// server_defer_accept.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netinet/tcp.h>
#include <sys/socket.h>

#define PORT 9090

int main() {
    int sockfd, connfd;
    struct sockaddr_in servaddr, cliaddr;
    socklen_t len;

    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) { perror("socket"); exit(1); }

    int opt = 1;
    setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));

    // TCP_DEFER_ACCEPT: wait until client sends data before accept() returns
    int timeout = 5; // seconds
    if (setsockopt(sockfd, IPPROTO_TCP, TCP_DEFER_ACCEPT,
                   &timeout, sizeof(timeout)) < 0) {
        perror("setsockopt TCP_DEFER_ACCEPT");
    }

    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"); exit(1);
    }

    if (listen(sockfd, 5) < 0) {
        perror("listen"); exit(1);
    }

    printf("Server waiting for client...\n");

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

    printf("Client connected!\n");

    char buffer[1024];
    int n = read(connfd, buffer, sizeof(buffer)-1);
    buffer[n] = '\0';
    printf("Received from client: %s\n", buffer);

    const char *reply = "Hello Client!";
    write(connfd, reply, strlen(reply));

    close(connfd);
    close(sockfd);
    return 0;
}