#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

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

// Max UDP payload size (safe cross-platform limit)
#define MAX_PACKET_LEN 65507  // (65535 - 8 byte UDP header - 20 byte IP header)
// MAX_PACKET_LEN 65507 - 15  => fails
// MAX_PACKET_LEN 65507 - 19  => okay
// MAX_PACKET_LEN 65507 - 20  => okay

int main() {
    int fd;
    struct sockaddr_in6 sa6;
    socklen_t sa_len;
    char *buf;
    ssize_t n;
    size_t len;

    // Create IPv6 UDP socket
    fd = socket(AF_INET6, SOCK_DGRAM, 0);
    if (fd < 0) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    // Setup target IPv6 address (loopback)
    memset(&sa6, 0, sizeof(sa6));
    sa6.sin6_family = AF_INET6;
    sa6.sin6_port = htons(9999);  // Change if needed
    if (inet_pton(AF_INET6, "::1", &sa6.sin6_addr) <= 0) {
        perror("inet_pton");
        close(fd);
        exit(EXIT_FAILURE);
    }
    sa_len = sizeof(sa6);

    // Allocate buffer (can adjust to go beyond MAX_PACKET_LEN)
    len = MAX_PACKET_LEN;  // Try MAX_PACKET_LEN + 1 to force EMSGSIZE
    buf = malloc(len);
    if (!buf) {
        perror("malloc");
        close(fd);
        exit(EXIT_FAILURE);
    }

    memset(buf, 'B', len);  // Fill buffer with dummy data

    // Send the packet
    n = sendto(fd, buf, len, 0, (struct sockaddr *)&sa6, sa_len);
    if (n < 0) {
        perror("sendto");
        if (errno == EMSGSIZE) {
            fprintf(stderr, "Error: Message too long (EMSGSIZE)\n");
        }
    } else {
        printf("Sent %zd bytes successfully over IPv6\n", n);
    }

    // Cleanup
    free(buf);
    close(fd);
    return 0;
}