test.c

/*
 * Compile with:
 * $ gcc test.c -o /tmp/test -g -O2
 *
 * $ gcc --version
 * gcc (Debian 13.2.0-24) 13.2.0
 *
 * Output:
 *
 * $ /tmp/test
 * Before: input.sa_family: 10, output.ss_family: 0
 * After:  input.sa_family: 10, output.ss_family: 0
 * Error: address family was not copied? 10 vs. 0
 */

#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <netdb.h>

static int sap_get_ip4_dst(const struct sockaddr_in *input, struct sockaddr_in *output)
{
	in_addr_t pdst = ntohl(input->sin_addr.s_addr);

	/* this breaks it, even for AF_INET6 case? */
	if (((pdst) & 0xffffff00) != 0xe0000000)
		return -1;

	output->sin_family = input->sin_family;
	return 0;
}

static int sap_get_ip6_dst(const struct sockaddr_in6 *input, struct sockaddr_in6 *output)
{
	output->sin6_family = input->sin6_family;
	return 0;
}

static int copy_family(struct sockaddr *addr, struct sockaddr_storage *sap_dst)
{
	switch (addr->sa_family) {
	case AF_INET:
		return sap_get_ip4_dst((struct sockaddr_in *)addr,
				       (struct sockaddr_in *)sap_dst);
	case AF_INET6:
		return sap_get_ip6_dst((struct sockaddr_in6 *)addr,
	                               (struct sockaddr_in6 *)sap_dst);
	}

	return -1;
}

static void sap_create_socket(void)
{
	struct sockaddr_storage output = { 0 };
	struct sockaddr input = { 0 };
	int ret;

	input.sa_family = AF_INET6;

	printf("Before:\tinput.sa_family: %i, output.ss_family: %i\n", input.sa_family, output.ss_family);

	ret = copy_family(&input, &output);
	if (ret < 0)
		return;

	printf("After:\tinput.sa_family: %i, output.ss_family: %i\n", input.sa_family, output.ss_family);

	if (input.sa_family != output.ss_family)
		fprintf(stderr, "Error: address family was not copied? %i vs. %i\n", input.sa_family, output.ss_family);

	return;

}

int main(int argc, char *argv[])
{
	sap_create_socket();
	return 0;
}