我通过 安装了 Xcode 4.6.2 命令行工具Xcode > Preferences > Downloads > Components > Command Line Tools，它的状态是“已安装”，但是在重新启动我的 mac 后，在 Mac OS 10.8.3 上运行，我没有/Developer文件夹，当我尝试运行时sudo gem install posix-spawn出现此错误：
Building native extensions. This could take a while...
ERROR: Error installing posix-spawn:
ERROR: Failed to build gem native extension.
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb
creating Makefile
make
xcrun cc -I. -I/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/universal-darwin12.0 -I/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/universal-darwin12.0 -I. -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -fno-common -arch i386 -arch x86_64 -g -Os -pipe -fno-common -DENABLE_DTRACE -fno-common -pipe -fno-common -Wall -c posix-spawn.c
xcrun: Error: could not stat active Xcode path '/Developer'. (No such file or directory)
cc -arch i386 -arch x86_64 -pipe -bundle -undefined dynamic_lookup -o posix_spawn_ext.bundle posix-spawn.o -L. -L/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib -L. -arch i386 -arch x86_64 -lruby -lpthread -ldl -lobjc
clang: error: no such file or directory: 'posix-spawn.o'
make: *** [posix_spawn_ext.bundle] Error 1
Gem files will remain installed in /Library/Ruby/Gems/1.8/gems/posix-spawn-0.3.6 for inspection.
Results logged to /Library/Ruby/Gems/1.8/gems/posix-spawn-0.3.6/ext/gem_make.out
其他问题建议安装 Xcode 命令行工具，但我已经这样做了。/usr/bin/gcc-4.2存在。