Re: [Patch/RFC] Fix libstdc++/9533

[Paolo Carlini <pcarlini at unitus dot it>]

Nathan Myers wrote:

> On Thu, Mar 13, 2003 at 01:54:48PM +0100, Paolo Carlini wrote:
>  
>
> > the below fixes the problem by implementing a non-trivial showmanyc(),
> > as discussed thoroughly on the list during the last weeks.
> >
> > In particular, I would appreciate by reassured that the new
> > __basic_file<>::showmanyc_helper deals correctly with the various
> > possibilities.
> >    
> This patch needs some cleanup.  I apologize for the delay in replying;
> I was sick as a dog all of last week.
Too bad. Anyway, thanks for your comments: I will try to prepare an 
updated version during
this week (while Benjamin is working on the 27_io testsuite).

> First, the above code can only reveal information about sockets, and 
> not, unfortunately, pipes.  (On a pipe it reports failure.)  So, the 
> comment is not right.  Still, sockets are important enough to merit 
> the special treatment.
Ok.

> Second, passing zeroes to the recv() arguments isn't portable.  I find
> that, on Linux, (1) we must actually provide a buffer for it to copy 
> the characters into, and (2) it won't return a value larger than the 
> indicated buffer size (even with MSG_TRUNC).   This means you can't 
> avoid having it copy the characters.  (Drat.) The buffer used might as 
> well be the actual stream buffer.  This is still less expensive than 
> an actual underflow, potentially, because we don't have to run the 
> codecvt operation afterward.  Further, we haven't actually messed with 
> the socket state.
I see. This is really unfortunate, that we have to actually read the data :(
How much data we must be prepared to? Even gigabytes, I'm afraid! Just 
to know with MSG_PEEK
how much data we'll actually extract later on (maybe, only maybe!).
Where do you suggest we store such data?

> I would prefer to express the above as
>  
>
> > +#ifdef _GLIBCPP_S_ISREG_OR_S_IFREG
> > +    // Regular files.
> > +    struct stat __buffer;
> > +    int __ret = fstat(this->fd(), &__buffer);
> > +    if (!__ret && _GLIBCPP_S_ISREG_OR_S_IFREG(__buffer.st_mode))
> > +      if (__stdio)
> > +	return __buffer.st_size - ftell(_M_cfile);
> > +      else
> > +	return __buffer.st_size - lseek(this->fd(), 0, ios_base::cur);
> > +#endif
> >    
>
> with a corresponding definition for _GLIBCPP_S_ISREG_OR_S_IFREG 
> constructed in configure.
Yes, this is much more clean. I have to learn a bit more about the 
configury machinery in order to
do that, but I will try...

> > +
> > +      streamsize
> > +      showmanyc_helper(bool __stdio);
> >    
> As in several other patches that have come through, I believe this 
> member name must be uglified.  I don't mind if they're all fixed up
> in a separate patch.
Ok. To be honest, I didn't follow your exchanges with Gaby about this 
and the conclusions you
arrived to. We'll deal with the issue in separate patch, as you say.

Paolo.