Mocking

Sometimes you need to unit test code that wasn’t written with testability in mind — for example, a plain “Hello, world!” program:

#include <stdio.h>

int main(void) {
    puts("Hello, world!");
    return 0;
}

Testing main() poses two problems:

1. You need to call main() from a test, while keeping the original main() callable — so there must be a way to override the entry point the linker/loader uses while still reaching the real function.
2. main() has an outgoing side effect (puts()). To verify it, you either replace puts() with a mock that records what was passed, or intercept stdout/stderr directly.

AceUnit’s examples show five different ways to solve this, each with different toolchain requirements. All five achieve the same test; they differ only in how they intercept main and puts.

Strategy	Renames main via	Intercepts puts via	Portable to
objcopy override	objcopy --redefine-sym	objcopy --redefine-sym	GNU/Clang toolchains
Link-time wrap	ld --wrap=main	ld --wrap=puts	GNU ld / LLVM lld
Compile-time macro	-Dmain=original_main	-Dputs=mocked_puts	Any compiler
Darwin interposer	objcopy --redefine-sym	DYLD_INSERT_LIBRARIES dynamic interposition	macOS/Darwin only
Stdout/stderr interception	objcopy --redefine-sym	pipe + read back STDOUT	POSIX systems

If you’re not sure which to pick: compile-time macro requires nothing beyond a standard compiler and works everywhere. Link-time wrap is the cleanest if you’re already committed to GNU/Clang toolchains and don’t want to touch object files with objcopy.