由于缺少libffi，无法在树莓派上安装加密库。

这个问题已经有了答案：

pip install failing with: OSError: [Errno 13] Permission denied on directory

我一直在尝试在我的 Raspberry Pi Zero 上安装\"flask-ask\"包。但是在尝试安装依赖项\"cryptography\"时出了问题:

Downloading/unpacking cryptography
  Downloading cryptography-2.2.2.tar.gz (443kB): 443kB downloaded
  Running setup.py (path:/tmp/pip-build-wg353po4/cryptography/setup.py) egg_info for package cryptography
    Traceback (most recent call last):
      File "", line 17, in 
      File "/tmp/pip-build-wg353po4/cryptography/setup.py", line 28, in 
        "cryptography requires setuptools 18.5 or newer, please upgrade to a "
    RuntimeError: cryptography requires setuptools 18.5 or newer, please upgrade to a newer version of setuptools
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
  File "", line 17, in 
  File "/tmp/pip-build-wg353po4/cryptography/setup.py", line 28, in 
    "cryptography requires setuptools 18.5 or newer, please upgrade to a "
RuntimeError: cryptography requires setuptools 18.5 or newer, please upgrade to a newer version of setuptools
----------------------------------------
Cleaning up...
Command python setup.py egg_info failed with error code 1 in /tmp/pip-build-wg353po4/cryptography
Storing debug log for failure in /home/pi/.pip/pip.log

(这些来自尝试单独安装密码学库的日志，但安装flask-ask时遇到同样的错误)

我尝试使用pip3 install -U setuptools来升级它，但那并不起作用:

pi@raspberrypi:~ $ pip3 install -U setuptools
Downloading/unpacking setuptools from https://files.pythonhosted.org/packages/7f/e1/820d941153923aac1d49d7fc37e17b6e73bfbd2904959fffbad77900cf92/setuptools-39.2.0-py2.py3-none-any.whl#sha256=8fca9275c89964f13da985c3656cb00ba029d7f3916b37990927ffdf264e7926
  Downloading setuptools-39.2.0-py2.py3-none-any.whl (567kB): 567kB downloaded
Installing collected packages: setuptools
  Found existing installation: setuptools 5.5.1
    Not uninstalling setuptools at /usr/lib/python3/dist-packages, owned by OS
  Can't roll back setuptools; was not uninstalled
Cleaning up...
Exception:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/pip/basecommand.py", line 122, in main
    status = self.run(options, args)
  File "/usr/lib/python3/dist-packages/pip/commands/install.py", line 295, in run
    requirement_set.install(install_options, global_options, root=options.root_path)
  File "/usr/lib/python3/dist-packages/pip/req.py", line 1436, in install
    requirement.install(install_options, global_options, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/pip/req.py", line 672, in install
    self.move_wheel_files(self.source_dir, root=root)
  File "/usr/lib/python3/dist-packages/pip/req.py", line 902, in move_wheel_files
    pycompile=self.pycompile,
  File "/usr/lib/python3/dist-packages/pip/wheel.py", line 214, in move_wheel_files
    clobber(source, lib_dir, True)
  File "/usr/lib/python3/dist-packages/pip/wheel.py", line 208, in clobber
    shutil.copy2(srcfile, destfile)
  File "/usr/lib/python3.4/shutil.py", line 244, in copy2
    copyfile(src, dst, follow_symlinks=follow_symlinks)
  File "/usr/lib/python3.4/shutil.py", line 108, in copyfile
    with open(dst, 'wb') as fdst:
PermissionError: [Errno 13] Permission denied: '/usr/local/lib/python3.4/dist-packages/easy_install.py'
Storing debug log for failure in /home/pi/.pip/pip.log

那么该怎么办？我已经尝试了很多不同的搜索，我看到其他人在树莓派上有密码学问题，但不是这个特定的问题。

编辑: 好的，现在我已经使用pip install --user -U setuptools进行操作，那很好。现在我遇到了完全不同的错误:

