SetPointerPlugin

SetPointerPlugin solves a common embedded-testing problem: swapping out a function pointer (or any global pointer) for a test stub, then automatically restoring it after the test.

Include “mu/tiny/test.hpp” — SetPointerPlugin is included transitively, and the MUTINY_PTR_SET macro is available without any additional install step.

The Macro

MUTINY_PTR_SET(pointer, replacement)

• Saves the current value of pointer.

• Replaces pointer with replacement.

• After the test body (in post_test_action()), restores pointer to the saved value.

Both pointer and replacement must be the same pointer-to-T type. The macro is type-safe: it uses a template that takes T*& a, T* b.

Example: Stubbing a Function Pointer

/* module.h */
extern int (*platform_read)(int fd, void* buf, size_t n);

/* module.test.cpp */
#include "mu/tiny/test.hpp"
#include "module.h"

static int stub_read(int, void* buf, size_t n)
{
    memset(buf, 0xAB, n);
    return (int)n;
}

TEST_GROUP(ModuleRead) {};

TEST(ModuleRead, UsesStub)
{
    MUTINY_PTR_SET(platform_read, stub_read);
    // platform_read now points to stub_read
    // after this test, platform_read is restored automatically
    CHECK_EQUAL(4, read_four_bytes());
}

Example: Replacing a Global Object Pointer

static Logger* fake_logger = new FakeLogger;

TEST(MyGroup, LogsOnError)
{
    MUTINY_PTR_SET(g_logger, fake_logger);
    trigger_error();
    CHECK(fake_logger->was_called());
}

Use Cases

• Function pointer stubs: swap platform I/O, timers, or OS calls.

• Global state: temporarily replace a singleton or global object.

• Contract checking: swap in an assertion-catching fake to test error paths.

Limit

SetPointerPlugin::max_set = 32 — at most 32 pointers can be saved simultaneously per test. Each MUTINY_PTR_SET call consumes one slot; the slots are released at the end of the test.

Exceeding 32 calls in a single test body will crash or corrupt memory. If you need more than 32, consider restructuring your test.

How It Works

SetPointerPlugin is a Plugin with a post_test_action() that iterates the saved location/value pairs and writes each original value back. The static storage array means no heap allocation is needed, which is safe even in environments without malloc.

Examples

File	Demonstrates
CheatSheet.test.cpp	MUTINY_PTR_SET swapping a function pointer in setup()
hello.test.c + hello.test.cpp	Function pointer stub pattern in C (manual save/restore instead of the macro, showing the underlying technique)