Using function pointers in FFF

gistfile1.md

Using FFF to stub functions that have function pointer parameter can cause problems when trying to stub them. Presented here is an example how to deal with this situation.

If you need to stub a function that has a function pointer parameter, e.g. something like:

/* timer.h */
typedef int timer_handle;
extern int timer_start(timer_handle handle, long delay, void (*cb_function) (int arg), int arg);

Then creating a fake like below will horribly fail when trying to compile because the FFF macro will internally expand into an illegal variable int (*)(int) arg2_val.

/* The fake, attempt one */
FAKE_VALUE_FUNC(int,
                timer_start,
                timer_handle,
                long,
                void (*) (int argument),
                int);

The solution to this problem is to create a bridging type that needs only to be visible in the unit tester. The fake will use that intermediate type. This way the compiler will not complain because the types match.

/* Additional type needed to be able to use callback in FFF */
typedef void (*timer_cb) (int argument);

/* The fake, attempt two */
FAKE_VALUE_FUNC(int,
                timer_start,
                timer_handle,
                long,
                timer_cb,
                int);

Here are some ideas how to create a test case with callbacks.

/* Unit test */
TEST_F(FFFTestSuite, test_fake_with_function_pointer)
{
    int cb_timeout_called = 0;
    int result = 0;
    
    void cb_timeout(int argument)
    {
      cb_timeout_called++;     
    }
    
    int timer_start_custom_fake(timer_handle handle,
                          long delay,
                          void (*cb_function) (int arg),
                          int arg)
    {
      if (cb_function) cb_function(arg);
      return timer_start_fake.return_val;
    }
    
    /* given the custom fake for timer_start */
    timer_start_fake.return_val = 33;
    timer_start_fake.custom_fake = timer_start_custom_fake;
    
    /* when timer_start is called 
     * (actually you would call your own function-under-test
     *  that would then call the fake function)
     */
    result = timer_start(10, 100, cb_timeout, 55);
    
    /* then the timer_start fake must have been called correctly */
    ASSERT_EQ(result, 33);
    ASSERT_EQ(timer_start_fake.call_count, 1);
    ASSERT_EQ(timer_start_fake.arg0_val,   10);
    ASSERT_EQ(timer_start_fake.arg1_val,   100);
    ASSERT_EQ(timer_start_fake.arg2_val,   cb_timeout); /* callback provided by unit tester */
    ASSERT_EQ(timer_start_fake.arg3_val,   55);
    
    /* and ofcourse our custom fake correctly calls the registered callback */
    ASSERT_EQ(cb_timeout_called, 1);
}