BEFORE_AFTER_BLOCK macro

before-after.c

#include <stdio.h>

#define EMPTY /* empty */

#define CONCAT_HELPER(x,y) x ## y
#define CONCAT(x,y) CONCAT_HELPER(x,y)

#define LABEL_MAKER(id) CONCAT(id,__LINE__)

#define BEFORE_AFTER_BLOCK(decl, before_block, after_block)		\
	if (0)								\
	LABEL_MAKER(finished): EMPTY;					\
	else if (0)							\
	LABEL_MAKER(handle_break):					\
		break; /* if you get a compiler error here, you used BLOCK outside of any loop */ \
	else								\
		for (decl; ;)						\
			if (1) {					\
				before_block;				\
				goto LABEL_MAKER(body);			\
			} else						\
				while (1)				\
					if (1) {			\
						after_block;		\
						goto LABEL_MAKER(handle_break); \
					} else				\
						while (1)		\
							if (1) {	\
								after_block; \
								goto LABEL_MAKER(finished); \
							} else		\
							LABEL_MAKER(body):


#define LOG_LOOP(var_to_print) \
	BEFORE_AFTER_BLOCK(FILE *log = stderr,				\
		{ fprintf (log, "entering block (" # var_to_print " = %d)\n", var_to_print);}, \
		{ fprintf (log, "exiting block, (" # var_to_print " = %d)\n", var_to_print); })

int
collatz (int n) {
	int g = 0;

	while (1) LOG_LOOP(n)
		{
			if (n < 0) {
				n = -n;
				continue;
			}
			if (n == 1)
				break;
			if (n % 2)
				n = 3 * n + 1;
			else
				n = n / 2;
			g++;
		}

	/* while (1) LOG_LOOP(g)
		{
			g++;
			if (g > 100)
				break;
		}
	*/

	return g;
}

int
main (int argc, char ** argv)
{
	printf ("took %d steps\n", collatz (-10));
}

References

Metaprogramming custom control structures in C by Simon Tatham