recvfrom() not filling the from IP address even for UDP messages in first call

问题

I am using recvfrom() to get the UDP datagram into my buffer and passing non-NULL from_addr structure & len argument expecting it to get the source IP. But for the first call, they are NULL. Subsequent calls are filling the addr structure and len field. Is this expected ?

#include <stdio.h>
#include <string.h>

#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>


int
main()
{
    struct  sockaddr_in myaddr, from_addr;
    char    from_ip[1024] = "", myip[2014] = "";
    char    rmsg[1024 * 1024] = "";
    int     sock_fd=-1,nbytes=0;
    int     len=0;
    unsigned short port=0;

    sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
    if(sock_fd == -1)
    {
        perror("Socket");
        return -1;
    }

    printf("IP Addr & Port to bound to :");
    scanf("%s%u", myip, &port);

    myaddr.sin_family = AF_INET;
    myaddr.sin_port = htons(port);
    inet_pton(AF_INET, myip, &myaddr.sin_addr);

    printf("Bound to %s:%u\n", myip, htons(port));

    nbytes = bind(sock_fd, &myaddr, sizeof(myaddr));
    if(nbytes == -1)
    {
        perror("Bind");
        return -1;
    }

    do
    {
        /**/nbytes = recvfrom(sock_fd, rmsg, sizeof(rmsg), 0, &from_addr, &len);/**/
        inet_ntop(AF_INET, &from_addr.sin_addr, from_ip, sizeof(from_ip));

        printf("Received %d bytes from %s@%d : %s\n", nbytes, from_ip, ntohs(from_addr.sin_port), rmsg);

        if(strcmp(rmsg, "Bye") == 0)
        {
            printf("Client Disconnected.\n");
        }
        memset(rmsg, 0, sizeof(rmsg));
    }while(1);

    return 0;
}

Output:

-------------------------------OutPut -----------------------------------------
root@ubuntu:/home/akumarkk/Desktop/distributedComp_Project/test# ./userver 
IP Addr & Port to bound to :127.0.0.1 12
Bound to 127.0.0.1:3072
Received 2 bytes from 0.0.0.0@0 : hi
Received 6 bytes from 127.0.0.1@49511 : second
Received 7 bytes from 127.0.0.1@49511 : message
Received 3 bytes from 127.0.0.1@49511 : Bye
Client Disconnected.

回答1:

You must populate len before calling recvfrom, you can't leave it 0:

len = sizeof from_addr;
nbytes = recvfrom(sock_fd, rmsg, sizeof(rmsg), 0, &from_addr, &len);

来源：https://stackoverflow.com/questions/23472533/recvfrom-not-filling-the-from-ip-address-even-for-udp-messages-in-first-call

标签

sockets

udp

udpclient