cffi can't parse memcpy from string.h

Question

I watched a very compelling lecture on unit testing C code with Python and I've been trying to write some test code using the strategy. When I attempted to run a very simple test, I receive the error:

cffi.api.CDefError: cannot parse "extern void *memcpy (void *__restrict __dest, const void *__restrict __src,":5:39: before: __dest

Is this a limitation of cffi that I can work around or am I just using it wrong?

For reference, the lecture is here: https://www.youtube.com/watch?v=zW_HyDTPjO0 and the slides with code snippets are here: https://ep2016.europython.eu/media/conference/slides/writing-unit-tests-for-c-code-in-python.html

My header file:

#include <string.h>

#ifndef _STRING_UTILS_H_

#define _STRING_UTILS_H_

extern int cmp(char* a, char* b);

#endif

My implementation:

#include <stringutils.h>

int cmp(char* a, char* b) {
    return (strcmp(a, b) == 0);
}

The python file with the test in it:

#!/usr/bin/python3

import unittest
import cffi
import importlib
import pycparser
import subprocess

def preprocess(source):
    return subprocess.run(['gcc', '-E', '-P', '-'],
            input=source, stdout=subprocess.PIPE,
            universal_newlines=True, check=True).stdout

def load(filename):
    print('Loading ' + filename)
    source = open(filename + '.c').read()
    includes = preprocess(open(filename + '.h').read())

    ffibuilder = cffi.FFI()
    ffibuilder.cdef(includes)
    print('21')
    ffibuilder.set_source(filename + '_', source)
    ffibuilder.compile()

    module = importlib.import_module(filename + '_')
    return module.lib

class StringUtilsTest(unittest.TestCase):
    def test_cmp(self):
        module = load('stringutils')
        self.assertTrue(module.cmp('howdy', 'howdy'))
        self.assertFalse(module.cmp('howdy', 'haudy'))

#run(StringUtilsTest)
if __name__ == '__main__':
    unittest.main()

Is this something to do with the *__restrict declaration it has?

I have never attempted to mix Python and C in this source-aware way before. Thanks in advance.