bug#25268: 25.1; configure.ac libpng search error

bug#25268: 25.1; configure.ac libpng search error

[James K. Lowden]

configure.ac uses libpng-config, if installed, to determine the include
directive for the PNG library.  Unfortuately, it does not correctly use
libpng-config for the linker flags.  Consequently, it may mismatch the
header file to the library.  In my case, configure misdiagnosed my
setup, and recommended --no-png.

On my system, a libpng-config was installed by the Anaconda scientific
Python package distribution.  Because I had allowed Anaconda to place
itself earlier in the path than /usr/bin, that is the version configure
used.  I suspect the error I describe below has not been detected before
because my setup is unusual.

First, let's see what libpng-config has to say on my system: 

$ for opt in cflags ldflags libs
do
        cmd="libpng-config --$opt"
        printf "%-25s: %-s\n" "$cmd" "$($cmd)"
done
libpng-config --cflags   : -I/usr/local/anaconda/include/libpng16
libpng-config --ldflags  : -L/usr/local/anaconda/lib -lpng16
libpng-config --libs     : -lpng16

Now, let's look at how libpng-config is used in configure.ac:

$ grep -n libpng-config configure.ac
3412:    png_cflags=`(libpng-config --cflags) 2>&AS_MESSAGE_LOG_FD` &&
3413:    png_libs=`(libpng-config --libs) 2>&AS_MESSAGE_LOG_FD` || {
3414:      # libpng-config does not work; configure by hand.

As you can see, png_libs relies on

   libpng-config --libs

not

   libpng-config --ldflags

The produced configure script therefore finds an include file under
ananconda, but does not provide the associated -L option to the
linker.  The configure test fails because a symbol declared in the
header file is not supplied by the (older) libpng found by the linker.  

I believe the fix is to change line 3413 of configure.ac to read:

    png_libs=`(libpng-config --ldflags) 2>&AS_MESSAGE_LOG_FD` || {

although IMO that's not quite sufficient, because it doesn't set RPATH.  

I worked around the problem before completely diagnosing it simply by
renaming /usr/local/anaconda to /usr/local/.anaconda, taking it out of
the PATH and causing configure not to find it.  The outputs are:

    $ /usr/bin/libpng-config --cflags --libs
    -I/usr/include/libpng12
    -lpng12

which worked just fine.  In fact, I used report-emacs-bug in the newly
built emacs to send this message.

The rest of this message was produced by report-emacs-bug.  I include it
in case there are details you can use.

Humbly submitted,

--jkl




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bug#25268: 25.1; configure.ac libpng search error

[Paul Eggert]

[-- Attachment #1: Type: text/plain, Size: 486 bytes --]

> I believe the fix is to change line 3413 of configure.ac to read:
>
>     png_libs=`(libpng-config --ldflags) 2>&AS_MESSAGE_LOG_FD` || {

Thanks, I did something along those lines by installing the attached patch into 
master, and I'm boldly marking the bug report as done.

> although IMO that's not quite sufficient, because it doesn't set RPATH.

--ldflags is supposed to output any -R options needed, no? If it's not, it 
sounds like the libpng-config configuration is incorrect.

[-- Attachment #2: 0001-Use-libpng-config-ldflags-not-libs.patch --]
[-- Type: text/x-diff, Size: 1777 bytes --]

From 5bdbbe3e89adbb2a2092cc73586ffc38ddbb95d9 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Sat, 24 Dec 2016 17:03:22 -0800
Subject: [PATCH] Use libpng-config --ldflags, not --libs

Problem reported by James K. Lowden (Bug#25268).
* configure.ac (LIBPNG): Pass --ldflags, not --libs, to libpng-config.
---
 configure.ac | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/configure.ac b/configure.ac
index 5aaf006..cd6c689 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3476,7 +3476,7 @@ AC_DEFUN
   elif test "${HAVE_X11}" = "yes" || test "${HAVE_W32}" = "yes"; then
     AC_MSG_CHECKING([for png])
     png_cflags=`(libpng-config --cflags) 2>&AS_MESSAGE_LOG_FD` &&
-    png_libs=`(libpng-config --libs) 2>&AS_MESSAGE_LOG_FD` || {
+    png_ldflags=`(libpng-config --ldflags) 2>&AS_MESSAGE_LOG_FD` || {
       # libpng-config does not work; configure by hand.
       # Debian unstable as of July 2003 has multiple libpngs, and puts png.h
       # in /usr/include/libpng.
@@ -3486,18 +3486,18 @@ AC_DEFUN
       else
 	png_cflags=
       fi
-      png_libs='-lpng'
+      png_ldflags='-lpng'
     }
     SAVE_CFLAGS=$CFLAGS
     SAVE_LIBS=$LIBS
     CFLAGS="$CFLAGS $png_cflags"
-    LIBS="$png_libs -lz -lm $LIBS"
+    LIBS="$png_ldflags -lz -lm $LIBS"
     AC_LINK_IFELSE(
       [AC_LANG_PROGRAM([[#include <png.h>]],
 	 [[return !png_get_channels (0, 0);]])],
       [HAVE_PNG=yes
        PNG_CFLAGS=`AS_ECHO(["$png_cflags"]) | sed -e "$edit_cflags"`
-       LIBPNG=$png_libs
+       LIBPNG=$png_ldflags
        # $LIBPNG requires explicit -lz in some cases.
        # We don't know what those cases are, exactly, so play it safe and
        # append -lz to any nonempty $LIBPNG, unless we're already using LIBZ.
-- 
2.7.4