Mocking: objcopy override

Example: share/doc/aceunit/examples/hello_world_mocked_obj

How it works

1. objcopy rewrites the compiled hello.o into mocked_hello.o, renaming symbols directly in the object file:
   • main → original_main
   • puts → mock_puts
2. A mock_puts.c provides the replacement mock_puts(), recording calls into a buffer, plus assert_puts() to check what was recorded.
3. The test calls original_main(), then assert_puts() to verify the expected output was “written”.

objcopy --redefine-sym main=original_main --redefine-sym puts=mock_puts hello.o mocked_hello.o

void test_hello(void) {
    original_main();
    assert_puts("Hello, world!\n");
}

When to use this

Works on any GNU/Clang toolchain with objcopy (or llvm-objcopy on Darwin). No special linker flags needed at link time — the renaming happens once, up front, on the object file itself.

See also: Link-time wrap for an approach that defers the renaming to the linker instead.