bug#10578: 24.0.92; No png images on OpenSUSE 12.1

[Eli Zaretskii]

> From: Lars Magne Ingebrigtsen <larsi@gnus.org>
> Cc: 10578@debbugs.gnu.org
> Date: Sun, 22 Jan 2012 17:50:02 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> Recent messages:
> >> PNG warning: Application was compiled with png.h from libpng-1.4.8
> >> PNG warning: Application  is  running with png.c from libpng-1.2.46
> >> PNG error: Incompatible libpng version in application and library
> >
> > Any questions?
> 
> Of course.
> 
> 1) Why didn't the warnings appear in the echo area instead of just
> hiding out in the *Messages* buffer?

Most probably because they are emitted during redisplay, and you
cannot have anything displayed in the echo area at redisplay time,
because doing so re-enters redisplay, which will again produce the
same error message, etc., ad nauseam (a.k.a. infloop).

> 2) If I have both libpng12 and 14 installed, why is Emacs building
> against 14 and then preferring 12 when running?

Because Emacs on GNU/Linux doesn't use the kind of machinery it does
on MS-Windows, whereby it will reject 12 if it was compiled against
14.  See w32-win.el:dynamic-library-alist, and in particular how it
consults the variable libpng-version (which is set at compile time).