Can’t get protocol while capture any interfaces with libpcap [duplicate]

libpcap will sometimes, depending upon the interface chosen, replace the layer 2 header (ethernet, in this case) with a Linux cooked header which does not have the same length as an ethernet header. You can check the datalink type of your pcap_t with the pcap_datalink function.

unsigned int layer_2_header_length;

switch ( pcap_datalink(handle) ) {
case DLT_EN10MB: // Ethernet header
    layer_2_header_length = 14;
    break;

case DLT_LINUX_SLL: // Linux cooked header
    layer_2_header_length = 16;
    break;

// other options
}