socket_mcjoin4 - join a multicast group

       socket_mcjoin4 joins the IPv4 multicast group groupip on interface.

       If something goes wrong, socket_mcjoin4 returns -1, setting errno appropriately.

       socket_mcjoin4 - join a multicast group

socket_mcleave4(3), socket_mcjoin6(3), socket_getifidx(3)

                                                                                               socket_mcjoin4(3)

#include<libowfat/socket.h>

       int socket_mcjoin4(int s,const char groupip[4],const char interface[4]);