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AUM: All Ur Mocks are belong to us

Introduction

AUM is a library that lets you easily write and execute unit tests in C with mocks.

Installation

You can install the prebuilt debian or rpm package. If you prefer, you can also build from source.

Debian package

sudo apt install ./aum_$VERSION.deb

RPM package

sudo yum install aum-$VERSION-$RELEASE.x86_64.rpm

Building from source

./configure
make
make test
sudo make install
ldconfig

Usage

First test suite

Create file test.c with this content:

#include <aum.h>

AUM_TEST(AUM_FAIL__should_fail)
{
  AUM_FAIL("Voluntary test failure\n");
}

AUM_TEST_SUITE(first_suite, &AUM_FAIL__should_fail);

AUM_MAIN_RUN(&first_suite);

Then compile, link and execute:

$ gcc -c test.c -o test.o
$ gcc test.o -o test.out -laum
$ ./test.out

This should run the test suite and report that 1 test fails. Indeed we purposedly used assert AUM_FAIL. The list of all asserts provided by AUM can be found in header aum/asserts.h.

First mock

This section explains how to declare and use a mock. Create file test_with_mock.c with this content:

#include <aum.h>
#include <aum_mock_create.h>
#include <stdlib.h>

AUM_MOCK_CREATE(void *, malloc, size_t);

AUM_TEST(aum_mock_will_return__should_set_return_code)
{
  aum_mock_will_return("malloc", 0);

  char *result = malloc(10 * sizeof(char));
  AUM_ASSERT_WAS_CALLED_WITH("malloc", AUM_PARAMETER_RAW(10));
}

AUM_TEST_SUITE(suite_with_mock, &aum_mock_will_return__should_set_return_code);

AUM_MAIN_RUN(&suite_with_mock);

Next, compile, link and execute the test suite as follows:

$ gcc -c test_with_mock.c -o test_with_mock.o
$ gcc test_with_mock.o -o test_with_mock.out -laum -Wl,-wrap,malloc
$ ./test_with_mock.out

Option -Wl,-wrap,malloc is required. It tells the linker to transfer all calls to malloc to function __wrap_malloc instead. This is the core technique that lets AUM intercept and redefine the behavior of mocked functions.

There are two steps when using mocks:

  • declaration,
  • exploitation

To declare a mock use macro AUM_MOCK_CREATE. It expects the return type, function name followed by each parameter type.:

AUM_MOCK_CREATE(void *, malloc, size_t);

The mock can then be used in the code of each test to interact with the mocked function in two ways:

  • to redefine its behaviour,
  • to check how it was called.

In the example, function malloc is made to return 0 when called:

  aum_mock_will_return("malloc", 0);

In the end of the test, we check that malloc was called with 10:

  AUM_ASSERT_WAS_CALLED_WITH("malloc", AUM_PARAMETER_RAW(10));

The mock declaration API is defined in header aum_mock_create.h. Header aum.h includes everything necessary to use mocks. It is split into:

  • aum/mock.h contains the functions that control the behaviour of mocked functions,
  • aum/asserts.h contains the list of assertions.

Xunit report for continuous integration

You may want to execute your test suite during continuous integration. To present the result, Jenkins will then need to input the test report in the xunit format.

In order to generate this report, call the compiled test program with option -o to choose the path to the report:

$ ./test.out -o aum_test_results.xml

Test suites organization

When dealing with larger test suites, we recommend to split your code into three types of files:

  • collections of mock declarations,
  • test suites,
  • the test runner.

A mock collection groups mock declarations (macro AUM_MOCK_CREATE) for a given header (for instance stdlib.h). It is written once and can be reused in different projects, every time any function provided by the header needs to be mocked.

A test suite contains several tests (macro AUM_TEST). It exports a function (declared with macro AUM_TEST_SUITE) to register them in the test runner.

The test runner executes all the test suites registered with macro AUM_MAIN_RUN. For the time being, it accepts only one option (-o) which turns on xunit report generation.

Known limitations

These limitations result from the technique used to intercept calls to mocked functions, the -wrap linker option:

  • the linker does not reroute calls between functions belonging to the same compilation unit. Hence, it is not possible to mock calls to a function that belongs to the same module as the caller.
  • it is recommended to compile unit tests without any optimisation (-O0 option). In fact, some optimisations remove unnecessary calls or replace a function call by another. For instance, a call to malloc followed by free can be removed by the compiler. Similarly, a call to malloc followed by memset to 0 can be automatically replaced by a call to calloc.
  • AUM does not handle variadic functions at all. So, functions like printf with a varying number of arguments can not be mocked. In such cases, there are two strategies. You can either wrap the variadic function or, on the contrary, extract its core implementation into another function with a va_list as its last argument. In the latter case, the variadic version can still be kept: it will simply converts its arguments into a va_list to then call this new function. For more details, see: http://c-faq.com/varargs/handoff.html.
  • calls to mocked functions with arguments that can't be stored in an unsigned long are not correctly registered. So, for these functions, macros AUM_ASSERT_WAS_CALLED_WITH and AUM_ASSERT_WAS_CALLED_WITH_AT will not work as expected. For instance, this is the case with vasprintf and longjmp, respectively because of their va_list and jmp_buf argument.
  • when declaring a mock with macro AUM_MOCK_CREATE be extra vigilant that the number and argument types correspond to the mocked function signature. In theory, AUM_MOCK_CREATE is written so that the compiler signals an error otherwise. In practice, there may still be cases where you end up with weird errors which pop only during tests run.

Meanings of AUM

The library name is a reference to an old video game with this phrase: All your base are belong to us. At the same time, AUM stands for Airbus Unit tests with Mocks for C.