03. Hello Server/Client

This chapter continues the introduction of socket programming. We’ll write 2 simple (incomplete and broken) programs to demonstrate the syscalls from the last chapter. The first program is a server, it accepts connections from clients, reads a single message, and writes a single reply. The second program is a client, it connects to the server, writes a single message, and reads a single reply. Let’s start with the server first.

First, we need to obtain a socket fd: int fd = socket(AF_INET, SOCK_STREAM, 0);

The AF_INET is for IPv4, use AF_INET6 for IPv6 or dual-stack socket. For simplicity, we’ll just use AF_INET throughout this book.

The SOCK_STREAM is for TCP. We won’t use anything other than TCP in this book. All the 3 parameters of the socket() call are fixed in this book.

Next, we’ll introduce a new syscall:

int val = 1;
setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &val, sizeof(val));

The setsockopt() call is used to configure various aspects of a socket. This particular call enables the SO_REUSEADDR option. Without this option, the server won’t able to bind to the same address if restarted. Exercise to reader: find out what exactly is SO_REUSEADDR and why it is needed.

The next step is the bind() and listen(), we’ll bind on the wildcard address 0.0.0.0:1234:

    // bind, this is the syntax that deals with IPv4 addresses
    struct sockaddr_in addr = {};
    addr.sin_family = AF_INET;
    addr.sin_port = ntohs(1234);
    addr.sin_addr.s_addr = ntohl(0);    // wildcard address 0.0.0.0
    int rv = bind(fd, (const sockaddr *)&addr, sizeof(addr));
    if (rv) {
        die("bind()");
    }

    // listen
    rv = listen(fd, SOMAXCONN);
    if (rv) {
        die("listen()");
    }

Loop for each connection and do something with them.

    while (true) {
        // accept
        struct sockaddr_in client_addr = {};
        socklen_t socklen = sizeof(client_addr);
        int connfd = accept(fd, (struct sockaddr *)&client_addr, &socklen);
        if (connfd < 0) {
            continue;   // error
        }

        do_something(connfd);
        close(connfd);
    }

The do_something() function is simply read and write.

static void do_something(int connfd) {
    char rbuf[64] = {};
    ssize_t n = read(connfd, rbuf, sizeof(rbuf) - 1);
    if (n < 0) {
        msg("read() error");
        return;
    }
    printf("client says: %s\n", rbuf);

    char wbuf[] = "world";
    write(connfd, wbuf, strlen(wbuf));
}

Note that the read() and write() call returns the number of read or written bytes. A real programmer must deal with the return value of functions, but in this chapter, I have omitted lots of things for brevity. And the code in this chapter is not the correct way to do networking anyway.

The client program is much simpler:

    int fd = socket(AF_INET, SOCK_STREAM, 0);
    if (fd < 0) {
        die("socket()");
    }

    struct sockaddr_in addr = {};
    addr.sin_family = AF_INET;
    addr.sin_port = ntohs(1234);
    addr.sin_addr.s_addr = ntohl(INADDR_LOOPBACK);  // 127.0.0.1
    int rv = connect(fd, (const struct sockaddr *)&addr, sizeof(addr));
    if (rv) {
        die("connect");
    }

    char msg[] = "hello";
    write(fd, msg, strlen(msg));

    char rbuf[64] = {};
    ssize_t n = read(fd, rbuf, sizeof(rbuf) - 1);
    if (n < 0) {
        die("read");
    }
    printf("server says: %s\n", rbuf);
    close(fd);

Compile our programs with the following command line:

g++ -Wall -Wextra -O2 -g 03_server.cpp -o server
g++ -Wall -Wextra -O2 -g 03_client.cpp -o client

Run ./server in a window and then run ./client in another window. You should see the following results:

$ ./server
client says: hello

$ ./client
server says: world

Exercise for readers: read manpages of APIs used in this chapter, or find online tutorials for them. Make sure you know how to find helps on API usage since this book won’t cover the details of API usage.

Source code:

• 03_client.cpp
• 03_server.cpp