Detect the operating system using UDP stack corner cases

which_os.c

#include <sys/socket.h>

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

int main(void)
{
	int fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
	if ( fd < 0 )
		err(1, "socket");
	if ( shutdown(fd, SHUT_RD) < 0 )
	{
		if ( errno == ENOSYS )
			puts("minix");
		else if ( errno == ENOTCONN )
		{
			struct sockaddr_in sin;
			memset(&sin, 0, sizeof(sin));
			sin.sin_family = AF_INET;
			sin.sin_addr.s_addr = 0;
			sin.sin_port = 0;
			if ( connect(fd, (const struct sockaddr*) &sin, sizeof(sin)) < 0 )
			{
				if ( errno == EADDRNOTAVAIL )
					puts("freebsd");
				else
					err(1, "connect");
			}
			else
				puts("linux");
		}
		else
			err(1, "shutdown");
	}
	else
	{
		union
		{
			sa_family_t family;
			unsigned char bytes[3];
		} u;
		memset(&u, 0, sizeof(u));
		u.family = AF_UNSPEC;
		if ( connect(fd, (const struct sockaddr*) &u, sizeof(u)) < 0 )
		{
			if ( errno == EINVAL )
				puts("netbsd");
			else if ( errno == EAFNOSUPPORT )
				puts("openbsd");
			else
				err(1, "connect");
		}
		else
			puts("dragonflybsd");
	}
	return 0;
}

$ ./a.out                                                                                                                    
openbsd
$ uname -a
NetBSD chieftec 8.99.4 NetBSD 8.99.4 (GENERIC) #0: Sat Oct 14 16:12:26 CEST 2017  root@chieftec:/public/netbsd-root/sys/arch/amd64/compile/GENERIC amd64