c - udp send and receive on the same socket

Question

I would like to send and receive packets on the same socket, is it possible or I have to create two socket, one to send and one to receive? If yes, can you give me an example?

Another question: how can I get the source ip from a received packet?

EDIT (code example):

int main(void) {
    struct sockaddr_in si_me, si_other;
    int s, i, slen=sizeof(si_other);
    char buf[BUFLEN];

    if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1)
        die("socket");

    memset((char *) &si_me, 0, sizeof(si_me));
    si_me.sin_family = AF_INET;
    si_me.sin_port = htons(1234);
    si_me.sin_addr.s_addr = htonl(192.168.1.1);

    if (bind(s, &si_me, sizeof(si_me))==-1)
        die("bind");

    if (recvfrom(s, buf, BUFLEN, 0, &si_other, &slen)==-1)
       diep("recvfrom()");
    printf("Data: %s \nReceived from %s:%d\n\n", buf, inet_ntoa(si_other.sin_addr), ntohs(si_other.sin_port));

    //now I want the server to answer back to the client

    close(s);
    return 0;
}

Remy Lebeau · Accepted Answer · 2014-05-19T02:19:09.780

26

Yes, you can use the same socket for sending and receiving. recvfrom() tells you the IP/port of the sender. Simply sendto() that IP/port using the same socket that you use with recvfrom(), eg:

int main(void) {
    struct sockaddr_in si_me, si_other;
    int s, i, blen, slen = sizeof(si_other);
    char buf[BUFLEN];

    s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (s == -1)
        die("socket");

    memset((char *) &si_me, 0, sizeof(si_me));
    si_me.sin_family = AF_INET;
    si_me.sin_port = htons(1234);
    si_me.sin_addr.s_addr = htonl(192.168.1.1);

    if (bind(s, (struct sockaddr*) &si_me, sizeof(si_me))==-1)
        die("bind");

    int blen = recvfrom(s, buf, sizeof(buf), 0, (struct sockaddr*) &si_other, &slen);
    if (blen == -1)
       diep("recvfrom()");

    printf("Data: %.*s \nReceived from %s:%d\n\n", blen, buf, inet_ntoa(si_other.sin_addr), ntohs(si_other.sin_port));

    //send answer back to the client
    if (sendto(s, buf, blen, 0, (struct sockaddr*) &si_other, slen) == -1)
        diep("sendto()");

    close(s);
    return 0;
}

edited May 19 '14 at 02:19

answered May 18 '14 at 21:40

Remy Lebeau

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can you please post an example? – user3574984 May 18 '14 at 21:45
Of what? sending/receiving? Or getting the source IP/Port? There are plenty of examples of both online if you look around. – Remy Lebeau May 19 '14 at 00:16
Server receive a packet => answer back on the same udp socket... I know how to receive and how to send, but not how to do it with the same socket! – user3574984 May 19 '14 at 01:09
What is so confusing about calling `sendto()` on the same socket handle that you call `recvfrom()` on? – Remy Lebeau May 19 '14 at 02:12
nothing, I was sending on the wrong port! ^^" ty for your help! – user3574984 May 20 '14 at 07:19
Is there any issue with doing this in a multi-threaded environment? or does the OS handle that? – Jordan Mackie Mar 30 '19 at 16:19
2

@JordanMackie you can send in one thread while reading in another thread, if that is what you are asking. Don't send in 2+ threads, or read in 2+ threads, unless you synchronize them to avoid overlap – Remy Lebeau Mar 30 '19 at 17:19

c - udp send and receive on the same socket

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