implies that you can't attempt to freopen() the
same stream again. (This may explain the "... any file ..."
wording.)


Don't do this. You can't assign to stderr. freopen() changes the file
associated with the stream rather than creating a new stream. Just
do

if(!freopen("/dev/errors","w", stderr))
...

Just to clarify, do you mean I can freopen() stderr with another
stream, even after the failure?

if(!freopen("/dev/errors","w", stderr))
{
stderr = freopen("/dev/screen","w", stderr);
}


I gather freopen() returns a null pointer if unsuccessful, does that
then mean (stderr==0) ? I read that sometimes stderr is implemented as
a macro e.g.

#define stderr (&iob[2])

But if so how can the call

stderr = freopen(..)

be legal?


Thanks,

 

[John Devereux]

But if so how can the call

stderr = freopen(..)

be legal?

Sorry, I now see it's not legal. I was confused because my
implementation did not complain.