From owner-freebsd-hackers@FreeBSD.ORG  Wed Jun 10 01:03:55 2015
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Date: Tue, 9 Jun 2015 21:03:53 -0400 (EDT)
From: Daniel Eischen <deischen@freebsd.org>
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Reply-To: Daniel Eischen <deischen@freebsd.org>
To: "Russell L. Carter" <rcarter@pinyon.org>
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Subject: Re: umtx_lock and friends
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On Tue, 9 Jun 2015, Daniel Eischen wrote:

> On Tue, 9 Jun 2015, Russell L. Carter wrote:
>
>> On FreeBSD 10-stable r283752 amd64, here is the compile line:
>> 
>> /usr/bin/cc -O -Wall tester.cpp -o tester
>> /tmp/tester-b681b3.o: In function `main':
>> tester.cpp:(.text+0x35): undefined reference to `_umtx_lock(umtx*)'
>> tester.cpp:(.text+0x5c): undefined reference to `_umtx_unlock(umtx*)'
>> 
>> Compilers don't matter, linking against libthr or libstdthreads
>> doesn't matter.
>
> Is cc doing something special because it thinks it is C++?
> I'm on 10-stable amd64 and that example (saved as tester.c)
> works just fine.

Try:

   /usr/bin/cc -O -Wall -x c tester.cpp -o tester

-- 
DE