Mimick

A KISS, cross-platform C Mocking library

Philosophy

Warning: this library is still experimental. APIs will change before the beta phase and things will break. Use at your own risk.

Mimick aims to be a simple of use and powerful mocking and stubbing library for C.

It doesn't rely on external code generation or compiler plugin to work -- simply link the library to your tests, and you're good to go!

Mimick works by poisoning the GOT of the target dynamic module at runtime, making any call to the target functions routed to one of its own stubs. The mock API is only sugar on top of the stub API, and provides a default stub suitable for testing interactions and specifying behaviour.

Samples

Here is a simple usage of Mimick to mock the malloc function:

#include <stdlib.h>
#include <assert.h>
#include <mimick.h>

/* Define the blueprint of a mock identified by `malloc_proto`
   that returns a `void *` and takes a `size_t` parameter. */
mmk_mock_define (malloc_mock, void *, size_t);

int main(void) {
    /* Mock the malloc function in the current module using 
       the `malloc_mock` blueprint. */
    mmk_mock("malloc@self", malloc_mock);

    /* Tell the mock to return NULL and set errno to ENOMEM
       whatever the given parameter is. */
    void *result = NULL;
    mmk_when(malloc(mmk_any(size_t)),
            .then_return = &result,
            .then_errno = ENOMEM);

    assert(malloc(42) == result && errno == ENOMEM);

    mmk_reset(malloc);
}

Or, alternatively:

malloc_mock mock = mmk_mock("malloc@self", malloc_mock);

void *result = NULL;
mmk_when(mock(mmk_any(size_t)),
        .then_return = &result,
        .then_errno = ENOMEM);

assert(malloc(42) == result && errno == ENOMEM);

mmk_reset(mock);

Other samples may be found in the samples directory.

Compatibility matrix

Supported Compilers: GCC 4.6+, Clang 3.5+, MSVC 14+

Legend:

✓: Supported.
✕: Unsupported.
?: Not Tested, but is expected to work.
∄: Does not exist / not applicable.
~: Works on some conditions.

Arch	Linux	OS X/iOS	FreeBSD	Windows
x86	✓	✓	✓	✓
x86_64	✓	✓	✓	✓
ARM	✓	?	✓	✕
ARM64	✓	?	?	∄
PPC	✕	✕	✕	∄

F.A.Q.

Q. Can I mock/stub static functions?
A. No. Static functions are, by definition, private implementations details, and because they are not exported nor visible to the outside world they cannot (and should not) be mocked/stubbed.

Q. Can I mock/stub functions inside a static library?
A. Maybe. Functions inside a static library are moved inside the executable and are not called using the PLT by default. You need to build your library to use Position-Independent Code, otherwise the functions cannot be stubbed nor mocked.

Q. I am mocking a standard library function, but mmk_when and mmk_verify are not working, why is this happening?
A. It's very possible that your compiler is optimizing away your function call inside mmk_when or mmk_verify since it has determined that there are no visible side effects in not calling the function. Compile your tests without optimizations (with a compiler flag or the mmk_no_optimize function attribute if it is available to your compiler), or use the function pointer returned by mmk_mock rather than the function itself.

Q. Does this run on embedded systems / bare metal?
A. Yes, but on very specific conditions. Your code must be position-independent, call functions using a global offset table or similar mechanism, and use an executable format that Mimick can handle (ELF, PE, Mach-O).
You also need to provide definitions for some functions like malloc, realloc, and free.