Google test: how to check a global C (C++) function was called

Question

I am writing test cases for a scripting language which is embedded in a C (or C++) application and one of the features is that the scripting language calls a method from the "host" program. The entire project is using the google test framework, and down here is one of the tests:

TEST(Functions, ExternalCalling)
{
    SCRIPT_START
    "                                      \
    extern void external_callee(int, int); \
    external_callee(1,2);                  \
    "
    SCRIPT_END
}

NAP_EXPORTS
void external_callee(nap_int_t a, nap_int_t b)
{
    fprintf(stderr, "\na=%"PRINT_d", b=%"PRINT_d"\n", a, b);
    if(a != 1 || b != 2) FAIL();
}

Do not mind the SCRIPT_START and SCRIPT_END macros, they just create/destroy scripting language objects (NAP_EXPORTS is defined as extern "C" so that the dynamic library loader can resolve the name).

As you can see the script defines an external method (from the host application) and then calls it. Right now I am sure that the method is called since I can see on the stderr/output the values of a and b but yeah... this has the feeling of manual testing :) How can I use the google test framework to make sure that the method actually was called without having to look on the screen? (I'd like to avoid hackish solutions, like use a global flag...)

Are you able to change the signature of `external_callee` to include a `FILE*` instead of hard-coding `stderr`? — John Zwinck, Apr 15 '14 at 12:08
@JohnZwinck not really since there must be a match between the scripting language types (`int`) and the C types (`nap_int_t`). — Ferenc Deak, Apr 15 '14 at 12:10
If it's plain C, it can get tricky with Google Mocks. Take a look at https://cmocka.org/ — Mawg says reinstate Monica, Sep 11 '15 at 08:39

score 3 · Answer 1 · edited May 23 '17 at 11:48

What you actually want to do is mock the function.

Look at mocking frameworks like google-mock.

The EXPECT_CALL macro lets you specify call occurences (with argument filtering).

See also This SO Question how to create C trampolins to hide C++ interfaces behind C code.

if you also want to have a real result you also might be interested in the invoke function that allows you to forward arguments to a real implementation.

the google mock cookbook has recipes for all use patterns above.

score 1 · Answer 2 · answered Apr 15 '14 at 12:13

Here are a few ideas, in order from least to most insidious:

Make your function return a string instead of printing it internally. Or return something, at least.
Add an (optional) FILE* parameter to external_callee() and write there instead of hard-coding stderr. Then your test harness can hook it up to a temporary file or something fancier.
Make a global variable which is the FILE* to write to. Default it to stderr but let the test harness change it. Basically like the previous idea but without modifying the signature.
Have your test harness reopen the stderr file descriptor as a pipe. Call the function and then try to read from that pipe.
Override printf() in your test program. Yes, you can probably do this--try it! Just define printf() yourself with the same signature and that one should get called. Then you can do anything you want.

Good ideas but you didn't include the correct/idiomatic/conventional one: mocking `external_callee`. — Lightness Races in Orbit, Apr 15 '14 at 12:47

Google test: how to check a global C (C++) function was called

2 Answers2