fix missing library on BSD systems

[tony]

On FreeBSD (haven't tried Linux yet) this library will be missing. I believe it's only available on OS X.

http://libcxxabi.llvm.org/

[tony]

Builds work in Linux, so this seems to be freebsd only.

I'm going to also test this on openbsd as well for good measure.

[tony]

OpenBSD 6.0

Let me brush up tomorrow and test across OpenBSD 6.0, FreeBSD 11-STABLE, MacOS and Ubuntu 16.10

[wjakob]

Hi @tony,

this looks great. One note: I've found that "-lc++abi" is needed on Linux (when using Clang), and your patch removes it for that platform.

Let me know when this is ready to merge.

Best,
Wenzel

[tony]

Hi @wjakob. I've tested on OpenBSD 6.0, FreeBSD 11-CURRENT, OS X, however have had issues with testing on Ubuntu. It does compile and link correctly on Ubuntu 16.10, but not run due to lack of OpenGL 3.3 support in VirtualBox.

[wjakob]

Hi @tony,

this looks great now. No worries about Ubuntu, I am using that frequently myself and everything works fine.

One quick question after looking at the PR once more: I saw that you actually completely disabled libc++ when using Clang as a compiler on *BSD. Is that really the right thing to do? To my knowledge, when using Clang, you'll generally want to combine it with libc++ that was developed specifically to be used with that compiler.

I've run into many incompatibilities in the alternative libstdc++ / Clang combination on Linux, hence this setting in the CMake script.

Best,
Wenzel

[wjakob]

(note that this comment is unrelated to the -lc++abi flag which indeed does not seem to be supported on *BSD)

[tony]

FreeBSD 11-STABLE will say cc: warning: argument unused during compilation: '-stdlib=libc++' as a warning. https://lists.freebsd.org/pipermail/freebsd-bugs/2013-October/054196.html

So I can add it back, but we'll get the warning. I have to check OpenBSD again.

I saw that you actually completely disabled libc++ when using Clang as a compiler on *BSD. Is that really the right thing to do? To my knowledge, when using Clang, you'll generally want to combine it with libc++ that was developed specifically to be used with that compiler.

I'm not 100% sure. I omit including libc++ by hand on OpenBSD and FreeBSD (as in my current patch), then how is it building and running on FreeBSD and OpenBSD?

I know CMake does some magic when it comes to adding flags on your behalf. I can't think of a time I've had the need to go in and add the standard lib stuff manually.

libc++abi-build.txt is my cmake --system-information output.

[wjakob]

Is this OpenBSD cc actually Clang?

[tony]

Default compiler on OpenBSD is going to be GCC 4.2... So yeah that's a caveat. My testing requires installing llvm (which includes clang).

This may change in OpenBSD 6.1 (https://www.phoronix.com/scan.php?page=news_item&px=OpenBSD-Adds-LLVM-To-Base)

On OpenBSD I am using CC=/usr/local/bin/clang CXX=/usr/local/bin/clang++ with python off. I was only really intending preliminary OpenBSD support in this patch. I may have promised too much 😄

[tony]

A few things,

1. current master @ 72cf9b5 is broken with FreeBSD. So this PR remedies that.
2. I'm currently in the process of going through OpenBSD setup and see what's needed. Default compiler isn't going to support C++11. It's common to install gcc5 and llvm (includes clang) after install due to these limitations.
3. I am going to write docs for installation on OpenBSD and FreeBSD and QA all systems once again.

[tony]

On OpenBSD 6.0 (clang 3.8) and FreeBSD 11-STABLE (system clang), this will raise a warning clang-3.8: warning: argument unused during compilation: '-stdlib=libc++'

[tony]

Even when removing these, OpenBSD 6.0 will do this on linking:

Scanning dependencies of target nanogui
[ 88%] Linking CXX shared library libnanogui.so
/usr/bin/ld: cannot find -lc++
clang-3.8: error: linker command failed with exit code 1 (use -v to see invocation)
*** Error 1 in . (CMakeFiles/nanogui.dir/build.make:157 'libnanogui.so')
*** Error 1 in . (CMakeFiles/Makefile2:107 'CMakeFiles/nanogui.dir/all')
*** Error 1 in /root/study/c++/nanogui/build (Makefile:128 'all')

[tony]

• Added docs
• Tested again on FreeBSD 11-STABLE, OpenBSD (GCC 4.7 and Clang 3.8) and MacOS Sierra
• Did some more research, the -stdlib=libc++ flag won't work on the BSD's. It will fail compilation phase. I think for the BSD's letting clang find the standard lib is best choice.

[wjakob]

Merged, thank you!