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

Mark Kettenis
2009-01-08T13:26:47Z

> 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.