    running install_egg_info
    running egg_info
    creating lib/PyYAML.egg-info
    writing lib/PyYAML.egg-info/PKG-INFO
    writing top-level names to lib/PyYAML.egg-info/top_level.txt
    writing dependency_links to lib/PyYAML.egg-info/dependency_links.txt
    writing manifest file 'lib/PyYAML.egg-info/SOURCES.txt'
    reading manifest file 'lib/PyYAML.egg-info/SOURCES.txt'
    writing manifest file 'lib/PyYAML.egg-info/SOURCES.txt'
    Copying lib/PyYAML.egg-info to /home/pi/.local/lib/python2.7/site-packages/PyYAML-3.12$
    running install_scripts
    writing list of installed files to '/tmp/pip-XYXo6X-record/install-record.txt'
  Could not find .egg-info directory in install record for PyYAML==3.12 (from flask-ask)
  Running setup.py install for cryptography
    Running command /usr/bin/python -c "import setuptools, tokenize;__file__='/tmp/pip-bui$
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    c/_cffi_backend.c:2:20: fatal error: Python.h: No such file or directory
     #include 
                        ^
    compilation terminated.
    Traceback (most recent call last):
      File "", line 1, in 
      File "/tmp/pip-build-2gl72v/cryptography/setup.py", line 319, in 
        **keywords_with_side_effects(sys.argv)
      File "/home/pi/.local/lib/python2.7/site-packages/setuptools/__init__.py", line 128,$
        _install_setup_requires(attrs)
      File "/home/pi/.local/lib/python2.7/site-packages/setuptools/__init__.py", line 123,$
        dist.fetch_build_eggs(dist.setup_requires)
      File "/home/pi/.local/lib/python2.7/site-packages/setuptools/dist.py", line 514, in $
        replace_conflicting=True,
      File "/home/pi/.local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 7$
        replace_conflicting=replace_conflicting
      File "/home/pi/.local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 1$
        return self.obtain(req, installer)
      File "/home/pi/.local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 1$
        return installer(requirement)
      File "/home/pi/.local/lib/python2.7/site-packages/setuptools/dist.py", line 581, in $
        return cmd.easy_install(req)
      File "/home/pi/.local/lib/python2.7/site-packages/setuptools/command/easy_install.py$
        return self.install_item(spec, dist.location, tmpdir, deps)
      File "/home/pi/.local/lib/python2.7/site-packages/setuptools/command/easy_install.py$
        dists = self.install_eggs(spec, download, tmpdir)
      File "/home/pi/.local/lib/python2.7/site-packages/setuptools/command/easy_install.py$
        return self.build_and_install(setup_script, setup_base)
      File "/home/pi/.local/lib/python2.7/site-packages/setuptools/command/easy_install.py$
        self.run_setup(setup_script, setup_base, args)
      File "/home/pi/.local/lib/python2.7/site-packages/setuptools/command/easy_install.py$
        raise DistutilsError("Setup script exited with %s" % (v.args[0],))
    distutils.errors.DistutilsError: Setup script exited with error: command 'arm-linux-gn$
    Complete output from command /usr/bin/python -c "import setuptools, tokenize;__file__=$
    Package libffi was not found in the pkg-config search path.
Perhaps you should add the directory containing `libffi.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libffi' found
Package libffi was not found in the pkg-config search path.
Perhaps you should add the directory containing `libffi.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libffi' found
Package libffi was not found in the pkg-config search path.
Perhaps you should add the directory containing `libffi.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libffi' found
Package libffi was not found in the pkg-config search path.
Perhaps you should add the directory containing `libffi.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libffi' found
Package libffi was not found in the pkg-config search path.
Perhaps you should add the directory containing `libffi.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libffi' found
c/_cffi_backend.c:2:20: fatal error: Python.h: No such file or directory
 #include 
                    ^
compilation terminated.
Traceback (most recent call last):
  File "", line 1, in 
  File "/tmp/pip-build-2gl72v/cryptography/setup.py", line 319, in 
    **keywords_with_side_effects(sys.argv)
  File "/home/pi/.local/lib/python2.7/site-packages/setuptools/__init__.py", line 128, in $
    _install_setup_requires(attrs)
  File "/home/pi/.local/lib/python2.7/site-packages/setuptools/__init__.py", line 123, in $
    dist.fetch_build_eggs(dist.setup_requires)
  File "/home/pi/.local/lib/python2.7/site-packages/setuptools/dist.py", line 514, in fetc$
    replace_conflicting=True,
  File "/home/pi/.local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 770, $
    replace_conflicting=replace_conflicting
  File "/home/pi/.local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 1053,$
    return self.obtain(req, installer)
  File "/home/pi/.local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 1065,$
    return installer(requirement)
  File "/home/pi/.local/lib/python2.7/site-packages/setuptools/dist.py", line 581, in fetc$
    return cmd.easy_install(req)
  File "/home/pi/.local/lib/python2.7/site-packages/setuptools/command/easy_install.py", l$
    return self.install_item(spec, dist.location, tmpdir, deps)
  File "/home/pi/.local/lib/python2.7/site-packages/setuptools/command/easy_install.py", l$
    dists = self.install_eggs(spec, download, tmpdir)
  File "/home/pi/.local/lib/python2.7/site-packages/setuptools/command/easy_install.py", l$
    return self.build_and_install(setup_script, setup_base)
  File "/home/pi/.local/lib/python2.7/site-packages/setuptools/command/easy_install.py", l$
    self.run_setup(setup_script, setup_base, args)
  File "/home/pi/.local/lib/python2.7/site-packages/setuptools/command/easy_install.py", l$
    raise DistutilsError("Setup script exited with %s" % (v.args[0],))
distutils.errors.DistutilsError: Setup script exited with error: command 'arm-linux-gnueab$
----------------------------------------
Cleaning up...
Command /usr/bin/python -c "import setuptools, tokenize;__file__='/tmp/pip-build-2gl72v/cr$
Exception information:
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/pip/basecommand.py", line 122, in main
    status = self.run(options, args)
  File "/usr/lib/python2.7/dist-packages/pip/commands/install.py", line 295, in run
    requirement_set.install(install_options, global_options, root=options.root_path)
  File "/usr/lib/python2.7/dist-packages/pip/req.py", line 1436, in install
    requirement.install(install_options, global_options, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/pip/req.py", line 707, in install
    cwd=self.source_dir, filter_stdout=self._filter_install, show_stdout=False)
  File "/usr/lib/python2.7/dist-packages/pip/util.py", line 716, in call_subprocess
    % (command_desc, proc.returncode, cwd))
InstallationError: Command /usr/bin/python -c "import setuptools, tokenize;__file__='/tmp/$

这个时候我完全不知道为什么会发生这种情况。