Re: [RFC] convert a host address to a string

[Kai Tietz <Kai.Tietz@onevision.com>]

gdb-patches-owner@sourceware.org wrote on 08.01.2009 14:26:15:

> > Date: Thu, 8 Jan 2009 14:19:11 +0400
> > From: Joel Brobecker <brobecker@adacore.com>
> > 
> > > An option would be to use the strategy used by phex_nz() to print 
host
> > > addresses.  Or we could use PRINTF_HAS_LONG_LONG, and always use 
%llx
> > > if it's available.
> > 
> > Unfortunately, I don't know how this could be made to work.
> > The problem is that GCC insists that the integer type that we
> > use to cast the host address to must have the same size.
> > At one point, hoping that GCC would kill the wrong branch,
> > I even tried:
> > 
> >   if (sizeof (void *) == sizeof (long))
> >     printf ("0x%lx", (long) address);
> >   else
> >     printf ("0x%llx", (long long) address);
> > 
> > But this didn't work, because GCC complained about the cast
> > in the "if" branch.
> 
> Ah, GCC is being a bit too helpful here.  Damn!
> 
> > Actually, it's only after writing the entire email that I realized
> > that we have another option. See option (3) below.
> > 
> > > I'd really like to avoid introducing another macro dealing with
> > > type-size issues if possible.  I especially dislike HOST_IS_LLP64
> > > since I fear its existence encourages people to write unportable 
code.
> > 
> > I can see several solutions:
> > 
> >   1. Use %p. To overcome the problem with 0x, we could use
> >      two alternatives:
> > 
> >        a. Import printf from gnulib. I looked at this a while ago,
> >           for some other issue, and I immediately stopped, as it
> >           looked like it might be a lot of work to do so (printf
> >           doesn't come alone, there's a bunch of other routines
> >           that printf uses which we probably want).
> 
> I'm not very excited about this option.  And if the gnulib printf
> doesn't actually implement the Microsoft-invented non-standard format
> specifiers it may even cause us more grief.
> 
> >        b. Strip the leading "0x" if %p already provides it. In other
> >           words:
> > 
> >             fprintf (buf, "0x%p", address);
> >             if (buf[2] == '0' && buf[3] == 'x')
> >               buf = buf + 2;
> >             return buf;
> > 
> >           There is no memory management issue in this case, because
> >           the buffer we return is more or less static. It's part
> >           of a bunch of buffers we cycle through each time we call
> >           this routine.  The caller never frees the memory we return.
> 
> Ugh, this is a bit ugly.  And we can't even be sure that there are
> even more variations on the format that %p generates.  I wouldn't be
> surprised at all if some platforms would use upper case for the hex
> digits for example.
> 
> >   2. Avoid the HOST_IS_LLP64 macro, but still do something similar
> >      inside host_address_to_string. Something like:
> > 
> >        #if defined(WIN64_)
> >            fprintf (buf, "0x%llx", (unsigned long long) address);
> >        #else
> >            fprintf (buf, "0x%lx", (unsigned long) address);
> >        #endif
> > 
> >      This eliminates the likeliness of re-using the HOST_IS_LLP64
> >      macro to write non-portable code.
> 
> Not really excited about this one either.
> 
> >   3. Work through uintptr_t.
> > 
> >        #ifdef PRINTF_HAS_LONG_LONG
> >          fprintf (buf, "0x%llx", (unsigned long long) (uintptr_t) 
address);
> >        #else
> >          fprintf (buf, "0x%lx", (unsigned long) (uintptr_t) address);
> >        #endif
> 
> This wouldn't be the first place where we'd use a double cast in
> connection with intptr_t/uintptr_t.  So I'd say that while this is a
> bit ugly, it's certainly acceptable.  It's by far the simplest way to
> fix things.
> 
> > I kinda like option 1b as being simple and avoiding the need to
> > cast the address to an integer.  Option (3) is my next favorite,
> > but I don't like the fact that we end up doing an unnecessary
> > integer promotion on the 32bit targets.
> 
> I'm not really worried about the integer promotion.  Printing host
> addresses is a fairly rare operation, and certainly not time critical.
> 

Please be aware that %llx isn't valid for x86_64 windows (as it isn't for 
32-bit standard). The options 'I' or 'I64' have to be used for those 
targets (at least for _WIN64).

Cheers,
Kai

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