clang error: unknown argument: '-mno-fused-madd' (python package installation failure)

Question

I get the following error when attempting to install psycopg2 via pip on Mavericks 10.9:

clang: error: unknown argument: \'-mno-fused-madd\' [-W         


        

Answer 1

At least Apple is aware of this as this issue is discussed in the Xcode 5.1 Release Notes (Compiler section). Expect an updated Python and Ruby in the next OS update (fingers crossed!). Also take note that the -Wno-error=unused-command-line-argument-hard-error-in-future flag is meant to be a temporary workaround and will go away in the future (hopefully after system Python is fixed!).

If you are a Python package maintainer like me and want to save your users the hassle of working around this themselves, here is a solution (besides providing binary wheels/eggs) that goes into your setup.py file:

from distutils.command.build_ext import build_ext
import subprocess
import sys

# Workaround for OS X 10.9.2 and Xcode 5.1+
# The latest clang treats unrecognized command-line options as errors and the
# Python CFLAGS variable contains unrecognized ones (e.g. -mno-fused-madd).
# See Xcode 5.1 Release Notes (Compiler section) and
# http://stackoverflow.com/questions/22313407 for more details. This workaround
# follows the approach suggested in http://stackoverflow.com/questions/724664.
class build_ext_subclass(build_ext):
    def build_extensions(self):
        if sys.platform == 'darwin':
            # Test the compiler that will actually be used to see if it likes flags
            proc = subprocess.Popen(self.compiler.compiler + ['-v'],
                                    stdout=subprocess.PIPE, stderr=subprocess.PIPE,
                                    universal_newlines=True)
            stdout, stderr = proc.communicate()
            clang_mesg = "clang: error: unknown argument: '-mno-fused-madd'"
            if proc.returncode and stderr.splitlines()[0].startswith(clang_mesg):
                for ext in self.extensions:
                    # Use temporary workaround to ignore invalid compiler option
                    # Hopefully -mno-fused-madd goes away before this workaround!
                    ext.extra_compile_args += ['-Wno-error=unused-command-line-argument-hard-error-in-future']
        build_ext.build_extensions(self)

setup(
    name="mypackage",
    ...
    cmdclass={'build_ext' : build_ext_subclass},
    ...
)