Re: Internal use of vsnprintf

[Florian Weimer <fweimer at redhat dot com>]

On 05/03/2016 06:57 PM, Joseph Myers wrote:
> On Tue, 3 May 2016, Florian Weimer wrote:
>
> > How do I do that?  We have an __vsnprintf definition in include/stdio.h, but I
> > can't find a libc_hidden_proto/libc_hidden_define for it, and I get a PLT
> > check failure as a result if I use it.
>
> Because __vsnprintf is an exported symbol (in libio/Versions).

Ahh, and _IO_vsnprintf is not.  That's what I was missing, thanks.  This 
is also how __snprintf works.

> > Or should I call _IO_vsnprintf instead (which is what the snprintf
> > implementation does)?  I don't see the usual PLT avoidance stuff in the
> > sources for _IO_vsnprintf, but it does happen.  So how does it work?
>
> If you call a function that's not exported from a shared library, then
> properly you should also declare it with attribute_hidden to get the same
> optimization as you get for an exported function using libc_hidden_* (you
> can use libc_hidden_* to create and use aliases for a non-exported
> function, that's just unnecessarily complicated compared to declaring it
> hidden).  For most architectures this doesn't matter (you don't get any
> better optimization that way), but declaring it hidden helps for 32-bit
> x86, where the ABI means a call to a function known at compile time to be
> in the same shared library can be more efficient (see bug 18822).

In the _IO_snprintf case, I have to include <libioP.h>, which is not 
great because it contains truly internal libio stuff as well.  Should I 
use this approach nevertheless, or somehow arrange for PLT avoidance for 
__vsnprintf (perhaps following the example of _IO_vsprintf)?

Florian