Program Statement

Create a pair of UDP sockets — a receiver bound to a well-known port and a sender that transmits a message and reads the reply.

/* udp_recv.c — receiver */
int main(void) {
    int fd = socket(AF_INET, SOCK_DGRAM, 0);
    struct sockaddr_in addr = {0}, cli; socklen_t len = sizeof(cli);
    addr.sin_family = AF_INET; addr.sin_port = htons(9877);
    addr.sin_addr.s_addr = htonl(INADDR_ANY);
    bind(fd, (struct sockaddr *)&addr, sizeof(addr));
    char buf[1024];
    int n = recvfrom(fd, buf, sizeof(buf)-1, 0, (struct sockaddr *)&cli, &len);
    buf[n] = '\0';
    printf("received: %s\n", buf);
    sendto(fd, "got it!", 7, 0, (struct sockaddr *)&cli, len);
    close(fd); return 0;
}

/* udp_send.c — sender */
int main(void) {
    int fd = socket(AF_INET, SOCK_DGRAM, 0);
    struct sockaddr_in srv = {0};
    srv.sin_family = AF_INET; srv.sin_port = htons(9877);
    inet_pton(AF_INET, "127.0.0.1", &srv.sin_addr);
    sendto(fd, "hello sockets", 13, 0, (struct sockaddr *)&srv, sizeof(srv));
    char buf[1024];
    int n = recv(fd, buf, sizeof(buf)-1, 0);          /* reply */
    buf[n] = '\0'; printf("server says: %s\n", buf);
    close(fd); return 0;
}

Running It

1. Terminal 1: gcc udp_recv.c -o udp_recv && ./udp_recv
2. Terminal 2: gcc udp_send.c -o udp_send && ./udp_send

Theory Link

Unit 1 "Elementary TCP Sockets" + Unit 2 "Elementary UDP Sockets". Note in your record: the receiver must bind to a known port; the sender's port is assigned implicitly (ephemeral) on the first sendto.