title: Mock a C function when unit testing with unity
body: <p>I have a C project with 2 modules - A and B.
  B has a function that calls a function from A.</p>
  
  <pre><code>int B_func() {
    if (1 == A_func()) {return 1;}
    return 2;
   }
  </code></pre>
  
  <p>I use unity to test these modules.</p>
  
  <pre><code>TEST(B, test_b) {
    TEST_ASSERT_EQUAL(1, B_func())
  }
  </code></pre>
  
  <p>When I test module B, I want to mock A_func so it will use my implementation and change the return value. Is there a way to do this without changing the source code of module B?</p>