glibc mock

Welcome to glibc mock, a solution of mocking glibc function (open, read, write … etc.) with Google Test and Google Mock!

Mock glibc function by changing GOT value to custom static function address.

As we know, glibc function address is store in GOT (Global Offset Table).
Change the GOT value to a custom static function address, then the glibc call will call the custom static function actually.
In the custom static function, we make it to call a Google Mock method, then we have mocked the glibc function.

Usage

Copy got_hook.h and got_hook.cc to your C++ test project.
Add #include "got_hook.h" to your test source code.

Sample 1

struct MockMalloc {
    MOCK_METHOD1(Malloc, void *(size_t));
};

static MockMalloc *g_mock{nullptr};

static void *Malloc(size_t size) {
    return g_mock->Malloc(size);
}

static std::mutex g_test_mutex;

TEST(MallocTest, ReturnNull) {
    std::lock_guard<std::mutex> lock(g_test_mutex); // not thread safe
    std::unique_ptr<MockMalloc> mock(g_mock = new MockMalloc());
    testing::GotHook got_hook;
    ASSERT_NO_FATAL_FAILURE(got_hook.MockFunction("malloc", (void*)&Malloc));
    // ... do your test here, for example:
    EXPECT_CALL(*g_mock, Malloc(testing::_)).WillOnce(testing::Return(nullptr));
    EXPECT_EQ(nullptr, malloc(1));
}

Sample 2

constexpr key_t shm_key = 0x123;

struct Turtle {
    void UseSharedMemory() {
        auto shm_id = shmget(shm_key, 0, 0);
        auto buffer = shmat(shm_id, nullptr, 0);
        EXPECT_STREQ("a fake shm buffer", (char*)buffer);
        // ...
        shmdt(buffer);
    }
};

struct MockTurtle : Turtle {
    MOCK_METHOD3(shmget, int(key_t, size_t, int));
    MOCK_METHOD3(shmat, void*(int, const void*, int));
    MOCK_METHOD1(shmdt, int(const void*));
};

static MockTurtle *g_turtle{nullptr};

static int my_shmget(key_t key, size_t size, int shmflg) {
    return g_turtle->shmget(key, size, shmflg);
}

static void *my_shmat(int shmid, const void *shmaddr, int shmflg) {
    return g_turtle->shmat(shmid, shmaddr, shmflg);
}

static int my_shmdt(const void *shmaddr) {
    return g_turtle->shmdt(shmaddr);
}

static std::mutex g_test_mutex;

TEST(TurtleTest, FakeSharedMemory) {
    std::lock_guard<std::mutex> lock(g_test_mutex); // not thread safe
    std::unique_ptr<MockTurtle> turtle(g_turtle = new MockTurtle());
    testing::GotHook got_hook;
    ASSERT_NO_FATAL_FAILURE({
        got_hook.MockFunction("shmget", (void*)&my_shmget);
        got_hook.MockFunction("shmat", (void*)&my_shmat);
        got_hook.MockFunction("shmdt", (void*)&my_shmdt);
    });
    constexpr int shm_id = 100;
    char fake_shm_buffer[2000]{"a fake shm buffer"};
    // ...
    EXPECT_CALL(*g_turtle, shmget(shm_key, _, _)).WillOnce(Return(shm_id));
    EXPECT_CALL(*g_turtle, shmat(shm_id, _, _)).WillOnce(Return(fake_shm_buffer));
    EXPECT_CALL(*g_turtle, shmdt(fake_shm_buffer)).WillOnce(Return(0));
    // ...
    g_turtle->UseSharedMemory();
    // ...
}

Sample 3

constexpr const char *prod_file = "/usr/local/xxx";
constexpr const char *test_file = "/tmp/xxx";

struct Panda {
    void OpenFile() {
        auto fd = open(prod_file, O_RDONLY);
        // ...
        close(fd);
    }
};

struct MockPanda : Panda {
    MOCK_METHOD2(Open, int(const char*, int));
};

static std::mutex g_test_mutex;

static MockPanda *g_panda{nullptr};

static int (*libc_open)(const char*, int, ...);

struct PandaTest : testing::Test {
    testing::GotHook *got_hook{nullptr};

    virtual void SetUp() {
        g_test_mutex.lock(); // not thread safe
        g_panda = new MockPanda();
        ASSERT_NO_FATAL_FAILURE({
            got_hook = new testing::GotHook();
            got_hook->MockFunction("open", (void*)&Open, (void**)&libc_open);
        });
        ON_CALL(*g_panda, Open(_, _)).WillByDefault(Invoke(libc_open));
        // ...
    }

    virtual void TearDown() {
        delete got_hook;
        delete g_panda;
        g_test_mutex.unlock();
    }

    static int Open(const char *pathname, int flags, ...) {
        if (strcmp(prod_file, pathname) == 0) {
            return g_panda->Open(test_file, flags);
        } else {
            if (flags & O_CREAT) {
                va_list arg;
                va_start(arg, flags);
                mode_t mode = va_arg(arg, mode_t);
                va_end(arg);
                return libc_open(pathname, flags, mode);
            } else {
                return libc_open(pathname, flags);
            }
        }
    }
};

TEST_F(PandaTest, OpenTestFile) {
    // ...
    EXPECT_CALL(*g_panda, Open(test_file, O_RDONLY));
    // ...
    g_panda->OpenFile();
    // ...
}

Requirements

For now support Linux 64-bit only. If you need a 32-bit version, contact me lonnywang@qq.com.