SO_REUSEADDR and UDP behavior in Windows

I'm aware th开发者_开发技巧at using SO_REUSEADDR with UDP in a *NIX environment, behaves like a multicast, where multiple clients bound to the same port can listen and receive broadcast datagrams simultaneously. Is this the behavior on Windows as well?

Multiple UDP sockets on Windows bound to the same port will all receive broadcast packets together.

Here's a demo program you can build for windows and Linux with GCC and test with Netcat as mentioned. In both systems, only one socket (either A or B) receives each datagram when a unicast address is used as the target. If a broadcast address is used then both sockets will receive the message.

/* Tested on linux and windows 7.
 * On windows use mingw-gcc:
 *    gcc -Wall -g -o udplisten udplisten.c -lws2_32
 * Test with:
 *    echo hello | netcat -u machinename 9898 (unicast)
 *    echo hello | netcat -u 172.16.255.255 9898 (broadcast)
 */
#ifdef WIN32
#include <winsock2.h>
#include <ws2tcpip.h>
#else
#include <sys/select.h>
#include <sys/socket.h>
#include <netinet/in.h>
#define SOCKET int
#define INVALID_SOCKET -1
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>

#undef max
#define max(x,y) ((x) > (y) ? (x) : (y))

static void
die(const char *str)
{
    perror(str);
    exit(1);
}

static SOCKET
mksocket(struct sockaddr_in *addr)
{
    SOCKET sock = INVALID_SOCKET;
    int opt = 1;
    if ((sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0)
        die("socket");
    if (setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, (const char *)&opt, sizeof(opt)) < 0)
        die("setsockopt");
    if (bind(sock, (struct sockaddr *)addr, sizeof(struct sockaddr_in)) < 0)
        die("bind");
    return sock;
}

static void
process(SOCKET sock, const char *label)
{
    char buffer[8192];
    struct sockaddr_in caddr;
    socklen_t caddr_size = sizeof(caddr);
    memset(&caddr, 0, caddr_size);
    int count = recvfrom(sock, buffer, sizeof(buffer), 0,
                         (struct sockaddr *)&caddr, &caddr_size);
    if (count < 0) die(label);
    printf("%s %d '", label, count);
    fwrite(buffer, 1, count, stdout);
    printf("'\n");
}

int
main(int argc, char *argv[])
{
    struct sockaddr_in addr;
    SOCKET socka = INVALID_SOCKET, sockb = INVALID_SOCKET;
    fd_set read_set;
#ifdef WIN32
    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2,2), &wsaData))
        return -1;
#endif
    addr.sin_family = AF_INET;
    addr.sin_port = htons(9898);
    addr.sin_addr.s_addr = INADDR_ANY;

    socka = mksocket(&addr);
    sockb = mksocket(&addr);

    for (;;) {
        FD_ZERO(&read_set);
        FD_SET(socka, &read_set);
        FD_SET(sockb, &read_set);
        if (select(max(socka,sockb)+1, &read_set, NULL, NULL, NULL) < 0)
            die("select");
        if (FD_ISSET(socka, &read_set))
            process(socka, "A");
        if (FD_ISSET(sockb, &read_set))
            process(sockb, "B");
    }
    return 0;
}