OS X getsockopt: no SO_PROTOCOL

OS X doesn't offer the SO_PROTOCOL socket option which allows the caller to "...retrieve the socket type as an integer." (http://linux.die.net/man/7/socket)

In other words the following program builds and works under linux but won't compile under OS X:

#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main(int argc, char **argv)
{
    int c, s, type, len;
    len = sizeof(type);

    s = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (s < 0)
    {  
        fprintf(stderr, "socket kaboom: %s\n", strerror(errno));
        return 1;
    }
    if (getsockopt(s, SOL_SOCKET, SO_PROTOCOL, &type, &len) < 0)
    {  
        fprintf(stderr, "getsosockopt kaboom: %s\n", strerror(errno));
        return 1;
    }
    printf("socket type: %d\n", type);
    return 0;
}

How to accomplish this under OS X?

The standard SO_TYPE socket option, which returns values like SOCK_STREAM (corresponding to TCP) and SOCK_DGRAM (corresponding to UDP), should suffice. With SCTP, SOCK_STREAM might correspond to TCP or SCTP and SO_PROTOCOL is useful to distinguish them, but MacOS X does not support SCTP.

Unix domain sockets do not use protocol numbers; therefore, SO_TYPE is the right choice there as well.