Xoltar

A blog by Bryn Keller about simplicity - in software design, and in life

Mac OS X C++ Development

I recently switched to a MacBook Pro. I have customers that use Linux and Mac, and I wanted to work in a similar environment. Also recently (a few months before the MacBook) I started working with C++ again after a long hiatus.

I had thought that the Mac, being a Unix, would be relatively close to Linux, and that things I was building for Linux would be much more likely to work there than on Windows. That might still be true, but it turns out that there are several things on Mac that are not obvious, and seriously complicate native code development compared with Linux. These are my notes on those differences and how to deal with them. Hopefully, it may be useful for other migrants to Mac as well.

Xcode

Apple includes something called Xcode. This is apparently a combination of a platform SDK, and an IDE with a user interface similar to iTunes. You have to have it, but you don’t have to use the IDE part. It must be installed from the App Store. Don’t fight it, just install it and move on.

Command line tools

You definitely want the Xcode command line tools. Run:

xcode-select --install

to install them. This will give you git as well.

Brew

There are actually two package managers for Mac OS X, MacPorts and Homebrew, and as a developer you’ll definitely need one of them. I use brew, because other people I know recommended it, and it’s been nice so far. You need it to install libraries and tools that don’t come with the Mac. Most notably gcc, cmake, and so on.

Clang and gcc

Apple ships the clang compiler with Mac OS X, so this is the considered the standard compiler for the platform. This means that some libraries (notably Qt) only support building with clang.

Some C/C++ projects assume (incorrectly) that everybody builds with gcc. For this reason (I guess), Apple did a really odd thing: they ship a gcc executable, which is actually clang in disguise:

> $ gcc
clang: error: no input files

This (I guess) works sometimes, since many flags work the same in both compilers. However, it is deeply confusing and causes problems as well. For example, gcc supports OpenMP, a powerful parallel computing tool, and crucial for the work I’m doing. Recent versions of clang support it as well, but Apple’s fork of clang that ships with Macs does not. So to use OpenMP, I have to have the real gcc. This will cause other problems down the road, we’ll get to them in a bit.

You’ll want to install gcc with brew:

brew install gcc gdb

Since clang is already masquerading as gcc, the Homebrew folks came up with a workaround - the gcc package installs executables called gcc-5 and g++-5 instead of gcc and g++. I added the following in my profile to encourage build systems to use these compilers instead of clang.

export HOMEBREW_CC=gcc-5
export HOMEBREW_CXX=g++-5
export CC=gcc-5
export CXX=g++-5

Note the Homebrew-specific ones. Homebrew generally installs binary, precompiled packages rather than compiling on your machine, but you can pass --build-from-source to it to make it recompile. If you do that, it will honor the HOMEBREW_CC and HOMEBREW_CXX environment variables and use those to do the build.

I also aliased cmake to ensure that cmake uses gcc-5 and g++-5 by default as well:

alias cmake=/usr/local/bin/cmake -DCMAKE_C_COMPILER=$CC -DCMAKE_CXX_COMPILER=$CXX

Compatibility

C++, unlike C, doesn’t specify a binary interface standard. This means that libraries that are compiled with different C++ compilers can have problems interoperating. So there’s that to consider when you use things that were compile with clang (such as anything you download using brew without recompiling) together with things you’ve been building with g++-5.

The most pressing example of this is related to the C++ standard library. There are, on Mac (and elsewhere too I suppose), at least two implementations: libstdc++, and libc++. By default, clang uses libc++ and gcc-5 uses libstdc++. In practice, this means that if you install a C++ based library with brew, you will be able to compile against it with g++-5, but when you get to the link stage, it will fail with lots of missing symbols. If this happens,

brew reinstall --build-from-source <package>

can often fix the problem. However, there are brew packages (e.g. pkg-config) that will fail to compile under g++-5, so there can be cases where this doesn’t work. One example: I was trying to build mxnet directly using the brew package for OpenCV support, and it failed with the aforementioned link errors. I tried to reinstall opencv with --build-from-source with brew, and it started recompiling all of opencv’s (many) dependencies, including pkg-config, which for some reason fails to compile. So in the end I had to pull opencv as well and build it manually, after which mxnet built fine too.

Next time

These were some important things to be aware of when starting to develop in C++ on Macs. In the next installment, we’ll talk about dynamic libraries, install names, and other such loveliness.