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Subject: Re: bug: configuration problem in perl with gcc libs
To: cygwin AT cygwin DOT com
References: <20160603161419 DOT GA5300 AT karasik2>
From: Ken Brown <kbrown AT cornell DOT edu>
Message-ID: <785ef8ef-4335-2d25-284e-6ece02fbd427@cornell.edu>
Date: Sun, 5 Jun 2016 18:28:02 -0400
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On 6/3/2016 12:14 PM, Dmitry Karasik wrote:
> 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.

I'm no perl expert, but this doesn't strike me as a good solution.  It 
means that a specific gcc version is hard-coded into perl.

It seems to me that the bug is in perl's algorithm (in 
/usr/lib/perl5/5.22/ExtUtils/Liblist/Kid.pm) for finding libraries of 
the form -lfoo.  There's a special case for Cygwin at line 171 that 
looks for foo.dll (if it hasn't found libfoo.dll.a), which is wrong for 
two reasons.  First, it should look for cygfoo*.dll instead (or in 
addition).  Second, it should look in the path, or at least in /usr/bin, 
rather than in libpth.

In the present case of -lgomp, it would have found /usr/bin/cyggomp-1.dll.

Ken


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