UDP 套接字不读取数据包

Question

So I have a device connected to my network card and it sends data to port 11678 and address 192.168.121.1 using IPv4 and UDP. I have checked that the device does actually send to that port and address using IPv4 and UDP by calling tcpdump. However my C socket does not receive any packets. Below I have a minimum non-working example that just runs an infinite loop until one packet is received. It does not receive any packets even though tcpdump does, so I assume something is wrong with my code.

#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

#define DEST_ADDR "192.168.121.1"
#define DEST_PORT 11678
#define PACKET_MAXSIZE 1024

int main(int argc, char **argv) {
    struct sockaddr_in dest_addr;
    bzero(&dest_addr, sizeof(dest_addr));
    dest_addr.sin_family = AF_INET;

    /* create socket */
    int fd = socket(AF_INET, SOCK_DGRAM | SOCK_NONBLOCK, 0); // use SOCK_NONBLOCK?
    if (fd < 0) {
        perror("Could not create socket.");
    }

    /* bind port and address to socket */
    dest_addr.sin_port = htons(DEST_PORT);
    inet_aton(DEST_ADDR, &dest_addr.sin_addr);
    int rc = bind(fd, (struct sockaddr*) &dest_addr, sizeof(dest_addr));
    if (rc != 0) {
        perror("Could not bind socket to local address");
    }

    /* read packets */
    void* buf;
    posix_memalign(&buf, 4096, 1024);
    while (true) {
        ssize_t read_size = read(fd, buf, PACKET_MAXSIZE);
        printf("%d\n", read_size);
        if (read_size > 0) {
            break;
        }
    }

    return 0;
}

The read just returns -1 and sets errno to 11 ( EAGAIN ) in every iteration. Any help is appreciated. Thanks in advance.

Answer 1

If you're on a system that uses iptables, check that you aren't dropping packets. tcpdump will show packets that are incoming before they get to iptables.

Another thing is that you should be using epoll or select to read from the socket in a more controlled way. EAGAIN isn't neccessarily wrong: it just means there's no data. But you're whizzing round that while loop without waiting, so I'd expect lots of EAGAIN's until something actually arrives at the port.