Weird result of getsockopt

[CodeSun via Digitalmars-d-learn]

Hello guys,
Today, I found a weird problem when I was learning to enable 
SO_KEEPALIVE for a specific socket. I use setsockopt to enable 
keepalive firstly, and then use getsockopt to show if it is 
enabled correctly.

My code snippet is listed below:

Dlang version:

import core.sys.posix.sys.socket;
import core.sys.posix.netinet.in_;
import std.c.stdio;

void main() {
	int sfd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
	int flag = 1;
	size_t size;
	setsockopt(sfd, SOL_SOCKET, SO_KEEPALIVE, &flag, 
cast(socklen_t)flag.sizeof);
	flag = 0;
	getsockopt(sfd, SOL_SOCKET, SO_KEEPALIVE, &flag, 
cast(socklen_t*)&size);
	printf("%d\n", flag);
}

C version:

#include <sys/socket.h>
#include <arpa/inet.h>
#include <stdio.h>

int main() {
	int sfd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
	int flag = 1;
	size_t size;
	setsockopt(sfd, SOL_SOCKET, SO_KEEPALIVE, &flag, 
(socklen_t)sizeof(flag));
	flag = 0;
	getsockopt(sfd, SOL_SOCKET, SO_KEEPALIVE, &flag, 
(socklen_t*)&size);
	printf("%d\n", flag);
	return 0;
}

Dlang version always prints 0, which means keepalive is not 
enabled, while C version can almost display 1  all the time as 
expected.

So is there anything wrong inside the code? If not, whose 
behavior is correct?