解决 Mac 无法编译带 C 扩展库的问题
当初为了无痛开发换到 Mac，之后大大小小也遇到过不少奇怪的问题，不过都没有这次的奇怪。考虑到之后也可能遇到，姑且记录一下。
问题的表现是无法安装任何带 C 扩展的 Python 库。在编译的过程中会提示 <stdio.h> not found。系统是 10.11 El Capitan。
于是就去网上搜，得知 Mac 下 stdio.h 应该位于 /usr/include。搞笑的是，include 文件夹消失了！去网上搜，发现很多人都遇到这个问题，估计是 Xcode 升级带来的 bug，然后基本所有人都说要通过 xcode-select --install 命令装 commandline tools。问题是我早就装了。
终于找到一个地方提示说可以手动把 Xcode 里面的文件夹链接过去。尝试执行
sudo ln -s /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include /usr/include
提示
ln: /usr/include: Operation not permitted
搜索发现原因是 Mac 默认开启了 Configuring System Integrity Protection，简单来说就是一些系统相关的文件夹被设为只读。解决方法参考这里：
- 重启电脑
- 在启动过程中按住
cmd + r，进入 Recover Mode - 选择 utilities > terminal，输入命令
csrutil disable
reboot
- 再次执行
ln，成功。
再次尝试安装，这里以 greenlet 为例
$ pip install greenlet
...
Building wheels for collected packages: greenlet
Running setup.py bdist_wheel for greenlet
Complete output from command /usr/local/opt/python3/bin/python3.5 -c "import setuptools;__file__='/private/var/folders/sy/msgzx60s2_332s1wdb92fqw80000gn/T/pip-build-amdm1ven/greenlet/setup.py';exec(compile(open(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" bdist_wheel -d /var/folders/sy/msgzx60s2_332s1wdb92fqw80000gn/T/tmp4a1nacfbpip-wheel-:
running bdist_wheel
running build
running build_ext
building 'greenlet' extension
creating build
creating build/temp.macosx-10.10-x86_64-3.5
clang -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/usr/local/include -I/usr/local/opt/openssl/include -I/usr/local/opt/sqlite/include -I/usr/local/Cellar/python3/3.5.1/Frameworks/Python.framework/Versions/3.5/include/python3.5m -c greenlet.c -o build/temp.macosx-10.10-x86_64-3.5/greenlet.o
Assertion failed: (!contains(New, this) && "this->replaceAllUsesWith(expr(this)) is NOT valid!"), function replaceAllUsesWith, file /Users/laike9m/Dev/C_CPP/Lib/cling/src/lib/IR/Value.cpp, line 343.
...
********************
error: command 'clang' failed with exit status 254
----------------------------------------
Command "/usr/local/opt/python3/bin/python3.5 -c "import setuptools, tokenize;__file__='/private/var/folders/sy/msgzx60s2_332s1wdb92fqw80000gn/T/pip-build-amdm1ven/greenlet/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /var/folders/sy/msgzx60s2_332s1wdb92fqw80000gn/T/pip-l6qwwxng-record/install-record.txt --single-version-externally-managed --compile" failed with error code 1 in /private/var/folders/sy/msgzx60s2_332s1wdb92fqw80000gn/T/pip-build-amdm1ven/greenlet
我不相信 greenlet 的代码有问题，遂怀疑是 clang 的问题。于是想用 gcc 来编译。尝试 export CC=gcc 未果，发现仍然在使用 clang，报同样的错。还是在 SO 上找到了解释
It has been this way for a long time already. The "GCC" that came with 10.8 was really GCC front-end with LLVM back-end.
解决方法是用 homebrew 安装 gcc，这样不会和系统的 gcc 冲突。
$ brew install gcc49
然后
export CC=/usr/local/Cellar/gcc49/4.9.3/bin/gcc-4.9
这下终于安装成功。
2017-2-9
最近发现有时候必须得在 virtualenv 里才能安装成功……
