defer.h

#include <stdlib.h>

/*
 * Defer an expression.
 * This requires a block in an outer scope to have called defer_init,
 * and the deferred statements will run once defer_run is called.
 */
#define defer(expr) \
	do { \
		__label__ _defer_label; \
		_defer_label: \
		if (_defer_rundefer) { \
			expr; \
			/* Go to the previous defer, or the end of the `it` block */ \
			if (_defer_labels.count > 0) \
				goto *_defer_labels.labels[--_defer_labels.count]; \
			else \
				goto *_defer_label_done; \
		} else { \
			_defer_labels.count += 1; \
			/* Realloc labels array if necessary */ \
			if (_defer_labels.count >= _defer_labels.size) { \
				if (_defer_labels.size == 0) \
					_defer_labels.size = 16; \
				else \
					_defer_labels.size *= 2; \
				_defer_labels.labels = realloc( \
					_defer_labels.labels, \
					_defer_labels.size * sizeof(*_defer_labels.labels)); \
			} \
			/* Add pointer to _defer_label to labels array */ \
			_defer_labels.labels[_defer_labels.count - 1] = \
				&&_defer_label; \
		} \
	} while (0)

/*
 * Init defer for the scope defer_init is called from.
 * It has to be the very first line in a block.
 */
#define defer_init() \
	void __attribute__((unused)) *_defer_label_done; \
	int __attribute__((unused)) _defer_rundefer = 0; \
	struct { \
		void **labels; size_t size; size_t count; \
	} _defer_labels; \
	_defer_labels.size = _defer_labels.count = 0; \
	_defer_labels.labels = NULL; \

/*
 * Run all the defers registered for the corresponding defer_init.
 */
#define defer_run() \
	do { \
		__label__ _defer_done; \
		_defer_done: \
		_defer_label_done = &&_defer_done; \
		_defer_rundefer = 1; \
		if (_defer_labels.count > 0) { \
			_defer_labels.count -= 1; \
			goto *_defer_labels.labels[_defer_labels.count]; \
		} \
	} while (0)

/*
 * Run all the defers, then return x
 */
#define defer_return(x) \
	do { defer_run(); return x; } while (0)