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Re: bug: configuration problem in perl with gcc libs

On 6/3/2016 12:14 PM, Dmitry Karasik wrote:

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/, 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/ 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.


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