// File: reuseport_server.c
// Compile: gcc reuseport_server.c -o reuseport_server
// Run: start multiple instances: ./reuseport_server & ./reuseport_server &

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

#define PORT 9090

int main() {
    int sockfd, connfd;
    struct sockaddr_in servaddr, cliaddr;
    socklen_t len;
    char buffer[1024];
    int opt = 1;

    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) { perror("socket"); exit(EXIT_FAILURE); }

    // Enable SO_REUSEPORT
    if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEPORT, &opt, sizeof(opt)) < 0) {
        perror("setsockopt SO_REUSEPORT");
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    memset(&servaddr, 0, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = INADDR_ANY;
    servaddr.sin_port = htons(PORT);

    if (bind(sockfd, (struct sockaddr*)&servaddr, sizeof(servaddr)) < 0) {
        perror("bind");
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    if (listen(sockfd, 5) < 0) { perror("listen"); close(sockfd); exit(EXIT_FAILURE); }

    printf("Server (PID %d) listening on port %d with SO_REUSEPORT...\n", getpid(), PORT);

    while (1) {
        len = sizeof(cliaddr);
        connfd = accept(sockfd, (struct sockaddr*)&cliaddr, &len);
        if (connfd < 0) { perror("accept"); continue; }

        int n = read(connfd, buffer, sizeof(buffer) - 1);
        if (n > 0) {
            buffer[n] = '\0';
            printf("Server (PID %d) received: %s\n", getpid(), buffer);
            write(connfd, buffer, n); // echo back
        }
        close(connfd);
    }

    close(sockfd);
    return 0;
}