gmock-module

Cross-platform GMock extension for mocking system API functions.

Platform	Technique	System APIs
Windows	IAT (Import Address Table) patching	Win32 API (GetCurrentProcessId, CreateFile, etc.)
Linux	PLT/GOT (Procedure Linkage Table / Global Offset Table) patching	POSIX API (open, read, write, stat, etc.)

Based on gmock-win32 by smalti. Extended with Linux support using ELF PLT/GOT patching via dl_iterate_phdr + mprotect.

Quick Start (Linux)

#include <gmock-module.h>
#include <fcntl.h>
#include <gtest/gtest.h>

// 1. Declare mock in global scope
MOCK_MODULE_FUNC(int, open, const char*, int);

// 2. Write test
TEST(OpenTest, MockReturnsCustomFd) {
    EXPECT_MODULE_FUNC_CALL(open, testing::_, testing::_)
        .WillOnce(testing::Return(42));
    
    int fd = open("file.txt", O_RDONLY);
    EXPECT_EQ(fd, 42);
    
    VERIFY_AND_CLEAR_MODULE_FUNC(open);
}

// 3. Initialize in main
int main(int argc, char* argv[]) {
    const gmock_linux::init_scope init{ };
    testing::InitGoogleTest(&argc, argv);
    return RUN_ALL_TESTS();
}

Building

mkdir build && cd build
cmake .. -DCMAKE_BUILD_TYPE=Debug
cmake --build .
ctest --verbose

API Reference

Macro	Purpose
MOCK_MODULE_FUNC(ret, name, args...)	Declare a mock for a function
EXPECT_MODULE_FUNC_CALL(name, matchers...)	Set expectation (must be called)
ON_MODULE_FUNC_CALL(name, matchers...)	Set default action
VERIFY_AND_CLEAR_MODULE_FUNC(name)	Verify and clear all expectations
RESTORE_MODULE_FUNC(name)	Restore original function
BYPASS_MOCKS(expr)	Execute expression with mocks disabled
REAL_MODULE_FUNC(name)	Get pointer to original function
INVOKE_REAL_MODULE_FUNC(name, args...)	Call original function directly

Known Limitations

• Only dynamically-linked library functions can be mocked (not raw syscall instructions)
• Not thread-safe: expectations must be set in the same thread as calls
• Full RELRO (-z now) may require mprotect override — build uses -z lazy by default
• Some functions like stat() may be macros/inlines in older glibc — use lstat() or __xstat() instead

License

MIT