Elementary TCP & UDP Sockets: System Call Lifecycles

Explanation

TCP and UDP sockets present distinct paradigms. Stream sockets (SOCK_STREAM) provide reliable byte-stream channels requiring explicit connection setup. The listening socket descriptor (listenfd) serves solely as a connection receptionist, while accept() yields a connected descriptor (connfd) for per-client data exchange. In contrast, Datagram sockets (SOCK_DGRAM) are connectionless. A single UDP socket can exchange datagrams with multiple distinct endpoints using recvfrom() and sendto(), which encapsulate destination addressing directly in the payload header.

Key points

• TCP Client Call Sequence: socket() -> connect() -> write()/read() -> close().
• listen(sockfd, backlog): Backlog specifies the maximum length of completed and incomplete connection queues.
• accept() blocks until a completed 3-way handshake arrives, returning a new connected socket descriptor.
• UDP Sockets use sendto() and recvfrom() without listen() or accept(); each packet specifies the destination sockaddr.

Example

Code examples

// Concurrent TCP Echo Server in C
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main() {
    int listenfd = socket(AF_INET, SOCK_STREAM, 0);
    struct sockaddr_in servaddr;
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port = htons(9000);

    bind(listenfd, (struct sockaddr *)&servaddr, sizeof(servaddr));
    listen(listenfd, 10);

    while (1) {
        struct sockaddr_in cliaddr;
        socklen_t clilen = sizeof(cliaddr);
        int connfd = accept(listenfd, (struct sockaddr *)&cliaddr, &clilen);
        if (fork() == 0) { // Child process
            close(listenfd);
            char buf[1024];
            int n = read(connfd, buf, sizeof(buf));
            write(connfd, buf, n); // Echo
            close(connfd);
            exit(0);
        }
        close(connfd); // Parent closes connected socket
    }
    return 0;
}

Exam questions

• [10 Marks] Write a complete C program to implement a concurrent TCP daytime/echo server using socket(), bind(), listen(), accept(), and fork().
• [6 Marks] Compare TCP socket functions with UDP socket functions (sendto/recvfrom vs write/read).