#define _GNU_SOURCE

#include <err.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
	int to, from, res;
	int pip[2];
	int siz = 65536;

	if (argc != 3)
		err(1, "usage: %s from_file to_file", argv[0]);

	from = open(argv[1], O_RDONLY);
	if (from == -1)
		err(1, "opening %s", argv[1]);

	to = open(argv[2], O_WRONLY | O_CREAT | O_TRUNC, 0644);
	if (to == -1)
		err(1, "opening %s", argv[2]);

	res = pipe(pip);
	if (res == -1)
		err(1, "creating pipe");

	while (1) {
		int num;

		res = splice(from, NULL, pip[1], NULL, siz, 0);
		if (res == -1)
			err(1, "splicing from %s to pipe", argv[1]);

		num = res;
		if (num == 0)
			break;

		do {
			res = splice(pip[0], NULL, to, NULL, num, 0);
			if (res == -1)
				err(1, "splicing from pipe to %s", argv[2]);

			if (res == 0)
				break;

			num -= res;
		} while (num);
	}

	res = close(to);
	if (res == -1)
		err(1, "closing %s", argv[2]);

	res = close(from);
	if (res == -1)
		err(1, "closing %s", argv[1]);

	return 0;
}