Atomic transaction lock in C w/ compile-time magic

transaction.c

#include <stdatomic.h>
#include <stdbool.h>
#include <stdlib.h>

#define TRANSACTION(name, ptr, lock, expect, desire) \
	struct { \
		union { \
			atomic_ulong *state; \
			struct { \
				char lock_bit[(lock)];   \
				char expected[(expect)]; \
				char desired[(desire)];  \
			} *T; \
		}; \
		unsigned long actual; \
	} name = { \
		.state = (ptr), .actual = sizeof(name.T->expected) \
	}; do { if (!atomic_compare_exchange_strong_explicit(      \
		name.state, &name.actual, sizeof(name.T->desired)  \
		| sizeof(name.T->lock_bit), memory_order_release,  \
		memory_order_acquire)) { name.state = NULL; }} while (0)

#define transaction_commit(name) \
	do { \
		name.actual = atomic_exchange_explicit(name.state, \
		  sizeof(name.T->desired), memory_order_release);  \
		name.state = NULL; \
	} while (0)

#define transaction_revert(name) \
	do { \
		name.actual = atomic_exchange_explicit(name.state, \
		  sizeof(name.T->expected), memory_order_release); \
		name.state = NULL; \
	} while (0)

#define transaction_active(name) (!!name.state)
#define transaction_commit_ok(name) \
	(name.actual == (sizeof(name.T->desired) | sizeof(name.T->lock_bit)))

enum {
	STATE_WAIT = 2,
	STATE_WORK = 3,
	STATE_LOCK = 1 << 30
};

bool begin_work(atomic_ulong *state_flags, void (*prepare_work)(void))
{
	TRANSACTION(xact, state_flags, STATE_LOCK, STATE_WAIT, STATE_WORK);

	if (!transaction_active(xact))
		return false;

	prepare_work();
	transaction_commit(xact);

	return true;
}