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X-HELO:	karasik.eu.org
Date:	Fri, 3 Jun 2016 18:14:19 +0200
From:	Dmitry Karasik <dmitry AT karasik DOT eu DOT org>
To:	cygwin AT cygwin DOT com
Subject:	bug: configuration problem in perl with gcc libs
Message-ID:	<20160603161419.GA5300@karasik2>
MIME-Version:	1.0
User-Agent:	Mutt/1.5.23 (2014-03-12)
Note-from-DJ:	This may be spam

Hello,

I'd like to report a configuration bug in perl.  The problem arises when a 3-rd
party module tries to build an extension using perl configuration with a
gcc-specific library.

Generally perl extensions don't have a way to specify library to link with
directly, they do that through ExtUtils::MakeMaker, the standard tool for that.
Which in turn tries to resolve '-llibname' using its own
compile-time-configured internal list of lib paths. Everything works so far
libraries are found in perl's libpth (see by running 'perl -V:libpth') which is
/usr/lib on my machine. The problem arises when I need to link together with
libgomp, which is not found there, being a gcc-version-specific library.

For example, the following minimal Makefile.PL configure script

   use ExtUtils::MakeMaker;
   WriteMakefile(NAME => 'foo', LIBS => '-lgomp');

will emit a warning

   Warning (mostly harmless): No library found for -lgomp

and removes -lgomp from the linker command, resulting in perl extension not
being able to compile.

The problem is confirmed, when, if I edit perl configuration file
/usr/lib/perl5/5.22/i686-cygwin-threads-64int/Config.pm, everything works:

     ldlibpthname => 'PATH',
-    libpth => '/usr/lib',
+    libpth => '/usr/lib /usr/lib/gcc/i686-pc-cygwin/5.3.0',
     osname => 'cygwin',

I believe perl needs to be built with the properly set/found libpth in advance.
The diff below is the closest thing resembling a patch for the perl source
package I could come with:

--- Configure.0	2016-06-03 17:45:43.102008000 +0200
+++ Configure	2016-06-03 17:46:10.077558700 +0200
@@ -4948,6 +4948,16 @@
 		   *) libpth="$libpth $j";;
 		   esac
 	       fi
+               # add gcc-specific libpath
+               if echo "$i" | grep -q "/usr/lib/gcc/"; then
+	           j="`$echo $i|$sed 's,/include$,,'`"
+	           if $test -d $j; then
+	               case " $libpth " in
+	               *" $j "*) ;;
+	               *) libpth="$libpth $j";;
+	               esac
+	           fi
+               fi
 	    done
 	    libpth="`$echo $libpth|$sed 's/^ //'`"
 	    for xxx in $libpth $loclibpth $plibpth $glibpth; do

The idea for the patch is taken from strawberry perl, which has the gcc libpath
included in configuration.  I couldn't find though exactly how they manage to
include the path, during the configuration or when building, but it seems they
somehow add it explicitly, using a custom tool for the MinGW perl build:

https://github.com/StrawberryPerl/Perl-Dist-Strawberry/blob/master/share/portable/portable.perl.32#L60-L65

Hopefully this makes sense.

-- 
Sincerely,
	Dmitry Karasik


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