Re: builtin echo command redirection misbehaves in detached scripts when terminal is closed

[Stephane Chazelas]

On Mon, Sep 10, 2007 at 11:56:33AM +0400, Dmitry Potapov wrote:
> On Sun, Sep 09, 2007 at 10:18:07PM +0100, Stephane Chazelas wrote:
> > Now, I'm not sure if we can say that the new glibc behavior
> > observed is bogus (other than it's different from the behavior
> > observed in all the libcs I tried with).
> 
> What libc have you tried?
> 
> To me, the new behavior makes much more sense, as dropping buffer on
> error is really weird thing to do. I have looked at the source code of
> newlib and dietlibc, none of them drops buffer on error, and I am not
> aware about any other implementation of libc that does.

Hi Dmitry,

thanks for replying, I gave a list in another email. I tried on
Solaris 7 and HPUX and both seem to flush the buffer upon an
unsuccessful fflush()

> > It is not a harmless
> > change, for sure as it seems to have broken at least bash, zsh
> > and possibly ksh93.
> 
> Unfortunately, you are right. I did not foresee that some shells may use
> "dup2(open("file.txt"), fileno(stdout))". It is a dirty hack, which may
> cause some other problems. Frankly, I am a bit surprised that bash uses
> printf instead of write(2).  BTW, you cannot use 'printf' in signal
> handlers, so it seems that you cannot use 'echo' in trap commands too.
> 
> Perhaps, we should rollback my patch and give some time for developers
> to fix their broken shells, but, in this case, what is actually broken
> are those shells, not libc!
[...]

On the other end, how would you force the flush of the buffer?

And how would you redirect stdout? We can use freopen() instead
of the hack above for files, but not for pipes or arbitrary fds
(as in >&3). Erik Blake was suggesting to use freopen(NULL) (not
to fix that very problem but because of the fact that if you
reassign stdout to some resource of a different nature, you need
to tell stdio as stdio may need to operate differently), but
that's not very portable according to POSIX. Would freopen(NULL)
flush the output buffer?

You cannot simply assign stdout to some value returned by
fdopen() as that's not portable either...

-- 
Stéphane