从Linux C向许多Android手机发送UDP消息

Question

Rewritten to try and be clear on what I need.

My goal is to duplicate the function of a device made by Digital Yacht in an embedded Intel Edison processor running C and Linux. The device sends via UDP to phone apps such as iRegatta and others. To set up the app, only the port number is entered. No ip address is entered in UDP mode on the app. I thought this was trivial but the experts here so far have said it is impossible so it must not be trivial. Perhaps that is why with all my hours of reading I cannot find an example. I am being voted down because, I am told, that what I am trying to do it impossible but it is not as it is done. I don't know how it is done, which is why I came to experts here.

I want to send nmea messages that might look like this:

$GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47

and I want any number of random Android phones to be able to receive them with the appropriate app. There are many apps that can be set up to receive UDP data where you just specify a port number. There is no ip address involved in the setup of the apps. Also, I do not wish to receive anything from the Android phones. This is one way and no ability to re-transmit so if a message does not get there, it has another chance next time. Everything is updated once a second.

I tried the following and I do not get data in the app. From the comments, I must need to add some kind of router function in my Linux machine.

void init_udp(){
    return;
    sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (sock < 0){
        printf("ER UDP Socket error\n");
    }
    else printf("UP Socket %d OK\n",sock);
}

void write_udp(char *buf){
    return;
// nmea data is in buff
    if (sock >= 0){
        int on = 1;
        setsockopt( sock, SOL_SOCKET, SO_BROADCAST, &on, sizeof(on) );
        struct sockaddr_in    address = {0};
        address.sin_family = AF_INET;
        address.sin_addr.s_addr = inet_addr( "255.255.255.255" ); //
        address.sin_port = htons( 3000 );
        if ( sendto( sock, buf, strlen(buf), 0, (struct sockaddr*)&address, sizeof(address) ) < 0) printf("ER UDP send error\n");
        else {
            printf("UP %s\n",buf);
        }
    }
}

I am not really sure what I need to do.

Answer 1

What you want to do is send a UDP packet to a broadcast IP address. This will send to thing in the subnet. eg 10.255.255.255 is the broadcast address for the 10.xxx subnet. You can also use the global 255.255.255.255 which should also send to your subnet and no router is going to pass that on to another one these days.

Also you need to make your socket able to send broadcast messages. In particular you need the option SO_BROADCAST set. The following is specifically Windows because of the BOOL . Its presumably an int for most platforms.

BOOL on = TRUE;
setsockopt( sock, SOL_SOCKET, SO_BROADCAST, &on, sizeof(on) );

Also you can't use send() for a UDP socket unless its "connected", so you should use sendto() and use the broadcast address.

To specify an address and port, use need to create a socket address.

struct sockaddr_in    address = {0};

address.sin_family = AF_INET;
address.sin_add.s_addr = inet_addr( "255.255.255.255" );
address.sin_port = htons( 3000 );

sendto( sock, buff, strlen(buff), 0, (struct sockaddr*)&address, sizeof(address) );

Note the use of the typecast. This is because sendto takes a sockaddr which is a generic address type, and here we are using an IPV4 address specifically. The sockaddr_in maps onto the sockaddr